The Pamphleteer/Volume 8/Some Remarks on the Mildew of Wheat
MILDEW OF WHEAT,
Choice of Seed Corn,
IN REFERENCE TO AN HYPOTHESIS.
SIR JOSEPH BANKS, K. B.
&c. &c. &c.
MILDEW OF WHEAT,
THE physician investigates the nature of his patient's case before he prescribes for it: if he misunderstands the disorder, he is not very likely to be successful in its cure. But I am not about to assume the character and perform the functions of the physician. I have no remedy to propose for this formidable disease, the mildew, believing it to be utterly irremediable, and that Doctor Solomon himself, in all his glory, cannot offer a specific. What cannot be remedied, however, may possibly be prevented. If the nature of the mildew, its origin, and its cause be rightly understood, means may be suggested by some ingenious observer, not to mitigate its effects perhaps, but possibly to resist its encroachment. In the present year [1811,] this Dæmon has taken a wide and destructive flight, shedding poison from its wings, and blasting with its breath the promise of the spring. I would not give a groundless alarm wantonly; but it is better that the public should suffer from an imaginary evil than a real one, from the apprehension of a deficiency in the wheat crop, than the existence of it. If the mildew has not extended its ravages so widely as I fear it has, all the better: the alarm is groundless, perhaps useless, but at any rate not mischievous, for it can do no harm to take timely precautions against an uncertain evil. The very precautions may repel the evil which, without them, would invade us. As the interval between the last harvest and the present was little more than eleven months, we may expect that the interval between the present harvest and the next will be nearly thirteen. Harvest is unusually early this year; but this circumstance, added to the abundant crop and fine quality and weight of the last year’s wheats, gives us reason to believe that the stock in hand is considerable. To economize this stock until the quality of the present year’s produce is ascertained cannot be bad policy, nor can any alarm be injurious which promotes an economy of consumption. The accounts from Sussex, Surrey, Hampshire, Dorset, Somerset, Essex, Kent, Buckinghamshire, Norfolk, and Suffolk, and many parts of Scotland, concur in representing the wheat crop as deficient in quantity, and injured as to its quality.
A few years ago Sir Joseph Banks published "A short account of the causes of the diseases in corn, called by the farmers the Blight, the Mildew, and the Rust." Without the assistance of the plates which accompany this ingenious paper, I shall scarcely be able to do justice to its contents. Botanists, says Sir Joseph, have long known that the blight in corn is occasioned by the growth of a minute parasitic fungus or mushroom on the leaves, stems, and glumes of the living plants. Of this fungus, in its different stages of growth and maturity, Mr. Bauer, botanical painter to the King, has made drawings from the original, very highly magnified, representing its destructive agency; in order to understand which, it is necessary to premise that the striped appearance of the surface of a straw is caused by alternate longitudinal partitions of the bark, the one imperforate and the other furnished with one or two rows of pores, which are shut in dry, open in wet weather. The intention of this arrangement, or the final cause, as some philosophers would call it, seems to be that whatever moisture is suspended in the atmosphere may be imbibed by the orifices when open, and afterwards that it may be retained by having the mouths of them closed. Through these pores it is presumed that the seeds of the fungus gain admission, and at the bottom of the hollows to which they lead, germinate and probably push their minute roots into the cellular texture beyond the bark, where it is supposed they draw their nourishment by intercepting the sap which was intended by nature for the nutriment of the grain. The corn becomes shrivelled in proportion to the number and activity of these fungi, and as the kernel only is abstracted from the grain while the cortical part remains undiminished, the proportion of flour to bran in blighted corn is always reduced in the same degree as corn is made light.
Sir Joseph goes on to observe, that the leaf is probably first infected in the spring or early in the summer, before the corn shoots up into straw, and that the fungus is then of an orange color. After the straw has become yellow, the fungus assumes that of a deep chocolate brown. Each individual is so small that every pore on a straw will produce from twenty to forty fungi, as may be seen in Mr. Bauer's plates, and every one of these will produce at least a hundred seeds. If each of these branches out, therefore, into the number of plants which are represented at the bottom of a pore in one of the plates, the increase must be incalculably great, and a few diseased plants scattered over a field must be sufficient to infect a whole parish. The seeds being very little heavier than air are wafted by every breeze, and are attached by the slightest moisture to the devoted plants.
Such is Sir Joseph's hypothesis concerning the cause of mildew or blight (which he seems to consider as the same thing) in corn. There seems room for suspicion that it is not perfectly correct. Sir Joseph appears to have mistaken, ab principio, an effect for a cause, or at least a proximate cause for a remote one.
On the surface of all diseased and putrid vegetable matter a mucor or mouldiness is formed. In wine vaults, in rotten timber, in decayed trees, &c. fungi are always to be found. Would Sir Joseph hazard the assertion, that the tree became diseased and decayed because a fungus was attached to it? Would he not rather suspect that because the tree was decayed, therefore it became the recipient of the fungus? There may be difficulties on both sides; but the hypothesis of Sir Joseph Banks has some very serious objections to encounter. How comes it to pass that the mildew, blight, call it what you will, should be so partially distributed that of two adjoining fields, nay that of two adjoining ridges in the same field, one may be entirely exempt from the disease, and the other severely suffer by it? But every farmer knows that this is no imaginary or even uncommon case: the track of the mildew is oftentimes remarkably distinct, which it would hardly be if the air were impregnated with this poisonous and prolific dust. Go into a hay-field when the anthers are shedding their pollen; see the cloud of virility which is diffused over the whole surface of the field by every undulation of the air. Were the disease, which we call the mildew, occasioned by such a cloud of contagious dust as this, we should as rarely see the precise extent and boundaries of the evil as we do of a shower of rain or a fall of snow.
Sir Joseph thinks it probable that the leaf is first infected in the spring, or early in the summer, before the corn shoots up into straw; but this cannot be accounted for consistently with his hypothesis. The increase of these fungi is allowed to be incalculably great, and the period of their pubescence and maturity is short. As the summer advances, therefore, the air must be more and more heavily laden with these seeds, and as the size of the cuticular orifices of the straw increases with its growth, the seeds of the fungus would find more room as the summer advances, and the mischief extend with an immeasurable rapidity over the whole country. We should never be free from it: many thousand acres would annually be destroyed in the summer, which were uninfected in the spring. The fact, however, is as Sir Joseph states it, namely that the leaf is first infected in the spring, or early in the summer. No one fears what is called a mildew on his wheat crop after the blossom is set. The season of flowering is indeed a very critical one: heavy rains and blustering winds may wash away or so disperse the pollen as to frustrate and render abortive the necessary process of fecundation. For the same reason it is injudicious to hoe corn during the time of blossoming; the clothes of those employed are very likely to brush away the farina and injure the impregnation. When this is the case, from whatever circumstance it arises, the farmer will find at harvest that much of his wheat is blind (that is to say), many of the coshes or seed vessels are destitute of kernels, while the straw is perfectly bright and free from any appearance of disease.
Wet lands are peculiarly subject to mildew, and if in any field one spot is wetter than the rest, that spot will be most frequently and earliest affected. Again, newly inclosed commons, which are often light and spongy, approaching to elastic, very much expose the wheat which grows upon them to mildew. How are these facts to be accounted for on the hypothesis of Sir Joseph Banks? If the atmosphere is charged with the seeds of these fungi, floating there like the dust of a puff-ball, they would be indiscriminately scattered, and no one particular spot be more affected than another; the first shower of rain, or the first fog that fell, must precipitate the seed with an equal and an even vengeance, attaching it to every straw in the field. I cannot but suspect, therefore, that the plant is diseased before the fungus seizes on it, and that those stems which are in sound health resist its advances, or counteract its activity.
Fungi find an appropriate nidus in diseased and decayed vegetable matter, and particularly if it remains in a state of moisture; the wood-work of vaults, as was before observed, and indeed their walls are always covered with them, rotten timber and the hollow trunks of trees are rarely free from them. Nature suffers no fit recipient for animal or vegetable life to remain void; microscopic beings of both kingdoms are always ready to seize on and make their prey of every thing which can furnish them with subsistence, and the decomposition of all animal and vegetable matter affords food for myriads. If the branch of a tree is rent by the wind, or blasted by the lightning, that branch will frequently become a prey to fungi and musci, while the sound healthy parts of the same tree resist the encroachments of the parasites. But if mildew is not caused by these parasitic fungi, to what is it to be attributed? I am inclined to believe that the disease so called, that is to say the dark and striped appearance of the straw and shrivelled kernel, is produced by the immediate operation of these fungi, and that they act precisely as Sir Joseph Banks has suggested. At the same time, I suspect that the remote cause is some disease in the plant, probably arising from an interrupted circulation of its sap, and totally unconnected with mildew. Is it inquired what this disease may be? I cannot answer the enquiry with any confidence, but the notice of a few facts may possibly suggest an answer.
Spring corn, Sir Joseph truly remarks, is less damaged by it than winter corn: the spring wheat of Lincolnshire was not in the least shrivelled in the harvest of 1804, although the straw was in some degree affected. About a fortnight ago I passed a few days in the country and examined the corn in the neighbourhood of the gentleman's house where I was staying. The barleys were very bright, very abundant, and heavy; some wheat which had been set on a newly-inclosed common where the land had not become firm, but was spongy and full of springs, was so much injured by the mildew that it seemed scarcely worth the trouble of reaping. This devoted field of wheat was only divided by a hedge from another, which was entirely exempt from it, although most of the neighbouring fields were partially affected. On enquiry I found that this fortunate field was not drilled until Christmas time. We rarely hear of oats or barley being mildewed. Now it is obvious that spring corn is free from the frosts of winter, and much more so from those of spring than the corn which is sown in autumn. Wheat is a very hardy plant, but it suffers probably more severely from the vernal than the winter frosts. If the season is mild the vessels of the young wheat begin to fill with fluids as early as the latter end of February or beginning of March; a severity of frost at this time, which in the winter would be perfectly harmless, is very likely to burst its tumid and tender vessels, and so materially injure the plant as to render it an unresisting prey to the ravages of this fungus. Thus we frequently see the young shoots of trees blighted, particularly the ash, the weeping willow, and the almond, while those of the preceding season, which are more hardy, remain uninjured: the juices of these succulent and delicate shoots are expanded by the frost until the vessels which contain them are ruptured, the organization is destroyed, and decomposition follows.
Sir Joseph says, "that the leaf is probably first infected in the spring, or early in the summer, before the corn shoots up into straw:" this is accounted for on the supposition now suggested, that the first injury arises from the severity of vernal frosts; but according to the hypothesis of Sir Joseph, it ought not to be the case, for, as was just now observed, the quantity of the fungus seed must be much greater in summer than in spring, and the size of the orifices into which it finds admission must also increase with the increased size of the straw, and thus afford a larger surface for attachment.
Again—the mildew is partial in its ravages—so is frost. Persons who are accustomed to the management of wall fruit-trees know how slight a screen is sufficient to protect them from its effects. Three or four trees standing together in a hedge-row may so mitigate the severity of frost on the ridges which they screen, that the plants on those ridges, if injured at all, would soon recover their vigor.
Once more—if there are any spots in a field particularly wet, there the corn is sure to suffer most. The reason is obvious: the more succulent a plant is, and the greater the quantity of moisture it has imbibed, the more susceptible must it be of injury from frost; the fuller its vessels are the more liable are they to be ruptured. Farmers, whose fears are always alive, anticipate a mildew from the continuance of a raw and foggy air in the spring or early part of the summer: nor are they often mistaken. In such a continued state of atmosphere, the grasses open their innumerable orifices, and imbibe a prodigious quantity of moisture; if a biting frost immediately succeeds, much mischief may reasonably be apprehended from the rupture of those vessels through which the necessary circulation is to be carried on.
It is said that wheat in the neighbourhood of barberry bushes seldom escapes the mildew: Sir Joseph supposes that the parasitic fungus which often attacks the barberry as well as the wheat, is one and the same species, and that the seed is transferred from the barberry to the corn. The plan which he proposes for preventing the spread of the disease is amusing enough, and reminds one of Dr. Last's celebrated cure for corns, "I pluck 'em up by the roots," quoth the Doctor; thus Sir Joseph Banks recommends the farmer to search diligently in the spring of the year for every infected plant, and to pull it up by the roots. Alas, the age of Hercules is gone!
I began with expressing an apprehension that the disease was irremediable: to ascertain the cause which produces it may in some few cases possibly lead to its prevention. Perhaps a very early luxuriance of the grass of wheat is to be deprecated: where the soil is light and fertile, therefore, and where the situation is sheltered and warm, it may be better to sow wheat at Christmas than at Michaelmas. An instance in which I saw the advantage of this was mentioned before. Perhaps it is unwise to sow wheat on newly broken-up lands: they are so rich and so spongy that an unhealthy luxuriance is produced. They should not be trusted with wheat until they are sufficiently drained, and are become firm: the roots of the young plants in such situations are particularly preyed upon by the wire-worm and other insects. This injures their growth, and produces a sickliness which unfits them for resisting the attacks of fungi.
I cannot take leave of Sir Joseph's paper without risking a few remarks on another subject on which he has offered, what appears to me, some very hazardous advice in rather a peremptory tone. He says, that although the seeds of wheat are rendered so lean and shrivelled by the exhausting power of the fungus that scarcely any flour fit for the manufacture of bread can be obtained by grinding them, these very seeds will, except perhaps in the very worst cases, answer the purposes of seed corn as well as the fairest and plumpest sample that can be obtained. The use of the flour of corn in furthering the process of vegetation, he continues, is to nourish the minute plant from the time of its developement until its roots are able to attract food from the manured earth; and for this purpose one-tenth of the contents of a grain of good wheat is more than sufficient. Sir Joseph Banks proceeds to say that the selection of the plumpest grains for seed is an unnecessary waste of human subsistence, and he advises that what is usually thrown aside as dross, and given to the farmer’s poultry, should be employed for that purpose. In support of this advice reference is made to an experimental paper in the Annals of Agriculture by Mr. Mackie, which I confess I have not had the opportunity of perusing.
I may take the liberty of remarking that advice from so high a quarter, and pregnant with such momentous consequences ought not to be given without the utmost caution—without the utmost certainty of its safety. Sir Joseph has taken upon himself a vast responsibility only to be justified by a long and careful series of experiments, personally conducted by himself.
It is one thing to bring children into the world, it is another to rear them. Adam Smith remarks of the Highland women, that they frequently bear more than twenty and have not two alive! Poverty does not prevent generation, but is extremely unfavorable to the rearing of a progeny. Can we expect a vigorous and thriving child when we see the miserable disappointed wretch pressing the dry, milkless breast of a half-famished mother? Flour is to the infant plant what maternal milk is to the babe: if the corculum, the speck of vitality, is not injured, a seed when committed to the earth will certainly germinate. The cotyledons or lobes of the seed appear to be merely organs of nutrition, communicating the farinaceous substance of which they are composed to the young plant: if this nutritious substance is liberally communicated, which we suppose to be the case when the cotyledons are large and plump, the plant surely must thrive better and grow more rapidly than when the cotyledons, shrunk and shrivelled, distribute a parsimonious and scanty mucilage.
The deficiency in this latter case may, doubtless, be in some measure supplied by imparting an additional fecundity to the soil, just as a mother without milk may bring up her babe by feeding it with pap. The cases are sufficiently analogous: in both, the offspring exchanges its natural for an artificial and vicarious food. It is much to be apprehended, that although shrivelled kernels may very well succeed as an experiment in a garden pot, or in a very rich soil and warm situation, they would miserably frustrate the expectation and punish the credulity of a farmer, if employed on an extensive scale, without peculiar advantages of soil, season, and situation, and without an extraordinary measure of manure. But I fear that experiments on the vegetation of suspected seeds have been too commonly made in some snug sheltered corner of a fertile garden; or we put a little rich mould into a pot, and bring it into the house: the seeds sprout, are carefully watered, and nursed up, and a hasty, dangerous inference is deduced, that refuse corn may as safely be employed for seed as the plumpest and the soundest kernels: thus are chickens cheated of their victuals, and the crops of the whole country exposed to imminent danger through a mistaken and pitiful economy. The late Mr. Benjamin Bell, of Edinburgh, communicated an Essay to the Highland Society, which was published in their Transactions, on the influence of frost and other varieties of bad weather on the ripening of corn. He instituted a series of experiments on a very large scale, and conducted them carefully: the perusal of his paper will make those shudder who have put to peril the agricultural produce of the country by the rashness of their theories. It is some years since I read it, but I well remember regarding it as a useful paper, intelligent and intelligible, on a most important subject.
On heavy lands the progress of infant vegetation is always languid: when a seed first germinates, it is obvious that the plumula and the radicle must be much longer in struggling through stiff clods of earth, than when they have only to insinuate their easy courses through a pulverized and unresisting soil; and at the time of year when our wheats are usually committed to the ground, the difference of a few days only in the time of sowing is on some soils succeeded by a difference of weeks in the first appearance of the crops. November may be considered as the commencement of our winter: the season now grows cold and rainy, and the wheat which at that time remains in a heavy, strong and undrained soil, or which just peeps through the surface, has many sad vicissitudes of weather to encounter in its first feeble state, and lies a long time exposed to the depredations of hungry birds. I suspect, that on such soils, and at such a season, to retard vegetation by a starveling seed, would be a most perilous experiment, the young plant requiring all the nourishment which the plumpest kernel can supply, in order to preserve its existence and enable it to bear the rigor of the season during a protracted infancy.
The harvest of 1799 was the wettest that had been known for many years: farmers brewed beer from barley which the rains of heaven had malted. It was the general opinion that black, or even discolored barley, was very unsafe to be used for seed. Maltsters found that it would not sprout on their floors, with all the advantages of constant moisture and artificial heat. I tried the vegetative powers of barley in different tints of discoloration, and with different proportions of farinaceous substance in the kernel, and found, precisely as I had expected, that these had little or nothing to do with the mere process of germination. Twenty kernels, the thinnest and most meagre which could be selected, were planted in some rich garden mould, and kept in a warm room: every one of them germinated, tardily indeed, and having plenty of pap, though very little maternal milk, the radicle fibres spread, and the plants grew luxuriantly. Some of the blackest barley which could be found was afterwards sown in a garden; a large proportion of it grew and was healthy; the corculum of some kernels had been injured, probably rotted by excessive rains, and these made no effort to germinate. The two following are curious facts:—1. Sixty kernels of barley were taken from the floor of a neighbouring maltster which had been nineteen days on the heap, and had totally refused to germinate: they certainly never would have germinated there; I planted them in my garden, and of the sixty, forty-five grew as rapidly and vigorously as any barley I ever saw. It is evident, therefore, that warmth and moisture, however essential to germination, are not of themselves sufficient to excite it. The desideratum was probably a larger proportion of oxygen.—2. The second remarkable fact was presented in another experiment, by which it appeared, that barley would grow in a garden-pot, even after incipient germination had taken place in the ear as it had lain in the field six months before. Here was a complete suspension of vitality during several months without the destruction of it. But I should nevertheless think moderately of that man's prudence and judgment who, relying on the success of such petty experiments as these, should risk his next year’s crop by sowing barley which was shrunk and shrivelled, or which had already sprouted, or was the refuse of a maltster's floor.
To return to the mildew: some ingenious observations on this subject were published four or five years ago by Mr. Egremont, who considers the attachment of these fatal fungi as a secondary cause of mildew, and as arising from a previously diseased state of the plant, which he seems to think, as I do, would resist their insidious advances in a healthy and sound state. To the question, "what occasions these diseased secretions? to what cause may we attribute the previous injury of those vessels by internal disease?" To these questions he answers, "I believe we must be assisted by the analogy of vegetable with animal life. What more likely to be a leading cause than the stimulus of heat returning perhaps in a greater rather than in a less degree after the excitability of the vegetable had been highly increased by a sudden abstraction of that stimulus in a previous extreme of cold, in a manner analogous to the sudden application of heat to a frozen limb? Here it may not be improper to state my own observations," he continues, "on the temperature of the atmosphere about the time when I conceive the injury began to take place. Travelling in the afternoon on the 11th of July, 1804, I found it so extremely cold as to induce a belief that the thermometer would have been near the freezing point in the shade. I had not the opportunity to ascertain the fact; but to my feelings it was a change unusually severe. The day following was much warmer, and for some days after the heat kept increasing: on the 16th, 17th, and 18th it was intolerably hot. Within about a week from that time the disease made its appearance. In my recollection, the wind during the cold was easterly, or towards the north. I am also much disposed to believe that the long drought previous to the cold was favorable to the effect."
Weakness, whether produced by internal or by external injury, appears to be the pre-disposing cause which favours the attachment and growth, or at least which incapacitates the plant from repelling the attachment and growth of parasitic fungi. Some circumstances, however, are wanting to make this case complete and satisfactory of the hypothesis which Mr. Egremont endeavours to establish, namely, that the previous disease does not arise from any rupture of the tumid vessels, which is a cause purely mechanical, but that it arises from the too rapid and sudden return of a stimulus previously abstracted from the plant, which is a cause entirely physical.
In the first place, Mr. Egremont draws his inference respecting the effect of a return of abstracted stimulus, without the least knowledge of the antecedent state of the plants—without being at all prepared to state that the vessels had not in fact been injured, perhaps ruptured by frost. A long previous drought certainly leads us to infer, that the plants were not in a very succulent state: but still what is there to show that the injury resulted from a sudden return of the stimulus of heat, rather than from a sudden abstraction of it?
Mr. Egremont has omitted to state whether the mildew in that part of the country where he made his observations affected the summer corns, barley, oats, &c. as well as wheat. He says that the disease made its appearance about a week after this extraordinary change in the temperature of the air. One is curious to know whether he had made any accurate observations on the state of the straw in its earlier stages, and whether the disease might not have existed long before it excited his notice? If the mildew makes its appearance for the first time so late as the last week in July, there seems no reason why it should not attack with equal frequency and fatality the summer corns as the winter ones; but it certainly does not: we rarely see barley or oats injured by the mildew. General as the complaint is of mildew among the wheats this year, we have heard of but little injury from that cause among the summer corns: their unusual brightness is presumptive of their exemption from mildew.
Mr. Egremont says that as far as his observation has extended, the degree of injury to the corn is not always nor necessarily connected with the appearance of the straw; having found both the straw and ear with a favorable appearance when the grain has proved exceedingly small and partially fed; "not unfrequently one end of the grain pretty well fed, while the other has been much shrivelled." Where the ear is partially blind, or where without any external appearance of injury to the straw, the corn is light and shrivelled, I should be much inclined to suspect that the mischief arose from an insufficient impregnation. The accidents of weather may have rendered the process of fecundation abortive; but this barrenness has no relation to the mildew. Or the injury may have arisen, as I have suggested to have been the case this year with the barley crop, from a sort of precocity, a premature ripening, a too rapid conversion of the milk of the young kernel into farinaceous substance, occasioned by unusual drought and heat.
The following case is produced as a strong one in favor of a physical to the exclusion of a mechanical affection: "In a district called Marshland, in the West Riding of Yorkshire, the culture of potatoes is carried to a very great extent; consequently great quantities are planted late in the summer, and depend on a favorable autumn for an abundant produce. Early frosts frequently disappoint the cultivator, sometimes by coming so soon as the beginning of September. After the frost, during the early part of the succeeding day the mischief is scarcely perceptible, and ultimately it is found to be in proportion to the temperature and brightness of that day. Should it be cold and gloomy the injury is less; but when warm and bright, the leaves of the vegetable become black and never revive. Had the effect been produced by the expansion of the water or juices of the vegetable by frost, it would have been uniform and not affected by the succeeding day; but here is an effect corresponding to the degree of the returning stimulus. It will be recollected that I stated the stimulus of heat to be a leading cause, thereby intending not to exclude the other stimuli of light and moisture. The great and sudden evaporation of moisture, independent of its generating cold, I am persuaded, is alone sufficient to bring on vegetable death."
Upon this it may be observed, if the disease arises merely from the too sudden return of the abstracted stimuli of warmth and light, and moisture, that when these are slowly and gradually restored; when the two or three days which succeed these biting frosts are "cold and gloomy," the plants ought not, according to Mr. Egremont, to receive any injury at all. But it seems, from the very case itself, that they do receive some injury; they only receive less than when the following days are warm and bright. The case, therefore, does not prove that no injurious effect was produced by "an expansion of the water or juices of the vegetable by frost," or by the sudden abstraction of stimuli; it rather proves that the sudden return of abstracted stimuli increases the injury which the plants had already received. Restore to a frozen limb as slowly as you please the abstracted stimulus of warmth, you will not restore it to soundness. The injury has been already sustained.
But it is not worth while to contend further about the influence of the abstraction of stimuli which, in his zeal to illustrate that of the too sudden restoration of them, Mr. Egremont seems, indeed, rather to have overlooked than to deny. The fact is, that plants, like animals, are morbidly affected by any violent transition either from heat to cold, or from cold to heat; by the too sudden abstraction or the too sudden accumulation of stimuli. Laurels and other shrubs, which have been carefully planted on the south side of a house for the sake of protection against the cold, are very likely to suffer in the winter for want of protection against the sun, while others which are exposed to the uniform severity of a northern aspect pass uninjured through the rigors of the season. The former experience more rapid changes of temperature than the latter are exposed to. Sometimes in winter the sun breaks out very warmly for a few hours and dissolves the snow which had lodged upon the plants on the south side of a wall or house: at night, the moisture is frozen on their leaves and stems, and the plants suffer from the violence of the transition from heat to cold.
It is well observed by Mr. Knight—a gentleman who, by his patient investigation and very delicate and ingenious experiments, has contributed more than any other individual in this country, since the time of Hales, to enlarge our acquaintance with the physiology of vegetation; that part of botany by far the most interesting and most instructive, and indeed without which it must be considered as little better than a dry and barren nomenclature, unworthy to be ranked among the sciences;—It has been well observed by Mr. Knight, that it is from attending to the effect of temperature on plants in different states of irritability that early fruits are to be obtained, the influence of climate on the habits of plants depending much less on the aggregate quantity of heat in any climate than on the distribution of it in different seasons. He conjectures, and not without plausibility, that if two plants of the vine, or other tree of similar habits, even if obtained from cuttings of the same tree, were placed to vegetate during several successive seasons in very different climates; if the one, for instance, were planted on the banks of the Rhine, and the other on those of the Nile, each would adapt its habits to the climate in which it was placed; and if both were subsequently brought in early spring into a climate similar to that of Italy, the plant which had adapted its habits to a cold climate would instantly vegetate, whilst the other would remain perfectly torpid.
The irritability of plants is unquestionably increased by the subduction of heat: vines in grape-houses, which have been exposed to the external air and all the cold of winter will become forwarder and more vigorous at spring than those which have been confined within the warm atmosphere of the house throughout the winter. Onions, potatoes, barley, &c. &c. will vegetate with a less degree of heat in spring than in autumn: roots introduced from southern latitudes germinate earlier in the season than those brought from more northern ones, retaining under different circumstances the habits they had previously acquired. Apple trees sent from this country to New England blossomed for a few years too early for that climate, and bore no fruit; but afterwards learned to accommodate themselves to their new situation. (See Bot. Garden, Part II. Cant. I. p. 322. Note on the Swallow.)
Mr. Knight, in a communication to the Transactions of the Horticultural Society, (No. I.) says he has found that the crops of wheat on some very high and cold ground which he cultivates ripen much earlier when he obtains his seed-corn from a very warm district and gravelly soil which lies a few miles distant, than when he employs the seeds of the vicinity. Barley, also, grown on sandy soils in the warmest parts of England, is always found by the Scotch farmer, when introduced into his country, to ripen on his cold hills earlier than crops of the same kind do when he uses the seeds of plants which have passed through Several successive generations in the colder climate of Scotland.
Farmers have certainly an indistinct and confused notion of some benefit to be derived from changing their seed: and accordingly, every three or four years, most of them do obtain from some neighbour a supply of seed-corn. Not knowing the principle, however, upon which the anticipated advantage is to be calculated, it is a matter of much uncertainty whether their expectations are gratified or disappointed. The general principle I think seems to be, a disposition in plants to retain under different circumstances their own former habits, and a disposition in seeds to acquire the habits of their parent plants. Thus, on rich luxuriant soils where the crops are much disposed to have great length of straw, and consequently are very liable to be laid during the summer, it seems desirable to select seed from a poor and naked soil where it has been the habit of the parent plants to bear deficient straw: on the contrary, upon those bare and barren heaths, where a meagre vegetation does not half conceal the parched-up ground, it is probably adviseable to obtain seed from the most luxuriant and fertile fields. The product of the hills should be spread upon the vallies, and that of the vallies on the hills. As the Scotch farmer has found advantage by obtaining seed-corn from the warmest districts of England, so the farmers of the northern and the eastern counties of England would probably reap similar benefit by having recourse to their brethren of the west and south. Whatever preserves corn in a healthy state, whether wheat, barley, oats, peas, or any thing else, arms it against the attack of those parasitic fungi which, when they have taken possession of the plant, we call the mildew, and which is rather to be considered as the consequence of previous unhealthiness than as a disease per se. Too great luxuriance and too little are alike injurious; a man may kill himself by eating too much as well as too little, and die of fat, instead of famine. The adaption of seed to soil and climate, by promoting the health may repel the mildew of the corn; and to protect the young crop as much as possible against excessive moisture is of the greatest consequence: no land that is not thoroughly drained should be entrusted with wheat.
In Mr. Egremont's pamphlet he endeavours to account for some effects upon vegetation, which atmospheric variations alone seem insufficient to explain.
Evaporation is the great generator of cold, and one cause, no doubt, why soils saturated with moisture have a languid vegetation in the vernal months: but will it explain, says Mr. E., why the comparative powers of supporting vegetables should be reversed in the autumnal ones? Vegetation at the spring of the year on limestone and sandy soils is at least a fortnight earlier than on clayey or even rich deep soils. But the case is reversed in autumn: for without an unusual degree of temperature, vegetation on limestone, and sandy soils nearly ceases in November, whereas on clayey or rich deep soils it continues to the end of January; and frequently after the first or second fall of snow (provided the soil has not yet been cooled down by previous frost) exhibits at its dissolution the verdure of April.
If the presence of a greater quantity of moisture on argillaceous soils and the consequent generation of cold be alone sufficient to account for the effect, since the same disproportion exists in spring, the effect ought to be the same at both seasons.
Mr. Egremont, in his ingenious pamphlet, endeavours to solve this apparent paradox by a reference to the different capacities for heat in different earths: the solution, however, as he does not appear to have subjected any of these earths to actual experiment, is merely an inference derived from reasoning on the difficulty. To this relative power in different earths of retaining and transmitting heat he has recourse for explaining why the presence of a greater quantity of moisture in clayey and deep soils, than in calcareous and sandy ones, should at one part of the year (independent of the cold generated by evaporation) prevent so great an accumulation of heat on their surface, when at another it is the means of retarding sudden refrigeration; and why comparative dryness should at the spring of the year be favorable to the quicker accumulation of heat and consequent earlier vegetation, when in autumn and at the approach of winter it should be favorable to the loss of heat and produce a consequent earlier decline of vegetation. The subject is curious, and not so unconnected with that which is before us, as it may at first sight appear; I shall therefore steal a leaf out of Mr. Egremont's book, and present it to the reader as worthy of his attention:—
"Land or earth, particularly when dry, according to Kirwan, receives heat from light very readily, but transmits or conducts it through its own substance very slowly. Dr. Hales, found the air and surface of the earth in the month of August to be at the temperature of 88°, when a thermometer placed at 16 inches deep stood at 70°. In the month of October, when the air and surface were at 35°, at 16 inches deep the temperature was 48°, and at 24 inches 50°, nearly the mean annual heat. Van Swieten also observed that the greatest cold, if it lasted only a few days, could not penetrate 20 inches.
"If heat be generated on land in proportion to the continued absorption and fixation of the rays of light, its accumulation on the surface will be retarded by the power which the soil has of transmitting it downwards, a power which may vary according to the nature of the substances of which the soil is composed, and to its comparative dryness. But generally I believe it will be found that land is heated or cooled sooner, and to a higher degree, than water: whence it will have a greater range of temperature, and the air incumbent on it, as well as other bodies in contact with it, will be hotter in summer and cooler in winter than that over the sea.
"This is agreeable to general experience as well as to the particular experiments of Dr. Raymond, who found land in the neighbourhood of Marseilles often heated in summer to 160° of temperature, while the sea did not exceed 77°; and in winter, when the land was cooled down to 14° or 15°, the sea was never lower than 44°.
"On the other hand, when the rays of light fall on water, fewer rays are combined, less heat is the result, and that which is produced being more readily mixed by undulatory motion, or transmitted downwards in a manner similar to its progress in solids, it cannot be accumulated either so soon or in the same degree, at the surface; hence, as before observed, air incumbent on the sea is cooler in summer than that on the land.
"From the absence of the sun’s rays in winter, sufficient heat is not generated to keep up the temperature of the atmosphere, reduced by cold winds from the poles, by evaporation, and other causes. At this time the equilibrium is in some degree restored, or at least the severity tempered by heat given back from the earth and its waters.
"The heat from the surface of the earth then is much sooner reduced to the temperature of the incumbent air than the surface of the sea, the surface having but a slow and imperfect supply of heat from the interior, while the sea, from the change of specific gravity and other causes of motion, is constantly presenting a warmer surface to the air; consequently that which is incumbent over the sea will be warmed more than that over the land.
"By these facts, and by a similar mode of reasoning, as much as by the principle of evaporation, I would explain why clayey and deep soils which are most retentive of moisture should be the longest in heating or cooling; why the returning sun-beams cannot so soon accumulate heat on the surface of those soils; and why, under such circumstances, vegetation at the spring is so much more languid than on drier lands. They will also be particularly important in accounting for the opposite effects which take place in the autumn, and at the approach of winter: nay by such alone, I conceive, can we assign any probable cause why calcareous and sandy soils which had previously exhibited greater fertility by earlier vegetation than clayey or deep ones should at that season so notoriously manifest a greater decline of vegetative power.
"On the same principle too, I would explain a fact, probably not generally noticed, though rather anomalous in the science of vegetation: it is, that meadow or pasture grounds which have been covered with water during a part of the winter, on being drained even early in the spring will for a short time shew a verdure which the adjoining drier lands cannot equal; when afterwards, on the continued action of the rays of light, the dry lands will far surpass them. This I beg to be understood as confined to land which had been covered with water not of a fertilizing quality; not such as having passed through a highly cultivated country might deposit its sediment.
"Nor will this principle be found less useful in accounting for the different effects which a sudden change in the heat of the atmosphere may produce on certain vegetables, in different or even on the same soils when under other modes of management. Whatever has a tendency to check a quick and great loss of temperature in the substances which surround such vegetables, particularly their roots, will be best calculated to save them from that effect and from vegetative death; consequently those earths which are the worst conductors of heat, or in other words, are the longest in heating and cooling, will be the most favorable in resisting any sudden alteration, and the vegetables growing on them will be the least injured when so assailed."
After all, evaporation is the great generator of cold, and although there may be some intrinsic and original difference in the capacities of different earths for retaining heat, abstracted from the retention of water, Mr. Egremont cannot avoid concluding, it is to that retentive power that the chief cause of difference is assignable, and that soils are good and bad conductors of heat according to the moisture they contain. The inference is that clay, being of all others the most retentive, will defend crops from the mildew better than drier soils: and allowance being made for local differences, that soils most liable to have their crops injured may be stated in the following order: Peat or moor, calcareous, sand, grey earth, and clay.
That the susceptibility of these soils to injury from mildew is actually diminished in the order here laid down is a hazardous statement: next to peat or moor, my observation justifies me in suspecting that strong, adhesive, clayey soils, especially those which are insufficiently drained, and indeed they scarcely ever can be sufficiently drained, are of all others the least capable of resisting the ravages of mildew. These plants become unhealthy from superabundant moisture, rather than from excessive cold, or sudden variations in temperature, and in consequence of that unhealthiness are attacked by parasitic fungi which they are incapable of repelling. Where there is a substratum of clay through which no water can filter, the land is soaked and saturated with it, and can only be relieved by the tardy process of exhalation. On such soils beans are perhaps the best preparative for wheat: for in the first place their leaves are shed on the ground and contribute to manure it, and in the second place, their strong tubular roots are left in the soil and contribute in a most essential manner to keep it loose and open.
After all that can be said concerning the mildew, as to its nature, character, or cause; even supposing it may occasionally be prevented by an intention to those circumstances of shelter, drainage, and general culture which contribute to the healthy vigor of the growing crops, since we are not possessed of such a command over the elements as the Philosopher Imlac enjoyed, the mischief will occasionally spread itself over our fields, as it has done in the present year, far and fatally. When this is the case, a question arises of some consequence, about which farmers differ, namely, should the infected crop be cut early or late? The general practice is to cut it early, before the corn is fully ripe. This practice is founded, so far as I have been able to learn from enquiry of those who have taken the trouble to reason upon it at all, on a supposition, that "as the mildew feeds upon the living straw, the sooner you destroy the life of the straw the sooner you check the progress of the mildew." This is very unsatisfactory: Sir Joseph Banks and Mr. Bauer have traced the action of the parasitic fungi in a manner described in the early part of this paper: their minute roots passing under the bark and into the cellular substance of the straw intercept a part of the nourishment which was destined for the kernel. But it is clear that they intercept only a part of that nourishment: if they intercepted the whole, there could be no flour in the kernel, which would then be an empty husk, nothing but bran. So long as there is a living principle in the straw sufficient to convey nourishment from the earth to the ear, it must be injurious to cut off the communication between them. The mildew intercepts a part of the nourishment, the sickle intercepts the whole.
- See Pamphleteer, No. XII. p. 401.
- A very curious and striking instance of a similar arrangement is exhibited in the Nepenthes Distillatoria, or Pitcher Plant, indigenous in the island of Java. Mr. Barrow gives the following account of it:—"Perhaps there is not," says he, "among the numerous examples that occur of the provident economy of nature in the vegetable part of the creation a more remarkable instance of contrivance adapted to circumstances, of means suited to the end, than what is evidently displayed in this wonderful plant. Being the inhabitant of a tropical climate, and found on the most stony and arid situations, nature has furnished it with the means of an ample supply of moisture, without which it would have withered and perished. To the footstalk of each leaf, and near the base, is attached a small bag, shaped like a pitcher, of the same consistence and colour as the leaf in the early stage of its growth, but changing with age to a reddish purple; it is girt round with a lid, neatly fitted, and moveable on a kind of hinge or strong fibre, which, passing over the handle, connects the vessel with the leaf. By the contraction of this fibre the lid is drawn open whenever the weather is showery, or dews fall, which would appear to be just the contrary of what usually happens in nature, though the contraction probably is occasioned by the hot and dry atmosphere, and the expansion of the fibre does not take place until the moisture has fallen and saturated the pitcher: when this is the case the cover falls down, and it closes so firmly as to prevent any evaporation from taking place. The water being gradually absorbed through the handle into the footstalk gives vigour to the leaf and sustenance to the plant. As soon as the pitchers are exhausted, the lids again open to admit whatever moisture may fall; and when the plant has produced its seed, and the dry season fairly sets in, it withers, with all the covers of the pitchers standing open."
- This was about the second week in August. The barleys are not found to be so good since they are housed as they were thought to be while in the field. The samples which come to market are much thinner and lighter than they were last year; but this inferiority is evidently not the effect of mildew, because the skin, so far from being dark and discolored, is much brighter than it was last year. I suspect that the barley crop this season ripened too rapidly: before the milk of the kernel was converted into farina much of it was dried up, and the kernel became shrivelled. A great deal of wheat has likewise the appearance of having been injured by this prematurity without any affection of mildew, being bright, although lean and shrivelled.
- It was published in the third volume of the Prize Essays of the Highland Society of Scotland.
- See White's Natural History of Selborne.
- Observations on the Mildew.
- There is an unhealthy vigor, a rank luxuriance of growth, which is very likely to be followed by disease and mildew.
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