DEFENSIVE EQUIPMENT OF PLANTS 139 



be seen dwarfed and cropped close to the ground, where 

 rabbits abound ; others, doubtless, are entirely extermi- 

 nated through the ravages of the resident animals. 



The structures found on plants which serve as a means 

 of defence against living enemies were not actually called 

 into existence for that purpose. The leaves of the hoUy, 

 for example, are not tough and spiny to protect the plant 

 against the hungry mouths of animals, although, being 

 prickly, they do as a matter of fact serve that purpose. 

 Nor does the gorse grow its thorns and the bramble its 

 prickles to make their bodies disagreeable eating. True, 

 the thorn and the prickle do secure their owners from 

 injury where browsing animals abound, and in the course 

 of time plants not so protected might in such localities 

 become exterminated. But the animal is not the cause 

 of the thorn. Thorny plants would grow even if no 

 browsing animals existed on the earth. They are char- 

 acteristic plants in dry, hot regions where extreme xero- 

 phytic conditions prevail. In a xerophyte the transpira- 

 tion-current is poor and slow. The plant is consequently 

 badly nourished, and the spiny or prickly habit is simply 

 the result of this defective nutrition. In xerophytic 

 regions vegetation is scanty, and the animals are not too 

 well suppHed with food. The few plants that are found 

 there would consequently stand a poor chance of surviving 

 if they did not possess some means of keeping off their 

 destroyers. But the very evils from which they suffer 

 provide a means of safety. Dearth of water means lack 

 of food. Lack of nourishment sets up all kinds of physio- 

 logical disturbances. It affects assimilation, transpira- 

 tion, respiration, and growth. As a result, leaves become 

 tough and indigestible ; buds become provided with scales, 

 and form gummy excretions ; spines, thorns, and prickles 

 develop at the expense of assimilating tissue, while gums, 

 waxes, oUs, and bitter and poisonous juices form and 

 collect in the organs. Once being produced, and being 

 found at the same time useful, these things have been 

 preserved by Natural Selection, and a specialization has 

 been marked out for them upon the lines of protection 

 and defence. Protective resemblance sometimes serves 

 as a means of defence against animals, as in the case of 

 certain succulent plants, like species of Sedum and Mesem- 

 bryanthemum, which have the appearance of stones. 



