APICAL (.'ELLS 



81 



appearance of longitudinal walls in the segment-halves leads, first, to 



the formation of quadrants, and, later, to the differentiation of peripheral 



and central cells, which in their turn become subdivided by further 



transverse walls. In this way each segment becomes converted, 



without undergoing any appreciable growth, 



into a multicellular structure, which soon 



passes out of the meristematic condition 



and becomes a part of the permanent tissue. 



The branches originate at an early stage 



arising indeed as lateral outgrowths of 



the apical cell and behave in precisely 



the same manner as the main axis. 



Apical cells of this kind, which carry 



out all their divisions in a single plane, 



are found in certain genera of Algae 



such as Sphacdaria, Chaetopteris, Clado- 



stephus, Stypocaulon, etc. 



More usually the apical cell divides 

 by oblique walls and in several planes ; 

 as a result it comes to be more or 

 less deeply embedded in the rest of the 



primordial meristematic tissue. This scheme of segmentation natur- 

 ally admits of considerable variation in detail. In some Liverworts 



Fig. 11. 



Branch of the thallus of Sphacelaria 

 scoparia, bearing lateral branches ; v, 

 apical cell. 



Fig. 12. 



A. Apex of the stem of Symphyogyna rhizoloba, as seen in longitudinal section; 

 .i, apical cell. B. Apex of a young stem of S. sinuata, as seen from above. After 

 Leitgeb. 



(Metzgeria, Aneura, Symphyogyna), in the stems of Salvinia and 

 Azolla, and in certain other instances, the segment-walls arise 

 alternately right and left of the median plane, so that two rows 

 of segments result. The accompanying figures (Fig. 1'2 A and B) 

 illustrate the behaviour of such a " two-sided " apical cell, as seen in 

 longitudinal section and in surface view respectively. Among the 



F 



