Popular Science Monthly/Volume 45/October 1894/Barberries: A Study of Uses and Origins II
By FREDERICK Le ROY SARGENT.
WHILE the vegetative organs of barberries exhibit, as we have seen, an abundant variety of form and many degrees of differentiation, the reproductive organs are, on the contrary, so very similar throughout the group that what we may find to be true of a single example, such as Berberis vulgaris, will apply very generally to all the other species.
In the flowers (Figs. 2 and 3) there is traceable in almost every feature some relation to the visits of insects. Thus the conspicuousness gained by the yellow color of every part, enhanced by the clustering of the flowers and supplemented by their sweet perfumes attractively advertise, the abundant nectar which visitors find provided for them through the activity of the twelve orange glands (Fig. 3, N). In time of rain these sweets are protected by the pendent or nodding attitude of the flowers. On the arrival of an insect the movements by which it obtains a sip of the nectar are turned to account in a way to secure an advantageous transfer of the pollen from anther to stigma.
It has long been known that the stamens are so sensitive that at the slightest touch on the filament there is a quick inward bending of the organ which brings the anther with its exposed pollen to the center of the flower. Subsequently the stamen regains its original position, and will now respond to another touch as before. Sprengel in whose classic work were first revealed some of Nature's most cherished secrets, considered this to be an arrangement whereby insect visitors brought about the self-pollination of the flower, thus making possible the setting of seeds. But later experiments have shown that while Sprengel was entirely right in supposing insect visits to be of the utmost importance in securing fertilization, nevertheless the barberry is no exception to the general rule announced by Darwin, that flowers which attract insects gain from their visits the advantages which come from the transference of the pollen of one flower to the stigma of another, the result of such cross-pollination being greater vigor in the offspring. Thus Prof. Halsted found that barberry flowers from which insects had been carefully excluded produced no fruit (others uncovered on neighboring branches fruiting abundantly), and this in spite of the fact that through jarring or as a result of age the stamens had curved inward as far as they ever could. Microscopical examination showed a considerable quantity of pollen to have been deposited among the viscid hairs which form a ring about the top of the pistil (see Fig. 3, H), but none whatever upon the cushion-like summit which was found to be the only part that served as stigma.
Barberry blossoms are great favorites among the insects. Few of our June flowers gather about them a larger number of bees, hornets, flies, butterflies, and beetles. The smaller bees and certain flies are especially abundant.
There is some reason to believe that the intense color of the glands may serve as a guide to the insect, directing it at once without loss of time to the nectar which collects in little hollows between the bases of the filaments and the glands, where it is held by capillary attraction. An insect in thrusting its proboscis into a nectar cavity must touch the base of two filaments, whereupon both stamens suddenly bend inward and strike the insect's head. Now, Müller calls attention to the fact that while large insects such as bumblebees pay no attention to this, but continue to make the circuit of the flower, smaller ones, like the hive bee, appear to be somewhat startled by this performance and fly away at once to another barberry flower. But the insect carries with it some pollen upon one side of its head, and if in the next flower this comes into the same relative position as before, more pollen will be added on the same part; but if, on the other hand, the flower is approached from the other side, then the pollen already collected will be deposited upon the stigma, while at the same time a new supply of pollen is being received which may in turn be carried to still another flower. As the smaller insects are the more common visitors, cross-pollination, which is so much the best for offspring, must therefore be the most usual result.
This sensitiveness of the stamens is exhibited by all the species of Berberis so far as known, but is not found in other members of the family, although a somewhat similar irritability of stamens has been observed in certain of the Portulacaceæ, Tiliaceæ, Cistaceæ, and Compositæ. The strikingly animal-like nature of the movement is well shown by the following facts: A chemical stimulus, such as ammonia gas, will induce contraction as effectually as a mechanical stimulus. The presence of oxygen and a suitable temperature are necessary conditions. Repeated stimulation at short intervals fatigues the organ, making it less and less responsive, until finally all signs of sensitiveness disappear, to return only after a period of rest. Certain chemical substances which are known to abolish or suspend the contractility of animal protoplasm have been found to affect in a corresponding manner the movements of barberry stamens. Thus nicotine, alcohol, and the mineral acids destroy all power of movement. A one-per-cent solution of morphine is similarly active, while curare, the powerful nerve poison which leaves the contractility of muscle unaffected, is found to exert no influence upon the stamens of Berberis. The effect of arsenic and corrosive sublimate is to render the filaments rigid and brittle, while if poisoned with prussic acid or belladonna they become relaxed and flaccid. By exposure for a short time to the vapor of chloroform or ether the
Fig. 13.—Series of anthers connecting; the primitive form with that having valvular dehiscence: A, Podophyllum emodi; B, Podophyllum peltatum; C, hypothetical transition form; D, the barberry form. All somewhat diagrammatic.
power of movement is suspended, but may return after removal from the influence of the anæsthetic.
Moreover, experiment shows that the part of the filament which contracts is not necessarily the part touched that is to say, there is a transmission of stimulus from cell to cell. So long as it was believed that the contents of neighboring plant cells were always completely separated by an imperforate wall, no satisfactory explanation could be given of such a transference of impulse, but now that modern microscopy has revealed the presence of protoplasmic threads passing through the cellulose walls of sensitive tissue, making the living matter continuous, the phenomenon in question may be understood as a manifestation of that fundamental property of protoplasm, irritability, to which we also refer the sensitiveness of animals, even though it be exhibited in a highly differentiated nervous system.
Since the irritability of the stamens is found so commonly throughout the genus, we may assume the ancestral herbaceous barberry to have had the same peculiarity, but how this remarkable degree of sensitiveness could have arisen is not so clear. It would seem as if insect agency in some way or other must have brought about a movement having such an obviously purposeful relation to insect visits; but when we reflect upon the almost universal absence of a similar movement in flowers similarly visited, and the very questionable usefulness of the pronounced irritability of "sensitive" leaves, it is apparent that such a simple general explanation really explains very little. The few conjectures that the writer has to offer on the subject will be best understood after we have considered what may probably have been the evolution of certain structural peculiarities of the flower.
The anthers (Fig. 3, A), opening as they do by little valves hinged at the top, present a form of dehiscence confined entirely to the Berberidaceæ, the Lauraceæ, and a few other nearly related families not represented in our native flora. Within the Berberidaceæ all the genera except Podophyllum (the May apples) and Nandina have the anthers thus characterized; hence, it is clear that the stamens of the ancestral berberis were already of this peculiar type, and so the antecedent stages should be thought of as occurring in that line of berberidaceous herbs which were the forerunners of the barberries. The herbaceous genus Podophyllum contains species exhibiting a difference in the stamens which affords us an important clew for the understanding of what these antecedent stages may have been like. In P. Emodi (Fig. 13, A) the dehiscence of the anthers is by a longitudinal slit down the middle of each lobe. In P. peltatum (B), our common species, there is likewise a vertical slit, but it is so near the inner side of the connective that there appears to be but one valve to each lobe, the other inner valve having been reduced to a mere vestige. To connect this condition with that common elsewhere in the family, we need only suppose the attachment of the enlarged valve to become gradually narrowed by a continuation of the slit from below (C) until there remains only the small hinge we find in the barberry stamens of to-day (D). It deserves notice that the hinge, instead of being quite at the top, is nearer the back of the anther, which is what might be expected according to the hypothesis.
In the other families we have mentioned as exhibiting a valvular dehiscence of the anthers there is found almost invariably a pair of nectar glands on each filament (see Fig. 15). Now, it is a curious fact that in certain of the less highly developed mahonias the filaments are each provided with a pair of appendages (Fig. 14, A) similarly placed but being, so far as we know. quite as functionless as are the cilia on the leaf margin of our common barberry. These cilia we saw good reason for believing to be rudimentary spines. The supposition that the stamen appendages are degenerate nectar glands would seem to be scarcely less probable, in spite of our inability to find as full a series of intermediate stages. For it should be remembered that the time
which has elapsed since the development of the floral peculiarities here considered is surely much greater than in the case of the foliar modifications, and consequently it would be strange indeed if the intermediate stages had not disappeared.
Although, in regard to the evolution of the floral organs, there is so much less opportunity than with the vegetative system to test the validity of our conjectures, yet it may not be entirely profitless to follow such clews as are available, and endeavor to reconstruct hypothetically the main features of those more primitive flowers from which the present barberry type was derived.
A multitude of stamens and pistils is generally recognized as characteristic of primitive flowers; hence, we shall probably be not far wrong in considering the remote ancestor of the barberries and their kin to be in this regard very like a marsh marigold (Caltha), although doubtless less conspicuous, and with the parts more definitely arranged. As it is a very general characteristic of berberidaceous flowers that the parts are in whorls of three, we should expect this to be the case with the common ancestor. Accordingly, we arrive at a generalized type of flower, the structure of which may be expressed diagrammatically as in Fig. 17. At this stage we should also expect the flowers to be solitary, arising each from the axil of a leaf very similar to the rest of the plant's foliage.
Competition in securing the benefits of insect visits, together with the possibilities of a more economical as well as more effective disposition of tissue-building material, would conspire to bring about through natural selection the following changes:
1. Those branches of the herb on which flowers appeared would be given up more and more fully to their function of flower production; their subtending leaves would be reduced in size, and through a shortening of the axis the flowers would be brought closer together, and thus their conspicuousness enhanced. At the same time, part of the material saved might go to form additional flowers in the cluster. With the assumption of the shrubby habit the floral branches (peduncle, rhachis, and pedicels) retaining their herbaceous nature, in consequence of their short-lived usefulness, would appear still less like the others. The formation of flower buds to last over the winter would favor the blossoming of the flowers more nearly together in the following
year. As the subtending leaves would now have lost almost the last vestige of their usefulness, we should expect their reduction to mere scales. The result of all this would be such a raceme as we find the barberry to possess (Fig. 2).
2. With the increase in the number of flowers in a cluster there would be less need for so many pistils in each flower. It might often happen that only a few of those in one flower would be fertilized, and in that case the store of food could be increased in the favored seeds, much to the advantage of the offspring produced. Pistils which ceased to have a use would gradually appear, until finally there would remain a single one of much, increased serviceableness (Figs. 18 and 19).
3. For the reasons already given we may suppose the stamens to have their anthers so modified as to open by hinged valves, while at the same time there was developed upon each filament a pair of nectar glands (Fig. 18). Insect visitors, finding an abundance of nectar in a flower, would be less likely to feed upon the pollen, which is so precious to the plant. As the position Fig. 19.—Diagram of euberberis flower. Bracts and sepals as before; petals, six, with nectar glands; stamens, six, with valves but no glands; carpel, one; ovules, few. of the nectar is nearer the center of the flower, the visitor comes to occupy a more definite place relative to the pistil and stamens. The six stamens of the inner row are for the most part the only ones which can have their anthers touched, for, as will be seen from the diagram, the remainder are so placed as to be directly behind the others. Being thus superfluous as pollen-producers, the anthers of the other stamens would naturally degenerate, and if they follow the general rule of stamens in flowers which are provided with an abundance of building material (as, for example, the "double" flowers of the florists), they would change into something very like petals. If these petaloid organs became slightly arched over the inner stamens, they might still be of use in the floral household by giving better protection to the pollen than it had previously had, and at the same time increase somewhat the conspicuousness of the blossom. While it is by no means clear that any advantage is gained by having such an organ bilobed at the upper end, it might be a not unnatural result of that special part's having been derived from a bilobed anther. A glance at Figs. 3 and 16 will show that just such a petaloid organ is situated behind each of the stamens in a barberry flower, The nectar glands of these hypothetical outer stamens, although situated behind the others, would not be disqualified in the least by such a position from continuing to perform their function. On the contrary, it might well happen that, as they no longer produced pollen, they would secrete all the more nectar in consequence, and thus relieve the inner stamens of so much of their work. No longer required, the glandular appendages of the latter would be reduced to rudiments (as in mahonias), or entirely disappear, as we have seen in the case of the true barberries.
Do the above considerations help us to any better understanding of how the irritability of the stamens came to be developed? In our previous consideration of this remarkable property, we saw reason to believe that such a peculiar manifestation of protoplasmic activity could only be satisfactorily explained as having resulted from a rare combination of favoring circumstances. Although, in the discussion of such a matter, we are confessedly treading upon uncertain ground, still, it may be worth while to inquire whether, supjoosing the barberry flower to have been evolved essentially after the manner indicated, there have not been thus happily combined the very factors we should deem necessary and adequate to produce this result. It should be remembered that we are not endeavoring to account for that fundamental property of protoplasm known as contractility, but only for its being in the stamens of the barberry so much more strikingly exhibited than in other organs of the plant, and in the great majority of other plants.
In the first place, in order that this or any other property of protoplasm should be especially well shown in any organ, it would seem to be a prerequisite that the organ should be unusually rich in protoplasm—a supposition which is confirmed by the comparative study of motile organs. Such a very considerable reduction of parts as we believe to have taken place in the barberry flower might well be connected with the enrichment of the remaining tissues.
Secondly, a mechanical stimulus applied repeatedly for innumerable generations, at a very definite part of the stamen, would seem to be also necessary in order to account for the fact that movement of the organ occurs in response to a touch only when applied to the front of the filament and near its base. From the position which the glands came to occupy in the flower, just such a stimulus was afforded by the proboscis of every insect that sipped the nectar.
Whether we are at liberty to suppose that the direct effects of such a repetition of stimuli may be accumulated through inheritance, or whether we must assume only the inheritance of fortuitous variations, is of comparatively small consequence in this particular case, because the movement in question is undoubtedly useful, and as such, variations in this direction, be they fortuitous or mechanically induced, would be preserved by natural selection. In other words, an ever-recurring mechanical stimulus is presupposed even on the theory which works entirely with accidental variations, responding more or less fortunately thereto, while, if the stimulus be a direct cause of the favorable variations, its importance as a factor becomes still greater.
Our theory of the origin of the peculiar movement in barberry stamens amounts, then, to this: Stimulation by contact at a definite part of the filament for innumerable generations, increase of the protoplasmic contents by the reduction of adjacent parts, and the usefulness of such a movement at every stage of its development—these three factors, although separately incompetent, have yet in combination been the ones chiefly concerned in bringing about through the agency of natural selection such changes in the protoplasm of the sensitive cells as make its fundamental property of contractility prominent to an extraordinary degree.
Fertilization being accomplished, the single pistil ripens into a berry. In Berberis vulgaris each of the two ovules ordinarily becomes a hard-coated seed flattened on its inner face by pressure (Fig. 20) in much the same way as happens with the two "beans" in a coffee berry. Sometimes (as in the so-called "male berry" coffee) one of the ovules aborts, thus leaving the other to form a seed proportionally richer in reserve food and correspondingly round in form. Occasionally there may be found barberry bushes producing fruit in which both ovules have aborted. But accordding to Buckhout such individuals "do not constitute a permanent variety, for stoneless barberries are only found on old plants, and it has been proved that young suckers taken from them and planted in fresh soil fruit with perfect seeds." Seed production in this case would thus seem to be a question of the plant's vigor at a given period, and so to be comparable with the case of ordinary seedless varieties (such as bananas, navel oranges, and the tiny seedless grapes sold as dried "currants") only on the supposition that in the latter also there had been a loss of vigor through long-continued non-sexual propagation.
The agreeable tartness of the barberry fruit, which makes it so generally and so highly esteemed, is due to the presence of Fig. 20.—Berberis vulgaris. Vertical section of berry, showing two seeds, each containing copious reserve food and a long, well-developed embryo. malic acid, a substance found also in the foliage. Besides being made into preserves and jellies, the ripe fruit is candied or may be dried like raisins. While yet green the berries are sometimes pickled as a substitute for capers. Barberry preserve is, moreover, often used as the basis of a refreshing summer drink—a sort of "barberryade." Finally, it is reported that in our Western States the fruit of Berberis aquifolium and certain other native species is made to yield upon fermentation an agreeable wine.
But, for all their attractiveness to us, the berries seem to be less in favor with birds than are many fruits which we care nothing for. So long as the more succulent or less acid fruits are to be obtained, birds visit the barberry but little. When winter comes, however, they are glad enough to profit by the barberry's offer of something to eat, and the bright scarlet clusters do not dangle in vain.
Kerner fed certain thrushes with barberries, and found that the resistant seeds not only passed unharmed through the digestive tract, but their power of germination was improved, as shown by comparing them with seeds which had not been eaten. Add to this advantage the long distances which birds are likely to carry the seeds they eat, and the likelihood of their depositing them in most favorable situations, and it will at once be apparent how much superior to other methods is this mode of dissemination.
There can be little doubt that in the primitive ancestors of the barberry family the fruits were dry capsules which depended upon the wind to distribute their numerous seeds, as is the case to-day in the majority of herbaceous Berberidaceæ. That is to say, if we suppose the six pistils of the primitive berberidaceous flower (see Fig. 17) to have ripened into as many capsules, we shall have a form of fruit from which not improbably may have been derived all the different forms of fruit exhibited in modern representatives of the family. Confining our attention to the line which culminates in the barberry, it will be seen that the supposition of such a fruit's having descended from the primitive form above mentioned involves the assumption of the following changes: (1) The disappearance of all but one of the pistils; (2) reduction of the number of seeds; (3) the abandonment of dehiscence; (4) increased hardness of seed coat; (5) the acquirement of succulence; (6) the development of an attractive color.
The first-named alteration we have already considered in connection with the evolution of the flower. As with this, so with the other changes, the best we can do is to imagine how they might have come about. Now, it is true of all capsular fruits that until fully ripe they are neither dry nor dehiscent. We know that variations in the time of ripening do occur, and experiments have shown that even unripe seeds will germinate and produce strong, healthy plants. In view of these facts it seems reasonable to suppose that not only might there arise varieties in which the capsule would retain something of its succulence until the seeds were nearly ripe, but if the fruit in this condition were eaten by birds or other animals the seeds might be disseminated by them, much to the benefit of the favored variety. There were doubtless seasons of scarcity in prehistoric times as well as now, when animals would be glad of even such comparatively unattractive fruits as we have described. Among the descendants of those plants whose fruits had become somewhat berrylike, those having the more succulent pericarp would, other things being equal, have most descendants, and thus in the course of many generations the present condition be reached.
The conspicuousness, depending as it does upon the same changes in the original pigment as occur in the transformations of chlorophyll in autumn leaves, may be looked upon as a result incidentally connected with the retention of succulence in the pericarp after growth had ceased, and as this tendency for the fruit to assume a color contrasting with the foliage would be beneficial as an advertisement to birds, natural selection would favor rather than hinder it.
The fact that in mahonias the berries are commonly of a dark purplish blue suggests that possibly this was the color first assumed by the fruit of the genus, the more conspicuous scarlet of the common barberry and its near relatives being acquired later, along with the higher differentiation of structure which it accompanies. Although this view gains some support from the occasional appearance of a blue-fruited variety of Berberis vulgaris (which might be thought of as a reversion to the ancestral type), still it should be remembered that our knowledge of the chemistry of plant pigments is at best too meager to justify much confidence in any theory of color change.
When once the good services of birds had been secured, there would be no need of having so many seeds in each fruit as must have been formerly necessary to compensate for the extreme wastefulness of wind as a distributing agent. At the same time a reduction in the number would permit, as we have already had occasion to notice, a higher development of each remaining seed—that is to say, an increase of reserve food and a thickening of the outer coat, features that we find to vary directly as the number of seeds, there being all the way from eight (in many mahonias) to two or even one in the true barberries.
However the characteristics of the barberry fruit may have arisen, the fact that they came to depend upon birds for their dissemination must have exerted an important influence upon all the subsequent differentiation of the group, for barberries were thus brought into widely separated regions which they might not otherwise have reached and so came to grow up in widely different surroundings.
We have already considered the extent of the migrations which are believed to have taken place in preglacial times. Among the forms which became adapted to the refrigerated climate that heralded the Glacial epoch, one of the most successful was probably a form almost if not quite identical with the modern Berberis canadensis, which despite its name does not grow in Canada, but is found only in the Alleghanies of Virginia and southward. Before the glacier came, however, the ancestral form we are speaking of probably did occur even to the north of Canada, and through the agency of birds was carried into Asia and distributed widely on that continent. Under the influence of their new environment it would not be strange if in the Asiatic descendants of the canadensis stock there appeared, even during the (geologically) short time since the beginning of the Glacial period, those slight differences which now distinguish Berberis vulgaris from the American descendants—differences which in the minds of some botanists entitle the two forms to rank only as varieties of the same species. After the retreat of the glacier, Berberis vulgaris extends into Europe to take the place of the mahonias previously exterminated. It now flourishes from England to Persia and from Persia to Japan. Our forefathers bring the plant to this country (largely for the sake of its fruit), and thus it finally returns to the ancestral acres. It would surely seem to be not a little invigorated by its journey around the world, since in the acquisition of American territory it appears to be in a fair way to outdo its stay-at-home relative, and has already fully justified with us its Old World name of "common barberry."