2o6 



BOTANICAL GAZETTE 



[SEPTEMBER 



history of any segment and the enlargement of the intercellular spaces 

 proceeds much more slowly. Moreover, the structure of the thallus 



makes it easier 

 to secure sections 

 through the apical 

 region, so that one 

 may readily ob- 



serve 



the 



ongm 



and follow the de- 

 velopment of the 

 air chamber. It 



probable 



seems 



Fig. 4. — Riccia natans. Nearly mature air chambers, 

 superficial and internal^ primary and secondary. 



from our observa- 

 tions that the time 

 and place of the 

 first splittin 



or 



is 



somewhat variable, depending upon the relation between cell division 

 and enlargement. 



The normal position of the cleft is either in the center of a segment 



of the apical cell near the surface or in the 



center of one or another of its primary sur- 

 face segments 

 formed by a 

 further dvision. 

 Apericlinal divi- 

 sion of the seg- 

 ment may cut it 

 into a superficial 

 and an internal 

 cell. This super- 

 ficial cell is often- 



^c--*- +u^ ^ 4.U Fig- ^.—Marchantia poly- 



est the mother , ^ . , ^^. r <, 



morpha: a, apical cell, /, 2, 

 cell of the cham- ^^ air chambers. 



Fig. 6.~Marchantia poly- ber(/fo-.p). Then 



morpha: a, apical cell; 2, 3, 

 air chambers. 



two anticlinal walls, ^ ^, and another, , 

 in the plane of the drawing, di\^de this 

 "mother cell" into four, a dextral and sinistral, an anterior and 





t 





