LEAF-BUDS. 23 



subsequently to the developement of the stem and leaves, and 

 without reference to the latter, are called latent, adventitious, 

 or abnormal. 



184. Adventitious Leaf-buds may be produced from any 

 part of the horizontal medullary system, or wherever cellular 

 tissue is present. It has been distinctly proved, that, while 

 roots are prolongations of the vertical or woody system, leaf- 

 buds universally originate in the horizontal or cellular system. 



185. They are formed'in the root, among the wood, and at 

 the margin or on the surface of leaves. 



186. They are constructed anatomically exactly as regular 

 buds, having pith in their centre, surrounded by a medullary 

 sheath of spiral vessels, and coated over by woody tissue and 

 cellular integument. 



187. Hence, as adventitious buds, containing spiral vessels, 

 can be produced from parts such as the root or the wood, in 

 which no spiral vessels previously existed, it follows that this 

 form of tissue is either generated spontaneously, or is produced 

 by some other tissue, in a manner unknown to us. It is most 

 probable, that spiral vessels are spontaneous modifications of 

 vesicles of cellular tissue. 



188. Embryo buds are woody nodules found in the bark of 

 trees, and apparently rudimentary branches formed without 

 leaves, within a space in which they are forcibly pressed upon 

 by the surrounding tissue. 



VI. LEAVES. 



189. A leaf is an expansion of the bark immediately below 

 the origin of a regular leaf-bud, and is an appendage of the 

 axis (64). 



190. Whenever a regular leaf-bud is formed, a leaf, either 

 perfect or rudimentary, is developed also ; and vice versa. 



191. Leaves are developed alternately 97 , one above and op- 

 posite the other, around their common axis ; but sometimes, 

 in consequence of the internodes being unequally developed, 

 leaves become opposite 98 or verticillate 103 . They are never 

 produced side by side, except by irregular developement. 



192. In Exogenous plants, the primordial or seed-leaves 

 (cotyledons) are opposite ; hence, in .such plants the supposed 



