﻿432 ELEMENTARY BIOLOGY. [CHAP. 



If the innermost part of the terminal bud, which con- 

 stitutes the free end of the axis, or stem, be examined, it 

 will be found to be formed by a single nucleated cell, 

 separated by a transverse septum from another. Beneath 

 this last follows another cell, which has already undergone 

 division into several smaller cells by the development of 

 longitudinal septa. This is the most newly-formed node. 

 Below this again is a single cell, which is both longer and 

 broader than those at the apex, and is an internodal cell. 

 Below it follows another node, composed of more numerous 

 small cells than in the first. Some of the peripheral cells 

 of this node are undergrowing growth and division, and thus 

 give rise to cellular prominences, which are rudiments of 

 the first whorl of leaves. In the still lower parts of the 

 stem the internodal cells get longer and longer, but they 

 never divide. The nodal cells, on the other hand, multiply 

 by division, but do not greatly elongate. From the first, 

 the nodal cells overlap the internodal cell, so as to meet 

 round its equator, and thus completely invest it externally. 

 And, as the internodal cell grows and elongates, the overlap- 

 ping parts of the nodes increase in length and become divided 

 into internodal and nodal cells, which take on a spiral 

 arrangement, and thus give rise to the cortical layer. 



Thus the whole plant is composed of an aggregation of 

 simple cells, just as is the case with Hydra among animals; 

 and, while it lives, new nodes and internodes are continually 

 being added at its summit, or growing point. The inter- 

 nodal cells which give rise to the middle of the stem un- 

 dergo no important change, except great increase of size, 

 after they are once formed. The nodal cells, on the con- 

 trary, undergo division with comparatively little increase in 

 size. And out of them, the nodes, the cortical layer, and 

 all the appendages, are developed. 



