CHAP. II. STEM. 57 



the gi'owing point clothes itself with leaves as it advances, and 

 then it becomes a branch ; but sometimes it simply hardens as 

 it grows, and forms a sharp conical projection called a spine^ 

 as in the Gleditschia, the Sloe, &c. 



The spine must not be confounded with the prickle or 

 aculeus already described, from which it differs in having a 

 considei'able quantity of woody tissue in its structure, and in 

 being as much in communication with the central parts of a 

 stem as branches themselves ; while prickles are merely su- 

 perficial conci'etions of hardened cellular tissue. Spines 

 occasionally, as in the Wliitethorn, bear leaves; in domesti- 

 cated plants they often entirely disappear, as in the Apple and 

 Pear, the wild varieties of which are spiny, and the cultivated 

 ones spineless. 



We ought to consider the spadix of the Arum, and several 

 forms of disk hereafter to be described, as moditications of the 

 growing point of the bud, and consequently as analogous to 

 spines. 



Linnaeus called the bud Hyhernaculum, because it serves for 

 the winter protection of the young and tender parts ; and dis- 

 tinguished it into the Gemma, or leaf-bud of the stem, and the 

 Bulb, or leaf-bud of the root. 



The leaf-bud has been compared by Du Petit Thouars and 

 some other botanists to the embryo, and has even been deno- 

 minated d, fixed embryo. This comparison must not, however, 

 be understood to indicate any positive identity between these 

 two parts in structure, but merely an analogous function, both 

 being formed for the purpose of reproduction ; but in origin 

 and structure they are entirely different. The leaf-bud con- 

 sists of both vascular and cellular tissue, the embryo of cellu- 

 lar tissue only : the leaf-bud is produced without fertilization, 

 to the embryo this is essential : finally, the leaf-bud perpetu- 

 ates the individual, the embr5fo continues the species. 



The usual, or normal, situation of leaf-buds is in the axil of 

 leaves ; and all departure from this position is either irregular 

 or accidental. Botanists give them the name of regular when 

 they are placed in their normal station, and they call all 

 others latent or adventitious. The latter have been found in 

 almost every part of plants ; the roots, the internodes, the 



