EVOLUTION 



261 



Chlorophyceae 



Among the primitive forms a schema such as the one below 

 (modified after Senn) will give an indication of the primary se- 

 quence of events : 



Protomastigineae -^ 

 (colourless) 



Primitive 

 Chrysophyceae 



Yellow, vary'ing types of 

 flagellae and nutrition 



-> Chloromonodineae 

 Yellow, green or 

 colourless. Leucosin 



Mycetozoa Ciliata Fungi / Dinophyceae 



Bacillariophyceae 



Cryptophyceae Xanthophyceae 



brown, /green, blue-green 



Phaeophyceae 



Euglenineae 



Chlorophyceae 



Rhodophyceae 



The origin of the Phaeophyceae and Rhodophyceae has already 

 been discussed in some detail and hence will not be considered 

 further. 



With this scheme as a basis it is now possible to consider 

 evolution among the Chlorophyceae, and here there are two starting 

 points. One hypothesis commences with the Chlamydomonadaceae 

 and branches out with a number of lines of evolution, whereas in 

 the other the Palmellaceae are regarded as the source of the group. 

 The first of these two theories is perhaps the more satisfactory. 

 The origin of the Siphonales offers a similar kind of problem to 

 that of the Fucales because there are very few satisfactory inter- 

 mediate forms. As a result they are illustrated in the schema as 

 having been evolved either directly from an unicellular organism or 

 else from the Siphonocladiales, the latter perhaps forming the more 

 attractive hypothesis. It has recently been emphasized that the 

 nearest affinities, even though distant, of the Siphonocladiales, are 

 with the Siphonales via Valonia and Halicystis on the one hand, 

 and with Ulothrix via Chaetomorpha on the other. 



