Popular Science Monthly/Volume 12/February 1878/The Chemistry of Fruit-Ripening
By ALBERT B. PRESCOTT,
PROFESSOR OF CHEMISTRY IN THE UNIVERSITY OF MICHIGAN.
TO form the seed seems to be the chief end of the plant. When in the vigor of its own maturity, and when receiving the sun's strongest rays and the earth's richest nourishment, the plant gathers all its resources, and devotes them to the building of the seed. When done, the seed itself, the embryo, commonly possesses little substance and serves little use beyond its primary purpose, the reproduction of the plant. But in the coatings and coverings of the seed we find a large and abundant supply of substances, in variety and quantity the rarest and richest stock in the vegetable commonwealth. Indeed, the wrappings of seed-germs constitute the especial provision for the nourishment of the human race. The seeds enveloped with starch and albuminoids, as in the cereal grains, make up "the staff of life" for man. Seeds with oily coatings, including the nuts, present a good supply of fats for food. The seeds with succulent coverings, the fruits, yield a great number of sharply-defined substances, most of which claim the approval of man, and some of which require for their due application the best efforts of the human intellect. Without the grains, the fruits, and the nuts, man would be left to browse with the ox and prey with the wolf.
In this abundant material gathered around the seed-germs, chemistry has achieved more success than elsewhere in the organic world. It is well understood that chemists have no reason to boast of what they can do with the products of living cells. In an analysis of vegetable or animal products, there is always a percentage, and often a large percentage, of unknown matter: It might be named "chemist's dirt;" not "matter out of place," but simply "matter unknown." It has weight, it may have color and consistence, but it responds to no inquiries and yields to no suggestions. Like an open polar sea, it baffles and invites and baffles again. But, with all due reservation for unknown bodies, the condition of organic analysis gives good ground for encouragement. Especially in this material about the seed, the analyst finds numerous compounds of clearly definite chemical character, many of them capable of sure identification and exact separation, even when taken in complex mixtures. Working with some of these compounds, an insight into their chemical structure has been obtained; so that the chemist can bring together the materials and conditions for their production. In the products of the peach, at every autumn's ripening, certain chemical changes occur in the kernel under your hand—changes as well known to science and capable of as exact quantitative statement as the local changes of the planets in the solar system. Forty-four years ago, Liebig and his fellow-workers discovered certain links in those chemical changes, in the products of the almond family, and the discovery was an era in chemical science.
The chemistry of the covered seed is of interest not only for the quality of the compounds found in it, but, quite as much, for the history of these compounds, the chemical changes of seed and fruit-ripening in the plant. These changes differ in their general character from other changes of plant chemistry, coinciding more nearly with the changes of animal chemistry. Taking for study the ripening of seeds with succulent coverings, the fruits—the proper subject of this article—we may undertake to compare fruit-ripening with vegetable nutrition on the one hand, and with animal nutrition on the other hand, as follows:
Fruit-ripening, then, coincides with vegetable nutrition in acting with the same substances, and coincides with animal nutrition in moving in the same direction.
To inquire, now, somewhat in detail, into the more obvious of the changes which constitute fruit-ripening, we may examine the proportion and formation of the following five classes of Fruit-Products:
1. Sugars (starches).
The analyses of fruits hitherto reported have mostly been made by European chemists. The fullest reports of ripe fruits, upon which I am in good part dependent, were made by Fresenius, from analyses under his direction, nearly twenty years ago, and represent the fruits of the Rhine district, obtained at Wiesbaden.
1. Sugars.—The prevailing sugar in fruits is glucose (dextrose), often termed grape-sugar. It is the same compound that is largely manufactured from starch, and called starch-sugar. It is much less sweet than cane-sugar, and less abundantly soluble in water, having an oily or "mealy" taste. As made from starch, it is now much used in certain candies. When in the uncrystallizable form, glucose (lœvulose) is the same as "fruit-sugar," the uncrystallizable product obtained to some extent in manufacturing cane-sugar, and which forms a part of the sirups of the market. Many of the fruits contain cane-sugar (which is the same as beet-sugar and maple-sugar), and certain rare varieties of sugar are found in some fruits.
Buignet decided that the apple, peach, plum, raspberry, orange, and pineapple, contain cane-sugar, with glucose (mostly as lœvulose). The sugar of the grape, cherry, gooseberry, and fig, consists wholly of glucose.
The average proportion of sugars in ripe fruits is given, by Fresenius, as follows (the smallest percentages being placed first):
Peaches, 1.6 per cent, (not varying very widely).
It is seen from this list that the sweetness of fruit has but slight correspondence with its proportion of sugar. Currants were found to have more sugar than raspberries, blackberries, or strawberries, and over three times as much as the peaches examined by Fresenius. All analysts agree in the predominance of grapes for their quantity of sugar. The sweetness of fruit is probably favored less by large proportions of sugar than by three other conditions, namely: 1. Small proportions of acids; 2. Large proportions of pectous substances; 3. Presence of cane-sugar instead of grape-sugar.
The sugar of fruits is chiefly formed or deposited in them during their ripening. Berard found that the pulp of cherries, unripe, contained only 1.1 per cent, of sugar; ripe, 18.1 per cent.; gooseberries, unripe, 0.5 per cent.; ripe, 6.02 per cent. In 1862 Hilger determined the sugar of grapes, at ten periods during their growth and ripening, as follows (Landw. Versuchsstat, xvii., 245; Journal of the Chemical Society, xxviii., 281):
Whether the sugar of fruits is formed within them, or introduced through the stem, and, if formed in the fruits, from what substance formed, are questions which have been investigated, but not wholly settled. It has been pretty generally held that starch in the unripe fruits is converted into sugar in the ripe fruits; the fruit acids inducing the change, as we know they have power to do. But starch is not found in the unripe stage of all fruits, and, in the cases where found, its quantity is sometimes too small to serve as the source of all the sugar of the ripened fruit. In the investigation of Hilger, above quoted, the immature fruit was at no time found by microscopic examination to contain starch. It appeared in the fruit-stalks in June; after August it almost wholly disappeared from the fruit-stalks, and was found only in the wood of the vines. Payen (Comptes Rendus, liii, 313) reported that he had demonstrated the presence of starch in unripe fruits and its conversion to sugar during ripening; but did not ascertain how much of the sugar of fruits is formed in this way.
It has been advanced that sugar is formed from malic and other acids, during ripening, either in the fruit or in the parts of the plant supplying juices to the fruit. Six molecules of malic acid and six molecules of tartaric acid, with nine molecules (eighteen atoms) of oxygen, would furnish the atoms for formation of four molecules of glucose, twelve molecules of water, and twenty-four molecules of carbonic anhydride. Mercadante (Gazetta Chimica Italiana, cxxv.; Journal of the Chemical Society, xxviii. , 904) made a series of determinations of the malic acid and sugar in plums, commencing May 20th. The quantities of both acid and sugar increased in the fruit so long as it was green and emitting oxygen in the daylight; the branches which bore the fruit containing acid and pectous substances but no sugar. During the same time, the pectous and gummy substances in the green fruit had decreased from six per cent, of the pulp to three per cent, of the pulp. The investigator believed the sugar of the green fruit to have been chiefly formed, in the fruit, from the pectous and gummy substances, under contact of the acids. As soon as the fruit, losing green color, began to emit carbonic acid in the daylight, the acid in it began steadily to decrease as the sugar increased. The increase of sugar at expense of the acid in the pulp of plums is shown as follows:
The green plums contain tannin, which commenced to diminish as soon as the fruit began to emit carbonic acid in the daylight, wholly disappearing by June 20th, the date at which the malic acid began to diminish. It is well known to every one that many green fruits are very astringent, and that their tannin decreases and sometimes disappears during ripening. Also, it is a familiar fact in the chemistry of tannin that it readily undergoes changes producing sugar. This, then, is the source of a portion of the sugar of many fruits. The formation of sugar from tannin will be discussed under the head of the glucosides of fruits.
Several chemists have reported the presence of sugar-producing substances peculiar to fruits. Buignet describes a fruit constituent, astringent like tannin, and combining with iodine like starch, and serving as the source of sugar.
The proportion of cane-sugar, in most fruits, is generally believed to diminish by transformation into glucose, as fruits become fully ripe or overripe. But Berthelot and Buignet (Comptes Bendus, li., 1094) found that, in oranges, the proportion of cane-sugar increased during ripening, the quantity of glucose remaining unchanged.
The increase of weight of fruits, during ripening, is no doubt largely owing to deposition of sugar. Berard found that 100 parts of unripe summer peaches yielded 179 parts of ripe fruit, and 100 parts of unripe apricots increased in ripening to 200 parts.
The maturity of fruit is the period of its maximum quantity of sugar. Sooner or later, the quantity of sugar begins to diminish, and then the fruit is overripe. It is safe to say that the sugar often begins to decompose during the life of the fruit; that is to say, fruit becomes overripe during its life. It would be difficult, however, to fix on the termination of the life of fruit. We certainly cannot say that life ceases when the circulation with the plant is cut off; and we cannot say that life continues in the sarcocarp until it is wholly disintegrated. Now it is within the limits of our subject to inquire by what changes the sugar begins to disappear.
In general terms, sugar suffers oxidation in ripe fruits, small portions being oxidized away even during the production of larger portions, and before perfect maturity. We do not know what fruit constituents, if any, result in this oxidation. The final products of oxidation, carbonic acid and water, are exhaled during ripening, and with greater rapidity after maturity has been passed.
It seems to be established that sugar in fruits is liable to traces of the alcoholic fermentation, even before maturity is passed. H. Gutzeit (Zeitscher Oest. Ap. Ver., 1875, p. 337; Pro. Am. Phar. Asso., 1876, p. 287) reports finding alcohol, or other simple compound of ethyl, in the fruits of a number of plants. Some of the fruits were not quite ripe, and none were overripe. De Luca (Comptes Rendus, lxxxiii., 512; Jour. Chem. Soc., 1876, ii., 649) reports obtaining products of the alcoholic and acetic fermentations from the fresh fruits, leaves, and flowers, of several plants. In all these cases the quantities of alcohol obtained were very minute. The investigator first above named found methyl-alcohol, in some cases, with the ethyl-alcohol. Pasteur states that the germs which excite alcoholic fermentation are very abundant on the bunches of ripe grapes, where very rare in the atmosphere. Also, that the fermentive germs are found on ripe strawberries, cherries, and currants, but not on the same fruits unripe. The formation of methyl-alcohol, above referred to, is closely allied to the formation of methyl-salicylate or wintergreen-oil. A number of the essential or volatile oils, with which plants and fruits are perfumed and flavored, contain alcohol radicals in union as compound ethers. It is probable, from every point of view, that the slight occurrence of the vinous fermentation in fruits belongs to an important class of chemical formations, by means of which a multitude of odor-giving substances are scattered throughout vegetation. We shall inquire more carefully into the fruit-flavor compounds and their formation further on.
2. The Pectous Substances.—These are, in general terms, the constituents of plant-jelly. As vegetable products, they correspond to the varieties of gelatine obtained from animal tissues. Unlike gelatine, however, they are non-nitrogenous. They are found in the soft parts of plants generally, as in the tuber of the potato and the root of the carrot; but it is in fruits that they have most importance for edible value. The immediate origin of the pectous substances is pretty well known, being due to a specific fermentation, a prominent feature in fruit-ripening. The material from which all the pectous substances proceed is the fermentable body called pectose, an insoluble, tasteless substance, found abundantly in unripe fruits, also to some extent in immature roots and tubers, and having no more value for food than cellulose. Now, there is formed along with this substance a "ferment," as it is called, a body which by contact induces a specific fermentation—a definite chemical change. Pectase is the name of the ferment. Just as, in the germinating seed, starch, by contact with diastase, suffers fermentation with production of sugar, and as, in bruised and wetted mustard-seeds, sinigrin, by contact with myrosin, splits up into pungent oil of mustard and sugar, etc., so the crude pectose of green fruits, by contact of their pectase at the time of ripening, changes to the edible plant-jellies or pectous substances. Long boiling with water alone effects the same change. Why this fermentation occurs just at the ripening-time, and not earlier or later, we do not precisely know. It may be that the pectose has just then become capable of fermentation, or the pectase then acquires potency for its office, or then, and not before, are other conditions of the change established. We know only that the fermentation gives us the before-mentioned pectous substances, which, moreover, succeed each other, during ripening, by repeated changes. It must be confessed that these products have been but imperfectly defined, but as a class their chief properties are known. They are given by chemists as follows (distinctions having value only in analysis being omitted):
Pectine: readily soluble in hot or cold water, gelatinizing when concentrated, and more perfectly by addition of sugar; changed by very long boiling to parapectine.
Pectic acid: gelatinous, insoluable in cold water, and but slightly soluble in hot water; hardened in jelly by solution of sugar, slowly changed by boiling to parapeptic acid, and afterward to metapeptic acid. Pectine and pectic acid result from long boiling of the crude pectose.
Parapectine: soluble in water, capable of gelatinizing slightly, changed by boiling to metapectine.
Parapeptic acid: soluble in water, the solution changing into one of metapeptic acid. Not gelatinous.
Metapectine: soluble in water, not gelatinous. (Found in overripe fruits.)Metapectic acid: soluble in water, incapable of gelatinizing. (Found in overripe fruits; produced by fermentation in overripening from all the other pectous substances. Also produced, from most of the other pectous substances, by long boiling, much more readily if acids are present.)
Alkalies change pectine and parapectine and metapectine to salts of pectic acid.
The properties of the separated pectous compounds represent certain well-known characteristics of fruits, as these are found in cooking. Moist heat, as In any mode of cooking, produces upon these substances the chief results of ripening, and, if continued long enough, the results of overripening. Unripe fruits are made more edible and wholesome by cooking, owing to its artificial (imperfect) ripening of pectose. Fruit-jellies owe their substance to pectic acid, pectine, and slightly to parapectine, the products of early maturity, with the coöperation of sugar. For jellies, it is well known, the use of over-ripe fruits must be avoided, and too long boiling in the preparation must be avoided. If the fruit be underripe, the juice should be boiled much longer than if the fruit be fully ripe, and if the fruit be overripe, boiling should be maintained no longer than necessary to clarify, and standing in hot solution should be avoided. Grapes bear full ripening for jellies.
The following statements of the quantities of pectous substances and of pectose are compiled from the reports of Fresenius. It should be mentioned that Fresenius found widely-different quantities in the different varieties of the same fruit, and the average here drawn from the varieties of each fruit would greatly vary from an average obtained from other varieties of the same. The percentage in the fresh fruit is first given, and then percentage of solids, or strictly dry fruit, as obtained by calculation from the percentage of water:
As food-materials, the pectous substances seem to be wellnigh indispensable to the health of man. They are not very nutritious; it is not known that they are fully digested into material which can be appropriated; and, being non-nitrogenous, they could scarcely yield tissue-building matter. What service they perform is not clearly understood. They may supply liquids important in digestion or assimilation. We obtain them in acidulous fruits, and in starchy tubers, and it is not clear how much of the value of each of these sorts of food is due to their pectous constituents; but, when all food containing pectine is cut off, the scurvy is liable to ensue, and then any food supplying pectine will serve as a remedy. At the same time it is found that pectous food is needed only in small quantities; large proportions proving not only innutritious but injurious, causing derangements of digestion and excretion.
3. Acids.—The principal fruit-acids, not astringent, are the following, given in the order of their importance:
Malic acid: Very widely distributed; predominating in apples, pears, cherries, gooseberries, strawberries, raspberries, and mountain-ash berries. Not extracted for use.
Citric acid: Found in lemons, oranges, tomatoes, currants, gooseberries, raspberries, strawberries, and a large number of other fruits, generally with malic and tartaric acids. Obtained from lemons for use.Tartaric acid: Also widely distributed in most fruits not forming the chief acid, but constituting the acid of the grape. Manufactured from the deposit of fermenting grape juice; used in baking-powders and in its salts, cream-of-tartar, and Rochelle salt.
Oxalic acid is sometimes found in small proportions in a few fruits. Reports vary as to its existence in the tomato.
Fresenius's analyses give the following as the average proportions of total acid, reduced to equivalent of malic acid:
The quantity of acids in fruits usually diminishes during ripening. The diminution is not, however, nearly so great as it appears to the taste, because the acid of ripe fruits is masked to the taste by the larger proportions of sugar and the pectous substances then present. The removal of acids is chiefly due to oxidation. It is not found that acids are neutralized, to any considerable extent, during ripening, by alkalies conveyed through the stem. The diminution of the acid in plums was shown definitely by the series of analyses before given from Mercadante. It is stated that the acids continue to oxidize away, after the sugar has reached its maximum and before it begins to diminish. Hence, perfect ripeness in fruit has been defined as that period during the maximum quantity of sugar when the quantity of acid is least. This will be, of course, just before the sugar begins to diminish.
It has been stated that both citric and malic acids are often found in unripe grapes, and are substituted by tartaric acid during the ripening. Oxalic acid is more often found in unripe than in ripe fruits. It is to be desired that closer determinations should be made as to the presence and proportion of oxalic acid in tomatoes and some other fruits. Any article of food containing oxalic acid (as the garden pie-plant) should probably be eaten with moderation, if at all.
A misapprehension sometimes occurs, from lack of reflection, as to the effect of sugar on the acidity of fruits. Sugar has no chemical effect upon acids. Its very sweet taste masks or overpowers to the sense the sour taste of free acids; but the acids remain free, all the same. Whatever effect the sugar eaten with fruits has on digestion and nutrition is due to the sugar itself; not to any change of the acids by the sugar, for there is no such change. Indeed, sugar approaches to the nature of an acid, though properly classed as a neutral body.
The varieties of tannic acids classed together as tannin are quite unlike the fruit acids above mentioned, both in sensible properties and in chemical relations. Only a few of the ripe edible fruits contain astringent acids, though these are found in many unripe fruits and in numerous ripe fruits not used for food. Most varieties of colored grapes contain a little tannin, deposited mostly in the skins and seeds, and imparting a slight astringency to the juice, retained after fermentation. In the red wines from 0.08 to 0.2 per cent, of tannin is found. The decomposition of tannin, by a fermentation producing sugar, has been mentioned under the head of sugars. Tannin is also liable to oxidation with various products not including sugar.
The vegetable kingdom furnishes numerous compounds, known as glucosides, which are capable of definite and distinctive fermentations, one of the fermentation products in each instance being sugar. A number of these glucosides are found in fruits. One of the most important of these is amygdalin, a glucoside found in the fruits, leaves, etc., of plants of the almond family, especially in the kernels of the bitter-almond, peach, and cherry, the leaves of the cherry-laurel, and the bark of the wild-cherry.
Amygdalin, when obtained pure, is a white, odorless solid, with a taste both sweet and bitter. Taken alone it is not poisonous, even in considerable quantities. But if mixed with a substance named emulsin, and wetted, amygdalin begins at once to break up, with formation of three other compounds, as follows:
In the plant, amygdalin is accompanied with the emulsin needful for its fermentation. During the ripening of the fruit, and in the maturity of the leaves and other parts, the amygdalin is constantly, though slowly, being transformed into the three products above named. The bitter-almond oil and hydrocyanic acid are volatile and odorous, and give the pleasant odor of peach-kernels, almonds, etc., familiar to every one. The rapidity of the chemical change is chiefly governed by the proportion of moisture, being greatly accelerated by wetting the bruised kernels or leaves, and stopped altogether by drying, while the moisture of the living plant permits only a gradual rate of the transformation. One of the products of this change is poisonous, the well-known hydrocyanic acid, or prussic acid, one-tenth of a grain of which is a full medicinal dose. The bitter-almond oil (known to chemists as benzoic aldehyde, and easily oxidized to benzoic acid) is not in the least poisonous (when separated from the hydrocyanic acid). It will be seen from the numbers of parts resulting from the change (as given above) that one part of hydrocyanic acid and four of bitter-almond oil are produced by sixteen parts of pure amygdalin. The amygdalin of the shops, in Europe, where it is somewhat used to generate hydrocyanic acid in medicine, yields from 20 to 25 of its weight of hydrocyanic acid. In exposure to the air, the hydrocyanic acid, being very volatile, is quickly dissipated, while the bitter-almond oil vaporizes more slowly. In most fruits of the almond family the amygdalin and its products are obtained chiefly or only from the kernel, hence the well-known flavoring effect of leaving in the stones, or a few cracked stones, in canned fruits. Some of these fruits, however, have the amygdalin deposited in the sarcocarp (or edible portion). This is stated to be the case with black cherries.
The almond-flavor is a very grateful accompaniment of fruits and flowers, and it is provided by Nature in safe and wholesome proportions, but it has been so tampered with by the art of man that its use is now beset with dangers of several sorts. In the first place, there is the danger in concentrating what the Creator has diluted. The oxygen of the air itself is poisonous when concentrated. Bungling art is almost sure to "o'erstep the modesty of Nature" by using good things in hurtful excess. The essential oil of bitter-almonds extracted from cherry-laurel leaves, or from bitter-almond kernels, is liable to retain a poisonous proportion of the hydrocyanic acid, and its use in flavoring extracts, for pastry, etc., has now and then produced illness and even fatal results, more frequently with children. If made free from hydrocyanic acid, as the manufacturers should do, the essential oil is harmless in any quantity, and the essences, extracts, waters, etc., made from it can be used with entire safety. If long exposed to the air, the oil deposits a slight sediment of benzoic acid, which is harmless. The danger in the use of bitter-almond oil from the amygdaline of plants lies in possible neglect of removing the hydrocyanic acid. Then, in the next place, there is another substance which has the same odor as bitter-almond oil, viz., a substance named nitrobenzine and sometimes designated "oil of mirbane," a body which is in itself very poisonous, either when taken into the stomach or inhaled into the lungs. It it is a very cheap substitute for actual bitter-almond oil, which it resembles only in the odor. It has been manufactured for twenty years, from coal-tar, great quantities of it being used in making aniline dyes. It is from this article that many cheap grades of soap have been saturated with the smell of almond, of late years, quite to the discredit of the flavor. Unscrupulous manufacturers have used it in confectionery, and the danger of its substitution in culinary extracts besets the public, who cannot employ analysts for the examination of every manufactured article purchased for the kitchen. But if chemical art furnished a temptation for the improper substitution of nitrobenzine, it has lately compensated for it by discovering the manufacture of actual bitter-almond oil itself, a pure article, at once real and artificial, and by means so cheap that they are likely to remove the temptation to use nitrobenzine. German samples of this new product were on exhibition at the Centennial last summer.
4. Flavoring Ethers.—Many other odor-giving constituents, besides that of the almond, are subjects of chemical manufacture. For example, oil of wintergreen (found in the berry and other parts) is well known to be chiefly salicylate of methyl, readily prepared from salicylic acid and wood-alcohol; and the oil or essence of pineapple is precisely butyric ether, manufactured largely from waste materials. Acetate of amyl and valerate of amyl are supposed to represent the flavor of the apple and the pear, but how accurately they coincide with the actual flavor-substances of these fruits has not been demonstrated. Formate of ethyl, another compound ether, is used in so-called peach-essence. Numerous fruit-flavors, used for culinary extracts and largely for soda-fountain sirups, are manufactured as mixtures of ethers, by recipes varying with different manufacturers. Many of these, resting on no due authority, are unwholesome mixtures, often spurious imitations of the true fruit-flavors, and again hurtful by reason of excessive proportions. As to the chemistry of the production of flavoring ethers in plants, some guesses were presented under the head of sugar fermentation.
5. Alkaloids.—Substances strongly affecting the nervous system, as medicines or poisons, of course do not occur in the edible fruits, and we are not in the habit of placing potent compounds among the constituents of fruits as a class; nevertheless, when we think of it, no small proportion of the banes and antidotes of the vegetable kingdom is matured in seeds and their coverings. In the poppy-fruit, the capsule or pericarp furnishes at least sixteen distinct alkaloids, including morphine, while the seeds are harmless, and yield an oil much used for food. In the fruit of the nux-vomica, the seeds are deadly with strychnine and other poisonous alkaloids, while the juicy pulp is but very slightly impregnated with these bitter poisons (Fluckiger and Hanbury, "Pharmacographia," p. 384). The seeds of henbane, and stramonium, and the Calabar-bean, contain potent alkaloids. The unripe tomato often contains traces of solanine, a poisonous alkaloid, which disappears during ripening, probably by a glucosic fermentation. The same alkaloid is sometimes found in the green or exposed parts of potato-tubers.
Many of the vegetable alkaloids are stable compounds, having clearly-marked chemical characteristics. Some of the opium-alkaloids closely resemble others in their composition. Different species of the same family often yield the same alkaloids. The theobromine of the chocolate-nut can be changed by the chemist into caffeine, the alkaloid of the coffee-berry. Such an insight has been obtained of the structure of conine, the alkaloid of the classic poison hemlock, that it has been formed from inorganic materials, through the processes of the laboratory. But no evidence has been obtained as to the steps through which alkaloids are formed in the living plants.
It is little enough we know of the productive chemistry of plants. As, at the beginning, we had need to plead ignorance of plant-constituents, still more, at the end of our brief survey, must we declare ignorance of the chemical genesis of those constituents. We can only obtain such glimpses of the progressive order of plant-chemistry, and we have only such a distant view of chemical action itself, as can give us some hints of the order, harmony, and grandeur, of the molecular changes going on in ripening fruits before us. None the less for our ignorance, the forces each season complete their work and drop their bountiful products into our hands.