CHAP. II. 
STEM. 
57 
the growing 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 
considerable 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 concretions of hardened cellular tissue. Spines 
occasionally, as in the Whitethorn, 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 modifications 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 a 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 embryo 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 
