FLOWER-GARDENING. 137 



leaf, can be discovered, there it is to be understood that the 

 rudiments exist of a system of life which may be, by favorable 

 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 Polypi 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 develop occasionally from all and any part 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 development of adventitious buds is ex- 

 tremely uncertain, such means of propagation can never be cal- 

 culated 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. 



Y. LEAVES. 



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



127. The veins consist of spiral vessels inclosed in woody 

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

 they are connected by loose parenchyma (7), 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. 



