156 FLOWERS. 



pose the greatest possible surface of their parenchyma to the 

 action of the atmosphere. 



134. Their cuticle is a non-conducting skin, which protects 

 them from great variations in temperature, and through 

 which gaseous matter will pass readily. 



135. Their stomata are pores that are chiefly intended to 

 facilitate evaporation; for which they are well adapted by a 

 power they possess of opening or closing as circumstances 

 may require. 



136. They are also intended for facilitating the rapid emis- 

 sion of air, when it is necessary that such a function should 

 be performed, 



137 The functions of stomata being of such vital impor- 

 tance, it is always advisable to examine them microsco- 

 pically in cases where doubts are entertained of the state of 

 the atmosphere which a particular species may require. 



13S. Leaves growing in air are covered with a cuticle. 



139. Leaves growing under water have no cuticle. 



140. All the secretions of plants being formed in the leaves, 

 or at least the greater part, it follows that secretions cannot 

 take place if leaves are destroyed. 



141. And as this secreting property depends upon specific 

 vital powers connected with the decomposition of carbonic 

 acid, and called into action only when the leaves are freely 

 exposed to light and air (279.), it also follows that the quan- 

 tity of secretion will bain direct proportion to the quantity 

 of leaves, and to their free exposure to light and air. 



142. The usual position of leaves is spiral, at regularly in 

 creasing or diminishing distances; they are then said to be 

 alternate. 



143. But if the space, or the axis, that separates two leaves, 

 is reduced to nothing at alternate intervals, they become op- 

 posite. 



144. And if the spaces that separate several leaves be re- 

 duced to nothing, they become verticillate. 



145. Opposite and verticillate leaves, therefore differ from 

 alternate leaves only in the spaces that separate them being 

 reduced to nothing. 



VI. Flowers. 



146- Flowers consist of two principal parts, viz. Floral 

 Envelopes (149.) and Sexes (VII.) 



147. Of these, the former constitute what is popularly con- 

 sidered the flower ; although the latter are the only parts that 

 are absolutely essential to it. 



148. However different they may be in appearance from 

 leaves, they are all formed of those organs in a more or less 

 modified state, and altered in a greater or less degree by 

 mutual adhesion. 



