98 BOTANY. [CHAP. in. 



the plant when no longer of service, but remain for a 

 long time in a rudimentary condition. 



(i.) Protection against climate. All climatic condi- 

 tions are not equally favourable for the development of 

 plant life, and as would be expected, in proportion as 

 plants have adapted themselves to live under more and 

 more unfavourable climatic conditions, the more marked 

 will be the modifications undergone to combat these con- 

 ditions. Taking as the typical plant structure living 

 under favourable conditions, we find in the majority of 

 instances a stem more or less erect, and bearing flattened 

 leaves, borne in a scattered manner on branches for the 

 purpose of exposing each leaf to the light, an indispen- 

 sable condition for the performance of its functions, as 

 already explained. A marked departure from this 

 general arrangement of stem and leaf is illustrated by 

 certain groups of plants that grow in very arid regions. 

 The cactus family, characteristic of and almost entirely 

 confined to the dry regions of Western North America, 

 is remarkable for the partial or complete suppression of 

 leaves, but to compensate for the arrest of these impor- 

 tant organs, the stem is furnished with chlorophyll, and 

 usually has prominent ridges or wings, for the purpose 

 of exposing a larger amount of assimilating surface than 

 would be the case with the ordinary cylindrical trunk. 

 Clusters of spines that originate from certain abortive 

 buds are usually present, and act as protective organs 

 against herbivorous animals that would otherwise browse 

 on the succulent and watery stems. In Central Africa a 

 similar habit is assumed by the members of a widely-sepa- 

 rated order of plants, the spurges ; in fact, so close is the 

 resemblance in habit that these plants are usually consi- 



