150 PHAEOPHYCEAE 



germinate to give a creeping basal thallus from which a new plant 

 arises. It is therefore suggested that the plants are wholly diploid 

 and that the haploid generation has been lost. Yendo (1919), 

 however, has reported that these zooids can develop after a resting 

 period into minute protonemal threads bearing antheridia and 

 oogonia which presumably produced gametes, although no sign of 

 fertilization was observed. If these observations are correct this 

 genus must be regarded as anomalous, because normally the 

 gametophytic generation arises from the products of unilocular 

 sporangia. It would therefore seem premature to accept this 

 peculiar life cycle without further evidence, and at present it would 

 be more in agreement with known life cycles if the plants are simply 

 regarded as being wholly diploid and without a haploid generation. 

 In the related genus Scytosiphon it would also seem that only the 

 diploid generation is present and that the reported protonemata are 

 not gametophytic as has been suggested by some workers. 



Dictyosiphonaceae: Dictyosiphon {dictyo, net; siphon, tube). Figs. 



103, 105. 



The filamentous plants arise from small lobed disks and have 

 either a few or many branches, the younger ones commonly being 

 clothed with delicate hairs. 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 plurilocular gametangia. The gametes 

 either develop parthenogenetically into a new protonema or else 

 two of them, coming from different gametangia, will fuse and the 

 zygote develops into a small ectocarpoid-like plant. This may either 

 reproduce itself by means of plurilocular zoosporangia or else it 

 develops into a plantule from which the adult sporophyte arises. 



AsPEROCOCCACEAE : Aspewcoccus (aspero, rough ; coccus, berry). Figs. 



104, 105. 



The structure of the adult plant is essentially the same as that 

 of the two preceding genera except that the central filaments 



