138 FLOWER-GARDENING. 



129. These cellules are arranged so as to leave numerous 

 open passages among them for the circulation of air in the inside 

 of a leaf. Parenchyma of this nature is called cavernous. 



130. Cuticle is formed of one or more layers of depressed 

 cellular tissue, which is generally hardened, and always dry and 

 filled with air. 



131. Between many of the cells of the cuticle are placed 

 apertures, called stomata, which have the power of opening and 

 closing as circumstances may require. 



132. It is by means of this apparatus that leaves elaborate 

 the sap which they absorb from the alburnum, converting it 

 into the secretions peculiar to the species. 



133. Their cavernous structure (129) enables them to expose 

 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 emission 

 of air, when it is necessary that such a function should be per- 

 formed. 



137. The functions of stomata being of such vital importance, 

 it is always advisable to examine them microscopically in cases 

 where doubts are entertained of the state of the atmosphere 

 which a particular species may require. 



138. 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 



