Evans: Air chambers of Grimaldia fragrans 



249 



vertical, the growth in the latter direction being often equalled 

 by the upward growth (accompanied by cell division) of the cells 

 forming the floors of the chambers; these in tuin remain more or 

 less united with one another and with the cells of the partitions 

 and in this way form the system of united cell-plates in the dorsal 

 chambers. At a later stage the margins of some of the plates 

 which end freely in the chambers give rise to teeth as shown in 

 Fig. 2. 



It is difficult to secure direct evidence from the vegetative 

 thallus that the partitions form surface-outgrowths. Fig. 14, 

 however, which is drawn from a sec- 

 tion of the young female receptacle, 

 shows that such outgrowths are pos- 

 sible. The section was cut parallel 

 with the upper surface of the recep- 

 tacle, and the figure shows two com- 

 plete chambers and parts of eight 

 others; two of the latter contain sec- 

 tions of the tubular epidermal pores 

 which hang down from the roofs of 

 the chambers. The partitions are one 

 cell thick but give the impression of 

 being thicker when cut obliquely. 

 The outgrowths originate as projec- 

 tions of cells which become cut off by 

 walls and then continue their growth 

 and cell-divisions. In the vegetative 



thallus such outgrowths evidently play a very minor part in the 

 development of the green tissue. 



The chambers below the dorsal layer make their appearance 

 very early, as seen in FiGS. 12 and 13, although they always appear 

 later than the dorsal chambers. As the thallus becomes differen- 

 tiated, these chambers increase rapidly in size through the growth 

 of the bounding cells, but the appearance of new chambers, except 

 in the apical region, has not been demonstrated and seems im- 

 probable. If schizogenous processes play a part in the enlarge- 

 ment of these chambers, it is only to a very limited extent. 



The green tissue in the thallus of Plagiochasma bears a strong 



Fig. 14. Section parallel with 

 the surface of a young female re- 

 ceptacle, just below the epidermis, 

 X 270. 



