2IO 



Cavers : Notes on Yorkshire Bryophvtes. 



Sections taken through the gTowing-point of the thallus 

 show, in the youngest parts, a single layer of air-chambers, each 

 opening on the upper surface by a small pore. In all the other 

 Marchantiaceai found in Britain, this condition of things remains 

 unaltered, that is to say, the thallus shows, even in the oldest 

 parts, only a single layer of air-chambers towards the upper 

 surface, each chamber opening by a pore. In Rehoiilia, how- 

 ever, each of the primary chambers becomes at an early stage 

 divided up in a rather complex manner by means of plates of 

 tissue which grow in from the sides, floor, and roof of the 

 chamber. The upper part of the thallus thus comes to consist 

 of a spongy mass of green tissue, made up of numerous 



Fig- 3' — Part of a transverse section of tl.e thallus, showing- the upper spong-y tissue with 

 two pores, and portion of the lower compact tissue with starch-containing cells. x 150. 



chambers, which vary considerably in form and size (Fig. 2, A., 

 E.; Fig. 3). Along the midrib of the thallus this spongy tissue 

 is seen in sections to become gradually merged below into the 

 compact ventral tissue. In the thin lateral wings of the thallus, 

 the air-chambers are usually reduced to a single series, and here 

 the underlying compact tissue consists of from one to three 

 layers of cells. Except in the neighbourhood of the growing- 

 point, it is not possible to make out the original limits of the 

 primary air-chambers, but the process by which they become 

 partitioned up can easily be followed, especially in serial micro- 

 tome-sections, in the region immediately behind the growing- 

 point. The superposed chambers communicate with each other 



Naturalist, 



