Popular Science Monthly/Volume 22/March 1883/The Pedigree of Wheat
|THE PEDIGREE OF WHEAT.|
WHEAT ranks by origin as a degenerate and degraded lily. Such in brief is the proposition which this paper sets out to prove, and which the whole course of evolutionary botany tends every day more and more fully to confirm. By thus from the very outset placing clearly before our eyes the goal of our argument, we shall be able the better to understand as we go whither each item of the cumulative evidence is really tending. We must endeavor to start with the simplest forms of the great group of plants to which the cereals and the other grasses belong, and we must try to see by what steps this primitive type gave birth, first to the brilliantly colored lilies, next to the degraded rushes and sedges, and then to the still more degenerate grasses, from one or other of whose richer grains man has finally developed his wheat, his rice, his millet, and his barley. We shall thus trace throughout the whole pedigree of wheat from the time when its ancestors first diverged from the common stock of the lilies and the water-plantains, to the time when savage man found it growing wild among the untilled plains of prehistoric Asia, and took it under his special protection in the little garden-plots around his wattled hut, whence it has gradually altered under his constant selection into the golden grain that now covers half the lowland tilth of Europe and America. There is no page in botanical history more full of genuine romance than this; and there is no page in which the evidence is clearer or more convincing for those who will take the easy trouble to read it aright.
The fixed point from which we start is the primitive and undifferentiated ancestral flowering plant. Into the previous history of the line from which the cereals are ultimately descended, I do not propose here to enter. It must suffice for our present purpose to say dogmatically that the flowering plants as a whole derive their origin from a still earlier flowerless stock, akin in many points to the ferns and the club-mosses, but differing from them in the relatively important part borne in its economy by the mechanism for cross-fertilization. The earliest flowering plant of the great monocotyledonous division (the only one with which we shall here have anything to do) started apparently by possessing a very simple and inconspicuous blossom, with a central row of three ovaries, surrounded by two or more rows of three stamens each, without any colored petals or other ornamental adjuncts of any sort. I need hardly explain even to the unbotanical reader at the present day that the ovaries contain the embryo seeds, and that they only swell into fertile fruits after they have been duly impregnated by pollen from the stamens, preferably those of another plant, or at least of another blossom on the same stem. Seeds fertilized by pollen from their own flower, as Mr. Darwin has shown, produce relatively weak and sickly seedlings; seeds fertilized by pollen from a sister plant of the same species produce relatively strong and hearty seedlings. The two cases are exactly analogous to the effects of breeding in and in or of an infusion of fresh blood among races of men and animals. Hence it naturally happens that those plants whose organization in any way favors the ready transference of pollen from one flower to another gain an advantage in the struggle for existence, and so tend on the average to thrive and to survive; while those plants whose organization renders such transference difficult or impossible stand at a constant disadvantage in the race for life, and are liable to fall behind in the contest, or at least to survive only in the most unfavorable and least occupied parts of the vegetal economy. Familiar as this principle has now become to all scientific biologists, it is yet so absolutely necessary for the comprehension of the present question, whose key-note it forms, that I shall make no apology for thus once more stating it at the outset as the general law which must guide us through all the intricacies of the development of wheat.
Our primitive ancestral lily, not yet a lily or anything else namable in our existing terms, had thus, to start with, one triple set of ovaries, and about three triple sets of pollen-bearing stamens; and to the very end this triple arrangement may be traced under more or less difficult disguises in every one of its numerous modern descendants. No single survivor, however, now represents for us this earliest ideal stage; we can only infer its existence from the diverse forms assumed by its various divergent modifications at the present day, all of which show many signs of being ultimately derived from some such primordial and simple ancestor. The first step in advance consisted in the acquisition of petals, which are now possessed in a more or less rudimentary shape by all the tribe of trinary flowers, or at least, if quite absent, are shown to have been once present by intermediate links or by abortive rudiments. There are even now flowers of this class which do not at present possess any observable petals at all; but these can be shown (as we shall see hereafter) not to be unaltered descendants of the prime type, but on the contrary to be very degraded and profoundly modified forms, derived from later petal-bearing ancestors, and still connected with their petal-bearing allies by all stages of intervening degeneracy. The original petalless lily has long since died out before the fierce competition of its own more advanced descendants; and the existing petalless reeds or cuckoo-pints, as well as the apparently petalless wheats and grasses, are special adaptive forms of the newer petal-bearing rushes and lilies.
The origin of the colored petals is almost certainly due to the selective action of primeval insects. The soft pollen, and perhaps, too, the slight natural exudations around the early flowers, afforded food to the ancestral creatures not then fully developed into anything that we could distinctively call a bee or a butterfly. But, as the insects flew about from one head to another in search of such food, they carried small quantities of pollen with them from flower to flower. This pollen, brushed from their bodies on to the sensitive surface of the ovaries, fertilized the embryo seeds, and so gave the fortunate plants which happened to attract the insects all the benefits of a salutary cross. Accordingly, the more the flowers succeeded in attracting the eyes of their winged guests, the better were they likely to succeed in the struggle for existence. In some cases, the outer row of stamens appears to have become flattened and petal-like, as still often happens with plants in the rich soil of our gardens; and in these flatter stamens the oxidized juices assumed perhaps a livelier yellow than even the central stamens themselves. If the flowers had fertilized their own ovaries this change would of course have proved disadvantageous, by depriving them entirely of the services of one row of stamens; for the new flattened and petal-like structures lost at once the habit of producing pollen. But their value as attractive organs for alluring the eyes of insects more than counterbalanced this slight apparent disadvantage; and the new petal-bearing blossoms soon outstripped and utterly lived down all their simpler petalless allies. By devoting one outer row of stamens to the function of alluring the fertilizing flies, they have secured the great benefit of perpetual cross-fertilization, and so have got the better of all their less developed competitors. At the same time, the exudations at the base of the petals have assumed the definite form of sweet nectar or honey, a liquid which is mainly composed of sugar, that universal allurer of animal tastes. By this means the plants save their pollen from depredations, and at the same time offer the insects a more effectual, because a more palatable, sort of bribe.
Passing rapidly over these already familiar initial stages, we may go on to those more special and distinctive facts which peculiarly concern the ancestry of the lilies and cereals. It is probable that the nearest modern analogue of the earliest petal-bearing trinary flowers is to be found in the existing alisma tribe, including our own English arrowheads and flowering rushes. As a rule, indeed, it may be said that fresh-water plants and animals tend to preserve for us very ancient types indeed; and all the alismas are marsh or pond flowers of an extremely simple character. They have usually three greenish sepals outside each blossom, inclosing one whorl of three white or pink petals, two or three whorls of three stamens each, and a number of separate ovaries, which are not united, as in the more developed true lilies, into a single capsule, but remain quite distinct, each with its own individual stigma or sensitive surface. Even within this relatively early and simple group, however, several gradations of development may yet be traced. I incline to believe that our English smaller alisma, a not uncommon plant in wet ditches and marshes throughout the whole of Southern Britain, represents the very earliest petal-bearing type in this line of development; indeed, save that its petals are now pinky-white, while those of the original ancestor were almost certainly yellow, we might almost say that the marsh-weed in question was really the earliest petal-hearing plant of which we are in search. It closely resembles in appearance, and in the arrangement of its parts, the buttercups, which are the earliest existing members of the other or quinary division of flowering plants; and in both we seem to get a survival of a still earlier common ancestor, only that in the one the parts are arranged in rows of three, while in the other they are arranged in rows of five; and concomitantly with this distinction go the two or three other distinctions which mark off the two main classes from one another—namely, that the one has leaves with parallel veins, only one seed-leaf to the embryo, and an endogenous stem, while the other has leaves with netted veins, two seed-leaves to the embryo, and an exogenous stem. Nevertheless, in spite of such fundamental differences, Fig. 1.—a. ovaries; b, stamens, inner whorl; c, stamens, outer whorl; d, petals; e, calyx-pieces. we may say that the alismas and the buttercups really stand very close to one another in the order of development. When the two main branches of flowering plants first diverged from one another, the earliest petal-bearing form they produced on one divergent branch was the alisma, or something very like it; the earliest petal-bearing form they produced on the other divergent branch was the buttercup, or something very like it. Hence, whenever we have to deal with the pedigree of either great line, the fixed historical point from which we must needs set out must always be the typical alismas or the typical buttercups. The accompanying diagram will show at once the relation of parts in the simplest trinary flowers, and will serve for comparison at a later stage of our argument with the arrangement of their degraded descendants, the wheats and grasses.
Our own smaller alisma has a number of ovaries loosely scattered about in its center, as in the buttercups, with two rows of three stamens outside them, and then a single row of three petals, followed by the calyx or inclosing cup of three green pieces. Its close ally the water-plantain, however, shows signs of some advance toward the typical lily form in the arrangement of its ovaries in a single ring, often loosely divisible into three sets. And in the pretty pink flowering rush (not of course a rush at all in the scientific sense) the advance is still more marked in that the number of ovaries is reduced to six, that is to say, two whorls of three each, accompanied by nine stamens, similarly divisible into three rows. In all these very early forms (as in their analogues the buttercups) the main point to notice is this, that there is as yet no regular definiteness in the numerical relations of the parts. They tend to run, it is true, in rows of three; but often these rows are so numerous and so confused that nature loses count, so to speak, and it is only in their higher and more developed members that we begin to arrive at any distinct symmetry, such as that of the flowering rush. Even here, the symmetry is far from being so perfect as in the later lilies. There are, however, a few very special members of the alisma family in which the approach to the true lilies is even greater. These are well represented in England by our own common arrowgrasses—inconspicuous little green flowers, with three calyx-pieces, three petals, six stamens, and either six or three ovaries. Here, too, the ovaries are at first united into a single pistil (as it is technically called), though they afterward separate as they ripen into three or six distinct little capsules. One of our British kinds, the marsh arrowgrass, has almost reached the lily stage of development; for it has three calyx-pieces, three petals, six stamens, and three ovaries, exactly like the true lilies; but it falls short of their full type in the fact that its pistil divides when ripe into separate capsules, whereas the pistil of the lilies always remains united to the very end; and this minute difference suffices, in the eyes of systematic botanists, to make it an alisma rather than a lily. In reality, it ought to be regarded as a benevolent neutral—a surviving intermediate link between the two larger classes.
The specialization which makes the true lilies thus depends upon two points. In the first place, all the parts are regularly symmetrical, except that there are two rows of stamens to each one of the other organs: the common formula being three calyx-pieces, three petals, six stamens, and three ovaries. In the second place, the three ovaries are completely combined together into a single three-celled pistil. The advantage which the lilies thus gain is obvious enough. Then bright petals, usually larger and more attractive than those of the alismas, allure a sufficient number of insects to enable them to dispense with the numerous stamens and ovaries of their primitive ancestors. Moreover, this diminution in number is accompanied by an increase in effectiveness and specialization: for the lilies have only three sensitive surfaces to their pistil, combined on a single stalk; and the honey is usually so placed at its base that the insect can not fail to brush off pollen at every visit against all three surfaces at once. Again, while the number of ovaries has been lessened, the number of seeds in each has been generally increased, which also marks a step in advance, since it allows many seeds to be impregnated by a single act of pollination. The result of all these improvements, carried further by some lilies than by others, is that the family has absolutely outstripped all others of the trinary class in the race for the possession of the earth, and has now occupied all the most favorable positions in every part of the world. While the alismas and their allies have been so crowded out that they now linger only in a few ponds, marshes, and swamps, to which the more recent lily tribe have not yet had time fully to adapt themselves, the true lilies and their yet more advanced descendants have taken seizin of every climate and every zone upon our planet, and are to be found in every possible position, from the arborescent yuccas and huge agaves of the tropics to the wild hyacinths of our English woodlands and the graceful asphodels of the Mediterranean hill-sides.
The lilies themselves, again, do not all stand on one plane of homogeneous evolution. There are different grades of development still surviving among the class itself. The little yellow gagea which grows sparingly in sandy English fields may be taken as a very fair representative of the simplest and earliest true lily type. It bears a small bunch of little golden flowers, only to be distinguished from the higher alismas by their united ovaries: for though both calyx and petals are here brightly colored, that is also the case in the flowering rushes, and in many others of the alisma group. On the other hand, though it may be said generally of the lilies that their calyx and petals are colored alike—sometimes so much so as to be practically indistinguishable—yet there are many kinds which still retain the greenish calyx-pieces, and that even in the more developed genera. But most of the lilies are far handsomer than gaarea and its allies: even in England itself we have such very conspicuous and attractive flowers as the purple fritillaries, which every Oxford man has gathered by handfuls in the spongy meadows about Iffley lock, with their dark spotted petals converging into a bell, and the nectaries at the base producing each a large drop of luscious honey. Some, like our wild hyacinths, have assumed a tubular shape under stress of insect selection, the better to promote proper fertilization; and at the same time have acquired a blue pigment, to allure the eyes of azure-loving bees. Others have become dappled with spots to act as honey-guides, or have produced brilliant variegated blossoms to attract the attention of great tropical insects. Our British lilies alone comprise such various examples as the lily-of-the-valley, a tubular, white, scented species, adapted for fertilization by moths; the very similar Solomon's-seal; the butcher's-broom; the wild tulip; the star-of-Bethlehem; the various squills; the asparagus; the grape-hyacinth; and the meadowsaffron. Some of them (for example, asparagus and butcher's-broom) have also developed berries in place of dry capsules; and these berries, being eaten by birds which digest the pulp, but not the actual seeds, aid in the dispersion of the seedlings, and so enable the plant to reduce the total number of seeds to three only, or one in each ovary. Among familiar exotics of the same family may be mentioned the hyacinth, tuberose, tulip, asphodel, yucca, and most of the so-called lilies. In short, no tribe supplies us with a greater number of handsome garden flowers, for the most part highly adapted to a very advanced type of insect fertilization.
Properly to understand the development of our existing wheat from this brilliant and ornamental family, as well as to realize the true nature of its relation to allied orders, we must first glance briefly at the upward evolution of the other branches descended from the true lilies, and then recur to the downward evolution which finally resulted in the production of the degenerate grasses. In the main line of progressive development, the lilies gave origin to the amaryllids, familiarly represented in England by the snow-drops and daffodils, a family which is technically described as differing from the lilies in having an inferior instead of a superior ovary—that is to say, with the pistil apparently placed below instead of above the point where the petals and calyx-pieces are inserted. From the evolutionary point of view, however, this difference merely amounts to saying that the amaryllids are tubular lilies, in which the tube has coalesced with the walls of the ovary, so that the petals seem to begin at its summit instead of at its base. The change gives still greater certainty of impregnation, and therefore benefits the race accordingly. At the same time, the amaryllids, being probably a much newer development than the true lilies, have not yet had leisure to gain quite so firm a footing in the world; though on the other hand many of them are far more minutely adapted for special insect fertilization than their earlier allies. They include the so-called Guernsey lilies of our gardens, as well as the huge American aloes which all visitors to the Riviera know so well on the dry hills around Nice and Cannes. The iris family are a similar but rather more advanced tribe, with only three stamens instead of six, their superior organization allowing them readily to dispense with half their complement, and so to attain the perfect trinary symmetry of three sepals, three petals, three stamens, and three ovaries. Among them, the iris and the crocus are circular in shape, but some very advanced types, such as the gladiolus, have acquired a bilateral form, in correlation with special insect visits. From these, the step is not great to the orchids, undoubtedly the highest of all the trinary flowers, with the triple arrangement almost entirely obscured, and with the most extraordinary varieties of adaptation to fertilization by bees or even by humming-birds in the most marvelous fashions. Alike by their inferior ovary, their bilateral shape, their single stamen, their remarkable forms, their brilliant colors, and their occasional mimicry of insect-life, the orchids show themselves to be by far the highest of the trinary flowers, if not, indeed, of the entire vegetable world.
From this brief sketch of the main line of upward evolution from lilies to orchids, we must now return to the grand junction afforded us by the lilies themselves, and travel down the other line of degeneracy and degradation which leads us on to the grasses and the cereals, including at last our own familiar cultivated wheat. Any trinary flower with three calyx-pieces, three petals, six stamens, and a three celled pistil not concealed within an inclosing tube, is said to be a lily, as long as it possesses brightly colored and delicate petals. There are, however, a large number of somewhat specialized lilies with very small and inconspicuous petals, which have been artificially separated by botanists as the rush family, not because they were really different in any important point of structure from the acknowledged lilies, but merely because they had not got such brilliant and handsome blossoms. These despised and neglected plants, however, supply us with the first downward step on the path of degeneracy which leads at last to the grasses, and they may be considered as intermediate stages in the scale of degradation, fortunately preserved for us by exceptional circumstances to the present day. Even among the true lilies, there are some, like the garlic and onion tribe, which show considerable marks of degeneration, owing to some decline from the type of insect fertilization to the undesirable habit of fertilizing themselves. Thus, while our common English rampsons or wild garlic has pretty and conspicuous white blossoms, some other members of the tribe, such as the crow allium, have very small greenish flowers, often reduced to mere shapeless bulbs. Among the true rushes, however, the course of development has been somewhat different. These water-weeds have acquired the habit of trusting for fertilization to the wind, which carries the pollen of one blossom to the sensitive surface of another, perhaps at less trouble and expense to the parent plant than would be necessary for the allurement of bees or flies by all the bribes of brilliant petals and honeyed secretions. To effect this object, their stamens hang out pensile to the breeze, on long, slender filaments, so lightly poised that the merest breath of air amply suffices to dislodge the pollen: while the sensitive surface of the ovaries is prolonged into a branched and feathery process, seen under the microscope to be studded with adhesive glandular knobs, which readily catch and retain every golden grain of the fertilizing powder which may chance to be wafted toward them on the wings of the wind. Under such circumstances, the rush kind could only lose by possessing brightly colored and attractive petals, which would induce insects uselessly to plunder their precious stores: and so all those rushes which showed any tendency in that direction would soon be weeded out by natural selection; while those which produced only dry and inconspicuous petals would become the parents of future generations, and would hand on their own peculiarities to their descendants after them. Thus the existing: rushes are all plain little lilies with dry, brownish flowers, specially adapted to wind-fertilization alone.
Among the rushes themselves, again, there are various levels of retrogressive development—retrogressive, that is to say, if we regard the lily family as an absolute standard: for the various alterations undergone by the different flowers are themselves adaptive to their new condition, though that condition is itself decidedly lower than the one from which they started. The common rush and its immediate congeners resemble the lilies from which they spring in having several seeds in each of the three cells which compose their pistil. But there is an interesting group of small grass-like plants, known as wood-rushes, which combine all the technical characteristics of the true rushes with a general character extremely like that of the grasses. They have long, thin, grass-like blades in the place of leaves; and, what is still more important, as indicating an approach to the essentially one-seeded grass tribe, they have only three seeds in the flower, one to each cell of the capsule. These seeds are comparatively large, and are richly stored with food-stuffs for the supply of the young plantlet. One such richly supplied embryo is worth many little unsupported grains, since it stands a much better chance than they do of surviving in the struggle for existence. The wood-rushes may thus be regarded as some of the earliest plants among the great trinary class to adopt those tactics of storing gluten, starch, and other food-stuffs along with the embryo, which have given the cereals their acknowledged superiority as producers of human food. They are closely connected with the rushes, on the one hand, by sundry intermediate species which possess thin leaves instead of cylindrical, pithy blades; and they lead on to the grasses, on the other, by reason of their very grass-like foliage, and their reduced number of large, well-furnished, starchy seeds.
In another particular, the rush family supplies us with a useful hint in tracing out the pedigree of the grasses and cereals. Their flowers are, for the most part, crowded together in large tufts or heads, each containing a considerable number of minute separate blossoms. Even among the true lilies we find some cases of such crowding in the hyacinths and the squills, or, still better, in the onion and garlic tribe. But, with the wind-fertilized rushes, the grouping together of the flowers has important advantages, because it enables the pollen more easily to fix upon one or other of the sensitive surfaces, as the stalks sway backward and forward before a gentle breeze. Among yet more developed or degraded wind-fertilized plants, this crowding of the blossoms becomes even more conspicuous. A common American rush-like water-plant, known as eriocaulon, helps us to bridge over the gap between the' rushes and such compound flowers as the sedges and grasses. Eriocaulon and its allies have always one seed only in each cell of the pistil; and they have also generally a very delicate corolla and calyx, of from four to six pieces, representing the original three sepals and three petals of the lilies and rushes. But their minute blossoms are closely crowded together in globular heads, the stamens and pistils being here divided in separate flowers, though both kinds of flowers are combined in each head. From an ancestral form not unlike this, but still more like the wood-rushes, we must get both our sedges and our grasses. And though the sedges themselves do not stand in the direct line of descent to wheat and the other cereals, they are yet so valuable as an illustration from their points of analogy and of difference that we must turn aside for a moment to examine the gradual course of their evolution.
The simplest and most primitive sedges now surviving, though very degenerate in type, yet retain some distinct traces of their derivation from earlier rush-like and lily-like ancestors. In the earliest existing type, known as scirpus, the calyx and petals, which were brightly colored in the lilies, and which were reduced to six brown scales in the rushes, have undergone a further degradation to the form of six small, dry bristles, which now merely remain as rudimentary relics of a once useful and beautiful structure. In some species of scirpus, too, the number of these bristles is reduced from six to four or three. There is still one whorl of three stamens, however; but the second whorl has disappeared; while the pistil now contains only one seed instead of three; though it still retains some trace of the original three cells in the fact that there are three sensitive surfaces, united together at their base into one stalk or style. Each such diminution in the number of seeds is always accompanied by an increase in the effectiveness of those which remain; the difference is just analogous to that between the myriad ill-provided eggs of the cod, whose young fry are for the most part snapped up as soon as hatched, and the two or three eggs of birds, which watch their brood with such tender care, or the single young of cows, horses, and elephants, which guard their calves or foals almost up to the age of full maturity. What the bird or the animal effects by constant feeding with worms or milk, the plant effects by storing its seed with assorted food-stuffs for the sprouting embryo.
In the more advanced or more degenerate sedges we get still further differentiation for the special function of wind-fertilization. Take as an example of these most developed types, on this line of development, the common English group of carices. In these the flowers have absolutely lost all trace of a perianth (that is to say, of the calyx and petals), for they do not possess even the six diminutive bristles which form the last relics of those organs in their allies, the scirpus group. Each flower is either male or female—that is to say, it consists of stamens or ovaries alone. The male flowers are represented by a single scale or bract, inclosing three stamens; and in some species even the stamens are reduced to a pair, so that all trace of the original trinary arrangement is absolutely lost. The female flowers are represented by a single ovary, inclosed in a sort of loose bag, which may perhaps be the final rudiment of a tubular, bell-shaped corolla like that of the hyacinth. This ovary contains a single seed, but its shape is often triangular, and it has usually three stigmas or sensitive surfaces, thus dimly pointing back to the three distinct cells of its lily-like ancestors, and the three separate ovaries of its still earlier alisma-like progenitors. In many species, however, even this last souvenir of the trinary type has been utterly obliterated, the ovary having only two stigmas, and assuming a flattened, two-sided shape. In all the carices the flowers are loosely arranged in compact spikes and spikelets, with their mobile stamens hanging out freely to the breeze, and their feathery stigmas prepared to catch the slightest grain of pollen which may happen to be wafted their way by any passing breath of air. The varieties in their arrangement, however, are almost as infinite among the different species as those of the grasses themselves; sometimes the male and female flowers are produced on separate plants; sometimes they grow in separate spikes on the same plant; sometimes the same spike has male flowers at the top and female at the bottom; sometimes the various flowers are mixed up with one another at top and bottom, a regular hotch-potch of higgledy-piggledy confusion. But all the sedges alike are very grass-like in their aspect, with thin blades by way of leaves, and blossoms on tall heads, as in the grasses. In fact, the two families are never accurately distinguished by any except technical botanists; to the ordinary observer, they are all grasses together, without petty distinctions of genus and species. Like the grasses, too, the sedges are mostly plants of the open, wind-swept plains or marshy levels, where the facilities for wind-fertilization are greatest and most constantly present.
And now, from this illustrative digression, let us hark back again to the junction-point of the rushes, whence alike the sedges and the grasses appear to diverge. In order to understand the nature of the steps by which the cereals have been developed from rush-like ancestors, it will be necessary to look shortly at the actual composition of the flower in grasses, which is the only part of their organism differing appreciably from the ordinary lily type. The blossoms of grasses, in their simplest form, consist of several little green florets, arranged in small clusters, known as spikelets, along a single common axis. Of this arrangement, the head of wheat itself offers a familiar and excellent example. If we pull to pieces one of the spikelets composing such a head, we find it to consist of four or five distinct florets. Omitting special features and unnecessary details, we may say that each floret is made up of two chaffy scales, known as pales, and representing the calyx, together with a pair of small white petals known as lodicules, three stamens, and an ovary with two feathery styles. Moreover, the two pales or calyx-pieces are not similar and symmetrical, for the outer one is simple and convex, while the inner one is apparently double, being made up of two pieces rolled into one, and still possessing two green midribs, which show distinctly like ribs on its flat outer surface. Here, it will immediately be apparent, the traces of the original trinary arrangement are very slight indeed.
But when we come to inquire into the rationale and genesis of these curiously one sided flowers, it is not difficult to see that they have been ultimately derived from trinary blossoms of the rush-like type. The first and most marked divergence from that type, for which the analogy of the sedges has already prepared us, is the reduction of the ovary to a single one-seeded cell, whose ripe, fruity form is known as a grain. At one time, we may feel pretty sure, there must have existed a group of nascent grasses, which only differed from the wood-rush genus in having a single-celled ovary instead of a three celled pistil with one seed in each cell; and even the ovary of this primitive grass must have retained one mark of its trinary origin in its possession of three styles to its one grain, thus pointing back (as most sedges still do) to its earlier rush-like origin. That hypothetical form must have had three sepals, three petals, six stamens, and one three-styled ovary. But the peculiar shape of modern grass-flowers is clearly due to their very spiky arrangement along the edge of the axis. In the wood-rushes and the sedges, we see some approach to this condition; but in the grasses, the crowding is far more marked, and the one-sidedness has accordingly become far more conspicuous. Suppose we begin to crowd a number of wind-fertilized lily-like flowers along an axis in this manner, taking care that the stamens and the sensitive feathery styles are always turned outward to catch the breeze (for otherwise they will die out at once), what sort of result shall we finally get?
In the first place, the calyx, consisting of three pieces, will stand toward the crowded stem or axis in such a fashion that one piece will be free and exterior, while two pieces will be interior and next the stem, thus:
Now, the effect of constant crushing in this direction will be that the two inner calyx-pieces will be slowly dwarfed, and will tend to coalesce with one another; and this is what has actually happened with the inner pale of wheat and of other grasses, though the midribs of the two originally separate pieces still show on the compound pale, like dark-green lines down its center. Thus, in the fully developed grasses, in place of a trinary calyx, we get two chaffy scales or pales, the outer one representing a single sepal, and the inner one, which has been dwarfed by pressure against the stem, representing two sepals rolled into one, with two midribs still remaining as evidence of their original distinctness.
Next, in the case of the petals, which alternate with the sepals of the calyx, the relation to the stem is exactly reversed; for we have here two petals free and exterior, with one interior petal crowded closely against the axis, thus:
Here, then, the two external petals will be saved, exactly as the one external sepal was saved in the case of the calyx; and these two petals are represented by the very small white lodicules under the outer pale in our existing wheats and grasses. On the other hand, the inner petal, jammed in between the grain and the inner pale (with the stem at its back), has been utterly crushed out of existence, partly because of its very small size, partly because of its functional uselessness, and partly because it had no other part with which to coalesce, and so to save itself as the inner sepals had managed to do. Moreover, it must be remembered that the sepals do still perform a useful service in protecting the young flower before it opens, and in keeping out noxious insects during the kerning or swelling of the grain; whereas the lodicules or rudimentary petals are now apparently quite functionless; and so we may congratulate ourselves that they are there at all, to preserve for us the true ground-plan of the floral architecture in grasses. Indeed, they have not survived by any means in all grasses; among the smaller and more degraded kinds they are often wholly wanting, having been quite crushed out between the calyx and the grain. It is only the larger and more primitive types that still exhibit them in any great perfection. On the other hand, one group of very large exotic grasses, the bamboos, has three regular petals, thus clearly showing the descent of the family as a whole from rush-like ancestors, and also obviously suggesting that the obsolescence of the inner petal in the other grasses is due to their small size and their closely packed minute flowers.
Among the stamens, one-sidedness has not notably established itself, for in wind-fertilized plants they must necessarily hang out freely to the breeze, and therefore they do not get much crowded between the other parts. A few grasses still even retain their double row of stamens, having six to each floret; but most of them have only one whorl of three. In some of the lower and more degraded forms, however, even the stamens have lost their trinary order, and only two now survive. This is the case in our own very degenerate little sweet vernal-grass, the plant which imparts its delicious fragrance to new mown hay. But in the cereals and in most other large species the three stamens still remain in undiminished effectiveness to the present day.
Finally, we come to the most important part of all, the ovary. This part, alternating with the stamens, has the same arrangement of styles relatively to the axis as in the case of the petals; and it has undergone precisely the same sort of abortive distortion. The two outer styles, hanging freely out of the calyx, have been preserved like the two outer lodicules; but the inner one, pressed between the grain and the inner pale (with the stem behind it), has been simply crushed out of existence, like its neighbor the inner lodicule.
Thus the final result is that the whole inner portion of the flower (except as regards stamens) has been distorted or rendered abortive by close pressure against the stem (due to the crowding of the florets in the spiky form), while the whole outer portion remains normal and fully developed. We have an outer pale representing a single normal sepal, and an inner pale representing two dwarfed and united sepals; we have two normal outer lodicules or petals, and a blank where the inner petal ought to be; we have three stamens, symmetrically arranged, among the faithless faithful only found; and we have finally two normal outer styles, with a blank in place of the absent inner style.Fig. 2. The accompanying diagram, compared with that already given, will make this perfectly clear.
Here, a1 represents the outer pale or normal sepal, while a2 and a3 represent the inner pale composed of the two united sepals. Again, b1 and b2 stand for the two lodicules or surviving petals, while b3 marks the place of the lost petal, now found in the bamboos alone. The stamens are lettered c1, c2, and c3. The two existing styles are shown by d1 and d2, while d3 marks the abortive inner style, now not even present in a rudimentary condition. It will be observed at once that all the outer side is normal, and all the inner side more or less abortive through pressure against the axis.
Thus it will be seen that the line of links which connects the grasses and cereals with the lilies is absolutely unbroken, and that it consists throughout of one continuous course of degradation. At the same time, by this one-sided and spiky arrangement, the grasses secured for themselves an exceptional advantage in the struggle for existence. No other race of small, wind-fertilized plants could compete with them for the possession of the open, wind-swept plains; and over all these they spread far and wide, rapidly differentiating themselves into a vast number of divergent genera and species, each adaptively specialized for some peculiar habitat, soil, or climate. At the present time, the grasses number their kinds by thousands; they extend over the whole world, from the poles to the equator; and they form the general sward or carpet of greenery over by far the larger portion of the terrestrial globe. Even in Britain alone, with our poor little insular flora, a mere fragment of that belonging to the petty European Continent, we number no less than forty-two genera of grasses, distributed into more than one hundred species. In fact, what may fairly be called degradation from one point of view may fairly be called adaptation from another. The organization of the grasses is certainly lower than that of the lilies, but it fits them better for that station of life to which it has pleased Nature to assign them.
The various kinds of grasses differ very little from one another in general plan; the flower in almost all is constructed strictly on the lines above mentioned; and the leaves in almost all are just the same soft, pensile blades, making them into the proper greensward for open, unwooded, wind-swept plains. But, like almost all other very dominant families, they have split up into an immense number of kinds, distinguished from one another by minute differences in the arrangement of the florets and the spikelets; and these kinds have again subdivided into more and more minutely different genera and species. One great group, with panicles of a loose character, and very degraded spikelets, has given origin to many southern grasses, from some of which the cultivated millets are derived. Another great group, with usually more spiky inflorescence, has given origin to most of our northern grasses, from some of which the common cereals are derived. This second group has again split up into several others, of which the important one for our present purpose is that of the Hordeineæ, or barley-worts. From one of the numerous genera into which the primitive Hordeineæ have once more split up, our cultivated barleys take their rise; from another, which here demands further attention, we get our cultivated wheats.
The nearest form to true wheat now found wild in the British Isles is the creeping couch-grass, a perennial closely agreeing in all essential particulars of structure with our cultivated annual wheats. But in the South European region we find in abundance a large series of common wild annual grasses, forming the genus Ægilops of technical botany, and exactly resembling true wheat in every point except the size of the grain. One species of this genus, Ægilops ovata, a small, hard, wiry annual, is now pretty generally recognized among botanists as the parent of our cultivated corn. There was a good reason, indeed, why primitive man, when he first began to select and rudely till a few seeds for his own use, should have specially affected the grass tribe. No other family of plants has seeds richer in starches and glutens, as indeed might naturally be expected from the extreme diminution in the number of seeds to each flower. On the other hand, the flowers on each plant are peculiarly numerous; so that we get the combined advantages of many seeds, and rich seeds, so seldom to be found elsewhere, except among the pulse family. The experiments conducted by the Agricultural Society in their College Garden at Cirencester have also shown that careful selection will produce large and rich seeds from Ægilops ovata, considerably resembling true wheat, after only a few years' cultivation.
Primitive man, of course, did not proceed nearly so fast as that. Of the very earliest attempts at cultivation of Ægilops, all traces are now lost, but we can gather that its tillage must have continued in some unknown Western Asiatic region for some time before the neolithic period; for in that period we find a rude early form of wheat already considerably developed among the scanty relics of the Swiss lake-dwellings. The other cultivated plants by which it is there accompanied and the nature of the garden-weeds which had followed in its wake point back to Central or Western Asia as the land in which its tillage had first begun. From that region the Swiss lake-dwellers brought it with them to their new home among the Alpine valleys. It differed much already from the wild Ægilops in size and stature; but at the same time it was far from having attained the stately dimensions of our modern corn. The ears found in the lake-dwellings are shorter and narrower than our own; the spikelets stand out more horizontally, and the grains are hardly more than half the size of their modern descendants. The same thing is true in analogous ways with all the cultivated fruits or seeds of the stone age; they are invariably much smaller and poorer than their representatives in existing fields or gardens. From that time to this the process of selection and amelioration has been constant and unbroken, until in our own day the descendants of these little degraded lilies, readapted to new functions under a fresh régime, have come to cover almost all the cultivable plains in all civilized countries, and supply by far the largest part of man's food in Europe, Asia, America, and Australia.—Macmillan's Magazine.
- The sedges are not, in all probability, a real natural family, but are a group of heterogeneous, degraded lilies, containing almost all those kinds in which the reduced florets are covered by a single conspicuous glume-like bract. It will be seen from the sequel that these bracts are not truly analogous to the glumes or outer palese of grasses.