238 Evans: Air chambers of Grimaldia fragrans 



bers are seen to be in three or four layers in the thickened median 

 portion of the thallus. As the margins are approached the thallus 

 becomes thinner, and the number of layers decreases until only 

 the uppermost layer is left. Except in this uppermost layer the 

 chambers are usually polygonal in outline and tend to be isodia- 

 metric. In the uppermost layer they tend to be elongated verti- 

 cally, as shown in the spaces a and b. That the spaces communi- 

 cate with one another is also indicated in the figure. The space 

 c, for example, is connected with a space nearer the epidermis, and 

 the space d probably represents a passageway to a chamber in 

 another section. The figure seems, at first sight, to confirm the 

 statements made by M tiller, that both filaments and cell plates 

 are present. Immediately beneath the pore there are apparently 

 three filamentous outgrowths, e,f, and g, and a plate-like outgrowth 

 is clearly shown at //. Of course, as Schiffner intimates, apparent 

 filaments may be nothing more than sections of cell plates. In 

 the section drawn careful focusing does indeed show that e and / 

 are in close contact with another apparent filament in another 

 plane, and the same thing is true of other apparent filaments in 

 the section. Som^ of the cell plates, moreover, appear to have a 

 fluted surface, so that a section cut parallel with the surface of the 

 plate might readily give the impression of a series of filaments. 

 At the same time there are many apparent filaments which seem 

 to be entirely free from one another, and it is impossible to deter- 

 mine their true status except by the study of other sections. It 

 will be noted that the more deeply situated chambers are free or 

 nearly so from outgrowths of any kind . 



The figure is of further interest in showing that some of the 

 apparent filaments and plate-like outgrowths end freely in the 

 chamber without reaching the epidermis, this being especially 

 true in the vicinity of the pores; others, as shown by the one be- 

 tween the spaces a and b, extend to the very epidermis and seem 

 to be connected with it. It is doubtful, however, if the connection 

 is ever anything more than a close contact, such as the free fila- 

 ments in Marchantia and Conocephalum often exhibit. No 

 instance has been observed where an outgrowth extends downward 

 from the epidermis and ends freely in a chamber, and there is no 

 adequate evidence that the epidermal cells themselves ever give 



