LEAVES. 127 



V. Leaves. 



126. Leaves are expansions of bark, traversed hy veins. 



127. The veins consist of spiral vessels enclosed in woody fibre ; they 

 originate in the medullary sheath and liber ; and they are connected by 

 ioose Parenchyma (7), which is full of cavities containing air. 



128. This parenchyma consists of two layers, of which the upper is com- 

 posed of cellules perpendicular to the cuticle, and the lower of cellules 

 parallel with the cuticle. 



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. B'jtween many of the cells of the cuticle are placed appertures 

 called stomata, which have the power of opening and closing as circum- 

 stances may require. 



132. It is by means of th-is apparatus that leaves elaborate the sap which 

 they absorb from the albernum, converting it into the secretions peculiar to 

 the species. 



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

 est 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 evap- 

 oration ; for which they aie well adapted by a power they possess of open- 

 ing 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 performed. 



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 ar6 

 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 fol- 

 lows that the quantity of secretion will be in 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 increasing 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 opposite. 



144. And if the spaces that separate several leaves be reduced to nothing, 

 they become verticillate. 



