369 



formed everywhere from the surface of the thallus. By means 

 of such discs the filaments of the plant, too, fix themselves mutu- 

 ally together to each other. The connection is so intimate that 

 mostly it is impossible to decide from which of the two connected 

 filaments the hapteron is given off. 



If we examine the apex of the filaments (Fig. 361 c) it ap- 

 pears that the thallus increases by means of several filaments placed 



Fig. 361. Wurdemannia selacea Harv. 

 a, transverse section of the thallus. b, longitudinal section of the thallus. 



c, apex of a filament, 

 (a, about 240:1; b, about 130:1; c, about 80:1). 



close together and from which the different cell-tissues are formed. 

 Upon a transverse section of the thallus (Fig. 361 a) three diffe- 

 rent layers are visible. In the middle a medullary tissue is present 

 composed of smaller thick-walled, roundish polygonal cells; from 

 a longitudinal section (Fig. 361 b) it appears that these cells are 

 rather long, cylindrical, but of variable length about 250 jn long 

 at their greatest ; the transverse walls are often somewhat oblique. 

 Then follows a parenchymatic tissue, whose innermost cells are 

 larger gradually diminishing outwards. A transverse section shows 

 these cells to be nearly circular in outline, while a longitudinal 

 section shows them to be actually oblong about 60 |u long. Finally, 

 the epidermal tissue consists of a single layer of cells only; the 

 cells are nearly as long as broad; when seen from above roundish 

 polygonal; their diameter is about 12 |^. 



24 



