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. The former 

 of these is the apical cell, from the divisions of which all the 

 other cells are derived ; the latter is the last segment cut off 

 from it. 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. Each nodal cell is the sister-cell of the 

 internodal cell next below it, both having arisen by the 

 transverse division of a single segment. Next follows an- 

 other node, composed of more numerous small cells than 

 the first. Some of the peripheral cells of this node are 

 undergoing 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 inter- 

 nodal 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 in- 

 ternodal cell grows and elongates, the overlapping parts of 

 the nodes increase in length and become divided into 

 internodal and nodal cells, which take on a spiral arrange- 

 ment, 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- 



