80 



MERISTEMAT1C S YSTEM 



several apical initials. The presence of such an apical cell-group com- 

 prising two or more initials naturally involves a scheme of cell-division 

 which is more complicated than any that is found in connection with 

 a single apical cell. But stress must at once be laid if only on 

 phylogenetic grounds upon the fact that growth by means of a 

 solitary apical cell cannot differ fundamentally from the process in 

 which several initials are involved. It must of course be left to onto- 

 genetic investigators to determine the features which are common to 

 the different forms of apical growth, and to demonstrate the existence 

 of intermediate stages between them. 



Fig. hi. 



Apex of the thallus of Stypopodium utomariiim ; semi-diagrammatic, the cell-rows, 

 which iu reality form a compact tissue, being delineated as separate filaments. 

 After Nageli and Schwcndener (from Nageli and Schwendener, Dos Mikroskop). 



B. GROWTH BY MEANS OF A SINGLE APICAL CELL. 



In a number of Algae, among the Bryophyta and in the majority 

 of Pteridophyta, apical growth is carried out by means of a single 

 apical cell, which at definite intervals cuts off daughter-cells termed 

 segments. The segments by their further division give rise to the 

 many-celled primordial meristematic tissue : in the meantime the apical 

 cell, the volume of which is of course diminished whenever a segment 

 is cut off, grows vigorously until it regains its original size, when it is 

 once more ready to undergo segmentation. 37 



In the simplest cases such as that represented in Fig. 11 the 

 apical cell becomes (Mvided by successive transverse walls (I.-IV.), 

 thereby giving rise to a simple longitudinal row of segments. Each 

 segment soon becomes bisected by another transverse wall (1-4). The 



