442 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



and 20 ju. This may be taken as their greatest diameter, 

 for after this they would he reckoned part of the conduct- 

 ing tissue. 



The assimilating cells vary very little in diameter, being 

 about 4-6 x 3-4/x near the apex, and rarely rising above 

 8 x 3-4/x in the lowest regions just above the basal disc. 

 Their longest diameter is generally parallel to the longi- 

 tudinal axis of the whole row of cells. 



The increase in thickness of the lower regions of the 

 shoot is brought about not by the addition of thick layers 

 of assimilating tissue, as is the case with Phyllopliora 

 Brodioei. The assimilating layer in Chondrus is 20-25/x 

 deep in a flat frond of about 350/x in thickness, but in a 

 frond which was 840/x thick, the thickness of the assimi- 

 lating layer was only 25-30/x. The increase in thickness 

 is in fact due to the assimilating cell rows forming new 

 cells at their tips, whilst their inner cells gradually pass 

 into the collecting cells, and these gradually pass into the 

 conducting cells. The increase in thickness is noticeable 

 in the central tissue only to any extent. It is taking place 

 here at the expense of the outer layers, which are, how- 

 ever, continually being renewed by the formation of new 

 cells at the tips of the rows of assimilating cells. 



It is probable that a good deal of sliding of cells occurs 

 as the growth in length takes place. The increase in 

 length is probably caused not by the central cells actively 

 growing in length, but by their being drawn out passively 

 during the active lateral extension of the assimilating 

 layer. But frequent longitudinal slits have failed to indi- 

 cate in what way tension is distributed in the tissues. 



The central tissue is very well separated by the 

 filamentous nature of its constituents in the younger parts 

 of the shoot, but in older parts it assumes more and more 

 a pseudoparenchymatous appearance. By this change the 



