Popular Science Monthly/Volume 46/February 1895/The Thorns of Plants
|THE THORNS OF PLANTS.|
THE seeming absence of means of defense in plants, putting them in contrast—to the eye—with animals, which are bountifully and variously armed, is only apparent. A not very close examination of the behavior of plants toward animals, their great enemies, will soon satisfy one that they have many protective organs, some of them very efficacious. The spines, thorns, and prickles with which the stems and the leaves of some plants bristle are known to all, and we can hardly fail to perceive a protecting function in the aggressive defense of plants against animals and against the hand of man put forth to pluck them. In the spring, for instance, when vegetation is very little forward, the plum trees would soon disappear completely under the attacks of cattle, sheep, horses, and other foliage-loving animals, if Nature had not provided them with those long, sharp spines that make browsing of them very difficult if not impossible. The shapes of thorns are various, but may always be brought back to a protuberance broad at the base and pointed at its free end, and of an extremely hard consistence. Usually simple, they are sometimes bifid or trifid. Their positions are various. All the organs of plants may be said to bear them: the stem, as in the rose; the base of the leaves, as in the barberry; the leaves, as in the thistle; the floral parts, as in the thistle; the fruit, as in Datura; and even the roots, as in Acanthus rhiza aculeata.
The body of plants is, we all know, composed of three members—the root, the stem, and the leaf. In determining with which part to class the thorns, an important distinction should be made, which, unfortunately, can be perceived only by the aid of the microscope. Some thorns, like the stem, leaves, and roots, contain vessels that bring up the sap, while others are destitute of them. The unvesseled thorns, simple risings of the superficial tissues, are scattered without visible order over the body of the plants. The vesseled thorns, on the other hand, are disposed in a fixed and regular manner, easy to be comprehended, for they are modified members, as the vessels running through them prove. Many thorns originate in transformations of branches; a form very evident-in the plum tree, on which the thorns frequently bear flowers. Sometimes, too, they proceed from leaves, as in the barberries, or from parts of leaves, as in the agaves, or from stipules, as in acacia. Often both leaves and branches are sharpened, as in the rush and the broom. In this case, if in any, we can say that Nature employs various means to reach its ends.
Thorns are interesting, not only on account of their functions and their morphology, but also on account of the modifications they exhibit in different situations. A plant, for example, richly armed with thorns in one region, will have fewer in another place, and none in a third. It is observed that the influence of the medium in these different regions makes itself felt in the same way on all the thorned plants that inhabit them. The flora of the steppes, which extends over vast arid plains, and the flora of deserts comprise more thorny species than the flora of forests. So it is in Senegal, a country remarkable for prolonged dryness of the atmosphere and the intensity of the solar light. M. Antoine Martin has remarked that similar conditions are observable in France, where in dry, bare places, as at the Grand Camp, near Lyons, the vegetable carpeting is constituted of plants with reduced leaves or thorns, such as Genista, Ononis spinosa, and Eryngium campestre; by which it is given an appearance comparable with that of desert regions. Thorny plants are especially prominent in deserts, where vegetation is subject to the triple stunting action of dryness of the air, aridity of the soil, and intense light. On the question as to which of these causes is the one that influences the production of thorns, an interesting memoir has been published by M. Lothelier, of the Sorbonne. He employed in his investigation the scientific and fruitful method adopted at the laboratory of Prof. Gaston Bonnier, which consists in subjecting many individuals of the same species, of plant to identical conditions of light, moisture, and temperature, and then introducing variations of one at a time. Then, whatever differences are observed in the results obtained at the end of a certain time, are evidently due to the factor which has been changed. In studying the modifications caused by the hygrometric conditions of the air, M. Lothelier capped two specimens of barberry with a long cylinder of glass, along which he placed broad-mouthed flasks of sulphuric acid to absorb the moisture of the air; and two other plants with a similar tube along which were flasks filled with water. These two lots of plants, growing side by side under
Plants with Thorns.—1 and 2, Berberis (barberry grown under moist and under dry conditions); 3 and 4, Genista or broom, moist and dry; 5 and 6, Ulex, or furze, moist and dry; 7 and 8, Cirsium, moist and dry; 9 and 10, Pyracantha, moist and dry; 11, Xanthium, upper sprig moist, lower sprig dry; 12 and 13, Gleditscha, or honey locust, grown in the sun and in the shade; 14 and 15, Robinia, or locust, grown in the sun and in the shade; 16 and 17, Rosa, grown in the sun and in the shade.
the same conditions of light, temperature, and watering, were then experimented upon for about six weeks, at the end of which it was found that the new leaves of the plants in dry air were spiny, while the leaves of the two in moist air were well developed, and had acquired long petioles. Like experiments were instituted on other plants, with always the same results; the differences of appearance presented by these plants as they grew in dry or moist air were really remarkable, and so great as almost to suggest that they were of different species.
An interesting observation was made that the disappearance of the thorns in moist air was effected in two different ways. The thorns, when they possess the morphological significance of a limb of the plant, whether of a leaf, as in Berberis, or a bough, as in Ulex, have a tendency in the saturated air to return to the normal type. When they proceed from organs that are not indispensable to the life of the plant, whether from a stipule as in Robinia, or from a stipule peduncle as in Xanthium, they tend constantly to disappear by retrogression.
The influence of light on the production of thorns was studied by M. Lothelier in a similar manner. His results were for the most part parallel. Shade tends to suppress the thorny parts of plants. The tendency is exhibited sometimes in a return to the normal form of the organ; but more frequently the thorns suffer a greater or less atrophy in the shade.
It is evident, then, that the conditions that most influence the production of thorns are especially dryness of the air and intensity of light. There are also, doubtless, other conditions of life that act in the same direction. I recollect having seen a cultivated and a wild olive tree growing side by side in the south. Only the latter had thorns. This even seems to me to be a general law which M. Lothelier has unfortunately not touched upon—that wild plants lose their thorns when they have been cultivated for several generations. It seems as if the plant, when brought under the protection of man, gradually gives up its defensive arms, which are thenceforward not needed, since its enemies are kept away by the care of its master.
The office of thorns is not limited to defending plants against herbivorous animals. With a number of plants, particularly those which form long shoots and live in thickets, thorns, usually curved downward, help hold up the stems. When thorns are localized on flowers, fruits, and seeds there seems generally to be a purpose of aiding in the dissemination of the seeds by enabling them to hook themselves in the fleeces of animals that come to graze among them. The thorns then serve for the defense of the species rather than of the individuals. The seeds which are aided by this mode of dissemination are called zoöphiles, while those which are adapted to dissemination by the wind are called anemophiles.—Translated for The Popular Science Monthly from La Nature.