200 Trunks and Props. 



ference of anatomical structure, and in all a very essential physi- 

 ological dissimilarity. This portion is called the neck or col- 

 lum, and has been thought by some to be the seat of vegetable 

 vitality, which is a very erroneous idea. At first it is a space 

 that we have no difficulty in distinguishing, so long as the em- 

 bryo or young plant has not undergone any considerable changes ; 

 but in process of time it is externally obliterated, so that in a tree 

 of a few years' growth its existence becomes a matter of theory, 

 instead of being actually evident to our senses. Immediately 

 consequent upon the growth of a plant is the formation of leaves. 

 The point of the stem whence these arise, is called the nodus ; 

 and the space between two nodi an internodium. In inter- 

 nodia the arrangement of the vascular and fibrous tissue, of what- 

 ever nature it may be, of which they are composed, is nearly 

 parallel, or at least experiences no horizontal interruption. At 

 the nodi, on the contrary, vessels are sent off horizontally into the 

 leaf; the general development of the axis is momentarily inter- 

 rupted while this horizontal communication is effecting, and all 

 the tissue is more or less contracted. In many plants this con- 

 traction, though it always exists, is scarcely appreciable ; but in 

 others it takes place in so remarkable a degree as to give their 

 stems a peculiar character, as for instance in the Bamboo, in 

 which it causes diaphragms that continue to grow and harden, 

 notwithstanding the powerfully rapid horizontal distension to 

 which the stems of the plant are subject. Ail the divisions of a 

 stem are called branches ; an assemblage of these, such as forms 

 the head of a forest tree, is called a coma. Some branches are 

 imperfectly formed, lose their power of distension, become un- 

 usually hard, and acquire a sharp point ; they are called spines. 

 Fig. 4, A, simple spines ; B, a triple spine ; occasionally, as in 

 the white thorn, they bear leaves. In domesticated plants they 

 entirely disappear, as in the apple and pear, the wild varieties of 

 which are spiny, and the cultivated ones spineless. These are 

 distinguishable by their woody vascular centre from prickles, with 

 which they must not be confounded. Prickles are rigid, opaque, 

 conical processes, formed of masses of cellular tissues, and ter- 

 minating in an acute point. They may be not improperly con- 

 sidered as very compound indurated hairs ; they have no con- 

 nection with the woody fibre, and are found upon all parts of a 

 plant, except the stipules and stamens; rarely they are found 



