Valoniacete 251 



by the increasing septation of the thallus. The first steps of such a line of 

 evolution are seen in the irregular walls which appear in the thallus of 

 Derbesia and in the walls which sometimes cut off a few small outgrowths of 

 the ' foliar shoots ' of Caulerpa. The formation of the plug-like septa in the 

 Codiacese is also a tendency in the same direction. The line of evolution 

 suggested by Oltmanns ('04) in which the Siphonocladiales are derived from 

 uninucleate groups such as the Ulotrichacere by repeated division of the 

 nucleus, rather than from the Siphonales by the septation of the thallus, is 

 an improbable hypothesis which would make the Siphonocladiales a more 

 primitive group than the Siphonales. Such a view ignores the siphonaceous 

 tendency exhibited by some of the Protococcales and also the significance 

 which should possibly be attached to a comparative study of Protosiphon and 

 a few genera of the Chlorochytriea3 (vide p. 223). 



Family Valoniacese. 



This is much the largest family of the Siphonocladiales, embracing a 

 number of exclusively marine genera which attain their greatest development 

 in tropical seas. 



The thallus is at first siphonaceous and of a very simple character, con- 

 sisting of a large simple or branched coenocyte which is attached to the 

 substratum by a branched system of rhizoids. In the more primitive forms 

 of the Valoniea3 the thallus develops very little beyond this stage, but in 

 the Siphonocladese, Boodlese and Anadyomenese there is a complex septate 

 thallus which is frequently differentiated into ' stalk ' and ' frond.' In Dictyo- 

 sphierm the thallus is very compact and almost globular. The segments 

 of the thallus are often firmly linked together by numerous holdfasts or 

 haptera. 



The chloroplasts are in the form of angular discs, disposed parietally in 

 more or less anastomosing chains to form an open network. 



Reproduction by zoogonidia is known to occur in several genera, and 

 aplanospores are found in Petrosiphon. 



Halicystis is removed to the Protosiphonacese (vide p. 224). 



Borgesen ('13), as the result of his extensive West Indian investigations, discriminates 

 between four sub-families of the Valoniaceae. 



Sub-family VALONIE.E. In this, the most primitive division of the 

 Valoniacese, the thallus consists at first, and in Valonia ventricosa always, of 

 a single large sac-like coenocyte which typically becomes branched. Septa 

 soon appear, cutting off small lenticular, multinucleate, peripheral portions of 

 the protoplasm (fig. 162 a, a). In Valonia these lenticular segments may 

 remain small or they may grow out and form segments as large as the parent- 



