74 
ORGANOGRAPHY. 
BOOK I. 
and to consist of a highly excitable mass of cellular substance, 
originating in the pith, and having a special power of exten- 
sion in length. Under ordinary circumstances, the growing 
point clothes itself with leaves as it advances, and then it be- 
comes 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 super- 
ficial concretions of hardened cellular tissue. Spines occa- 
sionally, as in the Whitethorn, bear leaves ; in domesticated 
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 parls; and 
distinguished 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 Jixed 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 cellular 
tissue only : the leaf-bud is produced without fertilization, to 
the embryo this is essential : finally, the leaf-bud perpetuates 
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 
