ORIGIN AND SIGNIFICANCE OF SPINES 73 



desert is found every evidence of similar stunting combined 

 with adaptations to resist the unfavorable conditions of defi- 

 cient water supply, excess of radiation, etc. The diminution 

 in size applies not only to stature, but to the leaves and 

 branches, especially the parenchymatous tissues or parts of 

 the plant engaged in aerial assimilation. Consonant with 

 these changes, the drought and other conditions produce a 

 hardening of the mechanical tissues, which is of great aid in 

 resisting the extreme heat and dryness of the desert. Some- 

 times a deposit of wax affords a similar protection. 



The reduction of the leaves takes place in various ways. 

 They may simply become smaller in every dimension and 

 finally be reduced to mere scales, or an aphyllous condition 

 may be established. They may grow narrower and narrower 

 until only the hardened veins or midrib remain; or leaves 

 may be developed only for a short time, and in the case of 

 compound leaves after the shedding of the leaflets a spini- 

 f orm leaf axis remains ; as in Astragalus Tragacantha (figures 

 55, 56). The suppression of ., branches tends toward the 

 same end ; namely, either to their complete disappearance or 

 to their partial suppression into hard spiniform processes or 

 thorns. Thus leaves, branches, and other parts of the plants 

 may become reduced to their axial elements, bringing about 

 what is commonly termed spinescence. 



The spiny character of these plants is therefore one of the 

 results of an arid environment, and it may or may not be of 

 sufficient frequency to give an especial character to a partic- 

 ular desert flora. There is, moreover, a secondary influence 

 which has an effect in determining the abundance of spinose 

 plants in desert as well as in many other situations. This 

 relates to the destruction of the edible unarmed species by 

 herbivorous animals, and the comparative immunity of the 

 spiny types. Thus, in old pastures, the prevailing flora is 

 apt to be one that is offensive to grazing animals. This 

 character is generally given by poisonous plants or those 

 having a disagreeable flavor, or by those whose form or spiny 

 structures afford protection. 



