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.
