EUPHYCOPHYTA I75 



or limiting layer, whilst inside there is first an outer and then an 

 inner cortex and then a mass of interwoven threads that form the 

 central medulla. The young sporeUng is at first a uniseriate fila- 

 ment, but as a result of cell divisions it becomes a flat monostro- 

 matic plate. Rhizoids emerge from the basal cell and the thallus 

 then becomes distromatic, the periclinal divisions that are re- 

 sponsible starting at the basal end. After the holdfast, stipe and 

 blade have been differentiated, further growth is restricted to the 

 intercalary meristem at the base of the blade. When a distromatic 

 lamina has been produced a central group of cells, which form the 

 primary tubes of the medulla, are cut off and separate the two 

 outside layers. Whilst the origin of the medulla in the stipe is clear, 

 it is not so for the lamina. It would seem though that in the lamina 

 the medulla arises as an intrusion of ri^edullary filaments from the 

 stipe. The intercalary growing zone forms a ring around the base of 

 the lamina, the central tissues not being meristematic. 



Later the primary limiting layer in the stipe is replaced by a 

 secondary meristem 4-8 cells deep in the cortex. This is responsible 

 for the subsequent growth in thickness by the production of 

 secondary cortical tissue. This meristem is persistent but its acti- 

 vity is seasonal and as a result in many species distinct growth 

 rings can be seen. 



The medulla consists of (a) primary medullary tubes, and (h) 

 two types of lateral connecting branches, the connecting threads 

 and the hyphae. The former arise first in the course of development 

 as papilloid outgrowths from individual cells. They meet and fuse 

 at their tips and so give rise to a system of cross connexions. With 

 further growth the cells divide and elongate, but even when mature 

 they are composed of relatively few cells. The hyphae, which arise 

 later as branches of small cells cut off from the original vertical cells, 

 remain free but may branch. Ultimately they contain numerous 

 cells which subsequently elongate very considerably. One of the 

 most characteristic features of the genus, and indeed of the order, 

 is the presence in the medulla of 'trumpet-hyphae', which usually 

 arise as modifications of some of the cells in the connecting threads, 

 more rarely of the hyphae. At a transverse cell wall the ends of both 

 cells swell out to form bulbs, the upper bulb always being larger, 

 but so far no satisfactory explanation of this peculiarity has been 

 advanced, though it may be due to purely mechanical requirements. 

 The transverse wall is perforated to form a sieve plate and a callus 



