Fig. 91 Dictyosiphon. A, plant. B, longitudinal section of thallus. 



C, sporangium and thallus cells in surface view. D, gametophyte. 



(A, After Oltmannsj B,C, after Murbeck; D, after Sauvageau.) 



four apical growing cells at the tip. There is a central medulla of 

 four rows of large elongated cells which in parts may be penetrated 

 by hyphae from the smaller cortical cells. In old plants the axial 

 cells may rupture and separate so that the axis becomes partially 

 hollow. There is a central medulla of large elongated cells and a 

 cortex of small cells, but in old plants the medulla is often ruptured 

 and the axis becomes partially hollow. On the macroscopic plants 

 only unilocular sporangia are found, each of which is formed from 

 a single subcortical cell. Meiosis takes place in these sporangia and 

 the zooids germinate to form microscopic prothalli : these represent 

 the gametophytic generation and reproduce by means of pluri- 

 locular gametangia. The gametes either develop parthenogene- 

 tically into a new protonema or else two of them coming from 

 different gametangia, will fuse and the zygote develops into a small 

 ectocarpoid plant. This may either reproduce itself by means of 

 plurilocular zoosporangia or else it develops into a plantule from 

 which the adult sporophyte arises. In D. chordaria meiosis appears 



164 



