293 



above quoted description of FALKENBERG, I shall here only men- 

 tion briefly the specimens found (Fig. 290). 



What mostly characterizes this genus is the dorsiventrally 

 built, creeping main filaments, from which the exogenous branches 

 are formed alternately in pairs on both sides; of these branches 

 the lower one becomes a short, unbranched branchlet, while the 

 other, the upper, becomes ramified in a similar way as the mother 

 filament. 



Dipterosiphonia dendritica has 5 pericentral cells which arc 

 mostly arranged in rather distinct rows. Of these 5 cells in each 

 segment, the two are placed on the somewhat flattened, lower, 





Fig. 291. Dipterosiphonia dendritica (Ag.) Falkenb. 



o, summit of a filament seen from above, b, transverse section of the thallus. 



(About 180:1). 



ventral side, while the three others are found on the upper dor- 

 sal side (Fig. 291 b). 



The main stem is about 115 // thick. From the ventral cells 

 vigorous short rhizoids issue; these end in irregularly lobed discs 

 by means of which the plant is fixed to the host plant (see Fig. 290). 



Of the branches placed in pairs the one is, as mentioned 

 above, developed like the main stem. It is ramified in the same 

 way but is generally less vigorous. Its branches are commonly 

 unbranched and it stops in most cases its longitudinal growth 

 after having developed 5 6 pairs of branches; but occasionally 

 under favourable conditions the branches grow out to real main 

 stems like the mother filament (Compare Fig. 290). 



The short branchlets remain undivided. Sometimes they are 

 short or quite rudimentary, sometimes they grow to a consider- 

 able length. They are about 60 , thick. A few trichoblasts were 

 found upon the short branchlets. 



By reason of the greatly varying development of the branches 

 and branchlets their original arrangement in pairs is often much 



