THE THORNS OF PLANTS. 499 



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 mem- 

 bers 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, con- 

 tain 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 stip- 

 ules, 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. An- 

 toine 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 oC plants with re- 

 duced leaves or thorns, such as Genista, Ononis spinosa, and 

 Eryngium campestre; by which it is given an appearance com- 

 parable 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 



