Popular Science Monthly/Volume 61/October 1902/A Study in Plant Adaptation
|A STUDY IN PLANT ADAPTATION.|
EVERY one interested in plants knows that they are very dependent upon their surroundings. The atmosphere and soil conditions that suit one species are often totally unsuited to another. In the process of development the different species become structurally and physiologically modified with the change of environment; they take on certain adaptions, where they succeed best, which particularly fit them to their surroundings.
Every plant in order to grow must receive material from the outside and must get rid of waste matter. The plant does not differ in any essential respect from the animal in this regard. So also, the plant, in order to continue from generation to generation, must bear offspring and leave them in situations favorable to their growth.
In all seed-plants the food materials are essentially the same. The ability of a plant, however, to avail itself of these materials depends very largely upon a close correlation between the structure and the physiological activities of the plant organs and its environment. Thus a plant like the apple will not succeed in a hot and arid climate, while, on the other hand, the date will not thrive beyond the limits of the desert.
The sensitiveness of many plants to a slight change in soil or climate and the necessity for a perfect adaptation to a particular environment are illustrated in the very restricted range of many of our native trees and shrubs.
On the Pacific coast the Monterey cypress is only found growing naturally over a strip of territory, south of the Bay of Monterey, about two miles long and two or three hundred yards wide, and over a still smaller area a little farther south. The California fan-palm only occurs in a few canyons of two mountain ranges in the southern part of the state. The western hop hornbeam is only known to grow over a few square rods of territory in the Grand Canyon of the Colorado river in Northern Arizona. In eastern United States, Torreya only grows in a narrow strip on the eastern bank of the Appalachicola River in Florida: while the Florida yew, which grows in the same region, occupies a still less extended area.
In such cases as these it is likely that the structural and physiological adaptions of the different plant organs have not kept pace with the natural changes in environment. As a result, these trees are not only unable to extend their present range, but are poorly fitted to persist where they now grow and consequently are disappearing. These old types of trees have in the course of ages become inflexible and fixed and are no longer in perfect accord with their environment. More modern types, as illustrated in the various genera of Cactaceae, are more generalized and very readily take on structural and physiological modifications which fit them better to their present environment. It is interesting to note that many of the species which appear to be out of accord with their natural environment often do well under cultivation. The gardener's care in subjecting them to different environmental conditions, particularly as regards food supply, seems to stimulate them and give them new vitality, thus causing them to succeed better than more modern types perfectly in accord with their natural environment. In the latter case overstimulation, induced by cultivation, may from the standpoint of vitality do more harm than good.
The Monterey cypress, although now nearly extinct as a wild plant, is one of the most successful and easily cultivated trees of the Southwest. It appears to be far better in accord with the artificial environment induced by cultivation than it is with its natural environment. The Franklinia of our gardens, a small tree first collected by John Bartram in 1765 on the banks of the Altamaha River in Georgia, is successful in cultivation, although as a wild plant it passed out of existence during the past century. It is far more successful in cultivation than the Loblolly bay, an allied species of the same genus which is now growing wild from Virginia to Florida. The Ginkgo, an Asiatic tree of ancient origin, grows remarkably well in cultivation, although at the present time it is not known to grow as a wild plant any where.
Modern plant types that have not yet reached the limits of their distribution and variation, as illustrated in many species of the Compositae, Rosaceae and Cactaceae, are so nicely adjusted to their natural environment that cultivation often tends to diminish their vitality rather than improve it.
It is not to be presumed that every variation in the structure of plant organs is a direct result of adaptations taken on by the plant to protect it from unfavorable factors in its environment. It is the natural, inherent tendency of plants to vary, and when the variation chances to be in a direction that fits it better to its environment, the variation is apt to persist in future generations. There is no apparent reason, however, why in many instances structures may not be present in the plant that are in no sense of direct aid. We should not expect to refer every variation in plant structures to variation in environment. We should, however, expect those species to do best that in their natural
tendencies to vary become so modified as to fit them most perfectly to their surroundings.
Each plant organ must not only be adapted for the kind of work that it has to do, but is must be adapted for doing its best under the external influences which enable it to persist in any given form. The foliage leaf bears a definite relation to light and moisture; the leaves of one plant, however, may have quite different requirements as to light and moisture than the leaves of another. Every traveler in our arid southwest has noticed that the leaves of the trees and shrubs are small and thick, or, in some instances, entirely absent as foliage. The reason for this is very clear. It arises from the necessity of the desert plant to expose a comparatively small surface to the intense sunlight and the desiccating action of the dry atmosphere.
The various species of cacti illustrate this necessary correlation between plant structures and environment probably better than any other large group of plants. Opuntia, the most important genus., is abundantly represented in the flora of our arid southwest (Figs. 1 and 2) and reaches its maximum development on the Tucson plains in southern Arizona. No less than ninety-two species of Opuntia are growing wild in southwestern United States and northern Mexico, selecting for the most part situations that are so dry that few other plants persist where they thrive.
In this article I desire in particular to call attention to the cholla (Opuntia fulgida Engelm.) a cactus which grows to the size of a small tree and which reaches its maximum development on the Tucson plains.
The cholla has probably not yet reached the limits of its variation and distribution, and is one of the most interesting and characteristic plants of the arid regions of the western continent. The organs of this plant are most wonderfully adapted for performing their various functions, to the best advantage of the plant, under what would be with most plants an extremely adverse environment.
The cholla is one of the largest of the cacti having numerous branches. It grows best where fully exposed to the intense glare and heat of the desert sun and where the annual rainfall averages from four to twelve inches. It grows on the dryest upland, on open, porous, limy soil that for months at a time is as dry as powder.
Where it grows best the summer temperature often reaches a maximum of 115 degrees F. and the daily temperature for weeks at a time exceeds 100 degrees F. during the hottest part of the day. Often for several consecutive months there is no precipitation whatever and much of the rain that does fall only penetrates the. soil to the depth of a few inches.
Not only is this cactus, as an individual plant, perfectly equipped by nature to withstand and thrive under this extremely hot and arid environment, but it is splendidly equipped for perpetuating itself by its successful distribution of offspring under conditions which enable them to succeed where on account of lack of moisture most plants would perish.
The roots of the cholla do not penetrate to great depths in the soil as one would at first thought suspect them to do. For the most part they spread out a few inches under the surface. It would be useless for this plant to send its roots to great depth into the soil, because only in rare instances is there any available moisture there. As most rains only penetrate the soil to the depth of a few inches the most moisture is found in the surface soil. These surface roots of the cholla have a different structure from that found in the deeper roots and in the roots of most plants. Their structure is splendidly adapted to enable them to take up water with great avidity when the soil is moist and to survive long periods of drought during which the surface soil is practically air dry.
It would be of no special value to the cholla to absorb large quantities of water when available, if there were no provision made by the plant for storing it, or if through transpiration it were readily given up to the surrounding atmosphere. This cactus is not only remarkably well equipped for storing water in large quantities, sufficient to carry it through months of continuous drought, but it is able to retain this water with wonderful tenacity, only giving it up to the hot and dry air a little each day and taking advantage of each rain to fill its storage tissue.
The structure of the young branches and stems of the cholla particularly adapts them for the storage of water in large quantities. At the height of the growing season or after a prolonged summer rain the stems of this cactus may contain as high as ninety-two per cent, of moisture. During a prolonged drought the percentage of moisture very perceptibly diminishes. The older stems and branches which give strength and support to the tree contain a much larger proportion of woody tissue and consequently serve to a less degree for the storage of water.
The ability of the plant to retain moisture results largely from the comparatively small surface exposed to the dry air and the remarkably thick epidermis and dense spine covering of the branches. The small surface is chiefly a result of the elimination of the leaves as foliage and the contraction of the branches into thick, short stems as shown in Fig. 3. During the growing season the cholla exposes to the atmosphere less Fig. 3. The Short, Thick, Succulent Stems of the Cholla bearing Clusters of Fruit and Flowers. than one fiftieth of the surface which is exposed by the maple of equal weight here in the east.
Aside from this remarkable diminution of surface, the thick epidermis of the plant almost precludes transpiration at times when the water in the storage tissues begins to run low. The following illustrates how well the plant is adapted for conserving the moisture previously stored in its thick stems. I have cut a branch from the tree in the spring prior to the season of blooming, at a time when the storage tissue was well filled with moisture. I have placed these branches in a perfectly dry room out of contact with moisture. I have seen them continue in growth, and ultimately blossom. I have placed branches of the previous season's growth in open boxes without soil and without access to moisture early in March, and in September have found the branches still succulent and in condition to root and grow when placed in the soil.
Several years ago I removed a large specimen, having a trunk diameter of eleven inches and a height of ten to twelve feet, from the open mesa where it was growing to my garden. The tree was moved in late May when in full bloom. Although the month was hot and dry, the roots were closely pruned and the top left unpruned. The flowers did not wilt as a result of this severe treatment and a full crop of fruit ripened in the fall. This tree, which is illustrated in Fig. 4, suffered no apparent harm in its removal, although probably fifty to one hundred years old.
As a summary it may well be said that the cholla is admirably adapted for absorbing water rapidly, storing large quantities of it, and even when exposed to a very dry atmosphere for a long time retaining it with wonderful tenacity. Provision is also made for undue loss of moisture at times of injury to the epidermis, and the consequent direct exposure of the storage tissue to the dry air. At such times a mucilaginous substance contained in the cells appears on the injured surface, quickly rendering it impervious to moisture. It is probable that the thick covering of spines is of some value to the plant in protecting it from the full force of the intensely bright sunlight and also of some value in checking transpiration. These ever present and formidable, barbed spines are well illustrated in Fig. 5. They serve their greatest usefulness to the cholla in preventing its destruction by animals and in the important part which they play in the dissemination of the species. All the younger branches of the cholla are soft and succulent and, were it not for their efficient armor
of barbed spines, would be quickly destroyed by herbivorous animals. In acquiring a condensed and succulent plant body in order to fit itself to a desert environment the cholla would have courted its own ruin were it not that it acquired a full equipment of spines at the same time. From every standpoint it is, as an individual, admirably equipped for its desert home. It is, however, more than this; it is the best equipped of all desert plants for rapid and wide dissemination. It makes ample provision for its offspring.
The seeds of the cholla, like those of several allied species, rarely if ever germinate on the open mesa. In eight years of observation in its center of distribution I never found a seedling of this plant growing wild. It spreads almost entirely by vegetal dissemination, i. e., by the ends of the branches becoming detached and transported often long distances from the mother plant.
The cholla is perfectly adapted not only for the easy detachment of the ends of the branches, but for their wide dissemination as well. The fruit which hangs from the tree in long, pendulous clusters as illustrated in Fig. 6 is within easy reach of cattle and other large animals.
These clusters of succulent fruits are without spines and are for the most part sterile. The fruit from the standpoint of seed production is of very little service to the plant in aiding in its perpetuation and dissemination. Its chief service appears to be to entice animals to the plant that the fragile ends of the branches which become detached at the slightest disturbance may adhere to them and become scattered far and wide.
In order that these fruits may best serve their purpose they are succulent, unarmed and as eagerly eaten by animals when green as when ripe. Moreover they often remain on the plant for two or more years if undisturbed. During periods of scanty forage in the region where the cholla grows it is not an uncommon sight to see the range cattle with their heads literally covered with these formidable cactus burs which became attached to them in their effort to get the fruit. The spine arrangement of these end branches or burs is such that when they finally become detached from the animals transporting them and fall to the ground, the lower end comes in contact with the soil. As the roots start from this end of the branch the necessity for this provision is very evident. It results from the spines being very short or wanting on the lower end of the short, thick branches.
The special adaptation of the fruit to aid in vegetal dissemination is confined, so far as I am aware, to a few species of the Opuntia and reaches its highest development in the plant that I have described above.
The fruit of the cholla is probably changing from its original seed-bearing condition to a condition of sterility. The abundant clusters of fruit hang from the plant within easy reach of cattle and it is interesting to note that since the advent of stock into the arid southwest the cholla has become more widely distributed and more abundant than ever before. It is as well equipped by nature to care for itself and perpetuate itself on the hot, dry sands of the desert, as is the New England elm of the humid east which bursts into foliage under April showers.