Page:Popular Science Monthly Volume 46.djvu/515

Rh 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