LEAVES. J O.J 



rudiments exist of a system of life which may be, by favoura- 

 ble circumstances, called into action. 



117. Hence, all parts upon which leaves have ever grown 

 may be made use of for purposes of propagation. 



118. From these considerations it appears that the most 

 direct analogy between the Animal and Vegetable Kingdoms 

 is with the Polypes of the former. 



119. Adventitious leaf-buds are in all respects like Regular 

 leaf-buds, except that they are not formed at the axillae of 

 leaves, but develope occasionally from all and any parts of a 

 plant. 



120. They are occasionally produced by roots, by solid 

 wood, or even by leaves and flowers. 



121. Hence roots, solid wood, or even leaves and flowers 

 may be used as means of propagation. 



122. But as the developement of adventitious buds is ex- 

 tremely uncertain, such means of propagation can never be 

 calculated on ; and form no part of the science of cultivation. 



123. The cause of the formation of adventitious leaf-buds 

 is unknown. 



124. From certain experiments it appears that they may be 

 generated by sap in a state of great accumulation and activity. 



125. Consequently, whatever tends to the accumulation of 

 sap in an active state may be expected to be conducive to the 

 formation of adventitious leaf-buds. 



V. Leaves. 



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

 127- The veins consist of spiral vessels mclosed in woody 



fibre ; they originate in the medullary sheath and liber; and 

 they are connected by loose Parenchyma (7 J which is full of 

 cavities containing air. 



128 This parenchyma consists of two layers, of which the 

 upper is composed 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. Between many of the cells of the cuticle are placed 

 apertures called s/omuia, 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 ex- 



