SPONGIOLES. 63 



than there is now demand for. Such buds are, therefore, merely adventitious. 

 On this account it would seem that those roots, commonly so called, which do 

 naturally and uniformly produce buds, are with propriety described by modern 

 writers as subterranean stems; as the root-stalk of the sweet flag (Calamus), the 

 bidb of the tuUp, or the tuber of the potato. 



141. The summit of the root, or that part which connects it to 

 the ascending axis, is designated as the collum, or neck. 



a. Strictly speaking, this is the only stationary part of the plant. Occupying 

 the centre of motion between the ascending and descending axis, every enlarge- 

 ment that takes place upon its upper surface arises into the air, while all below it 

 descends into the earth. 



142. The parts of the root which require especial notice, are 

 the caudex, fibrils, and spongioles. 



a. The caudex (stock) is the main body of the root. 



h. The FIBRILS are the finer branches of the root, sent off 

 from the caudex. These are the true roots. 



c. The SPONGIOLES are the tender and delicate extremities of 

 the fibrils; and, since the latter lengthen only by accretions 

 made to these extremities, these are their growing points. 



143. The form of the root is much diversified in different 

 plants, but the principal varieties which have received distinct- 

 ive names, are the following : — 



144. i2<2mo5e (branching). This root consists of ramifications 

 sent off' from the main root, like the branches of a tree, but in 

 no determinate order. Such are the roots of most trees and 

 shrubs. (Fig. 20.) 



a. There is a strong analogy betvveen the roots of a tree and its branches. In 

 many instances they may be made to perform, each the functions of the other; 

 that is, the tree may be inverted, and the branches ^vill become roots and the roots 

 put fortli leaves like the branches. The willow and the maple may be thus 

 inverted without injuring their vitality. 



b. A branch may often be made to put forth roots instead of leaves. If a 

 brancli (offset) of the willow or currant (Kibes) be inserted into the ground, cither 

 by the lower or the upper end, or by both at once, it >vill take root and flourish. 

 Other trees, as tlie mulbeiTy (Moms) may be multiplied by layers. A branch is 

 bent and inserted into the ground by the apex. When it lias taken root it is 

 severed from the parent stock, and becomes a perfect tree. 



c. Tlie roots of a tree extend in all directions, and to v^onsiderable distances. 

 This distance is at least equal to the extent of the branches, and often much 

 greater. Tliose of the elm embrace an area of 300 feet diameter, of the poplar, 



G* 



