REPRODUCTION 255 



(b) Some of the primitive forms, e.g. Lithoderma and Nemo- 

 derma, have tv^^o equal generations in the Hfe cycle and a similar 

 state of affairs is also found among other brown algae, e.g. Dtctyota, 

 Zanardinia, 



{c) The ultimate decision must obviously be largely determined 

 by the condition of affairs found in the sources from which the 

 Phaeophyceae arose. On general grounds it is to be supposed that 

 the Phaeophyceae all arose from one common ancestor, but it must 

 not be forgotten that the group may have had a polyphyletic 

 origin, although at present there is hardly any evidence in support of 

 such a view. Two possible sources of origin for the Phaeophyceae 

 have been suggested in the past. One is that they arose, as did the 

 Chlorophyceae, from a flagellate ancestry with intermediate forms 

 such as Phaeococciis and Phaeothamnion (cf. p. 123). On the basis 

 of their pigments the Chrysophyceae (cf. p. 122) show a close 

 resemblance to the more primitive Phaeophyceae and this is not 

 without significance. If this theory is correct, one must almost 

 certainly consider that the primitive species, as in the primitive 

 Chlorophyceae, were wholly haploid and that the diploid state has 

 been interpolated subsequently. The other hypothesis is that they 

 arose from some member of the Chlorophyceae, probably among 

 the Chaetophoraceae. This latter group is characterized by 

 heterotrichy, a feature which is possessed by some of the primitive 

 Phaeophyceae, whilst another point in favour of this view is the 

 lack of any satisfactory existing series between the few known 

 phaeophycean-like flagellates and the primitive filamentous 

 Ectocarpales. If we accept an origin of the Phaeophyceae from the 

 Chlorophyceae, two possible sources may be suggested : 



{a) From a member of the Chaetophoraceae which possessed the 

 heterotrichous habit and two morphologically similar or nearly 

 similar generations. 



{h) From a member of the Siphonocladiales which had a life 

 cycle with two equal generations, such as is now shown by Chaeto- 

 morpha or Cladophora Suhriana, 



It is tempting to consider whether the Phaeophyceae have not 

 been derived from a form such as Trentepohlia, and it is much to 

 be regretted that at present the life cycle of Trentepohlia, so far as 

 cytological details are concerned, is wholly unknown. Until we 



