236 THE ALGAE 



are multiaxial. There is a whole group in which there is ex- 

 tensive Hme encrustation and considerable reduction of the thallus, 

 e.g. Lithothamnion, from the basic fountain type of construction. 

 Definite auxiUary cells are produced on accessory branches prior to 

 fertilization and these serve not only for nutrition but also as start- 

 ing points for the gonimoblast filaments. 



The order divides naturally into two main subdivisions. In the 

 first group the primary gonimoblasts fuse with nutritive cells in the 

 carpogonial branch and secondary gonimoblasts then fuse with the 

 auxiUary cells. In the second group the primary gonimoblasts fuse 

 directly with auxiliary cells. In the first group the Dumontiaceae 

 have the carpogonial branch and the auxihary cell branch widely 

 separated. In the remaining famihes of this group the carpogonial 

 branches are borne in groups in nemathecia. The second group is 

 also subdivided according as to whether the carpogonia and aux- 

 iliary cells are borne on separate branches (Grateloupiaceae) or on 

 adjacent branches. 



Dumontiaceae : Dudresnaya (after Dudresnay de St. Pol-de-Leon). 

 Fig. 131 

 The cylindrical, much branched thallus arises from a prostrate 

 disc. It is soft and gelatinous and consists when young of a simple, 

 articulated, filamentous axis with whorls of dichotomously 

 branched ramuH of Hmited growth. In older plants the axis be- 

 comes polysiphonous and densely beset with whorls of branches. 

 The polysiphonous condition is produced by enlargement of the 

 basal cells of the primary branch whorls and of cortical threads 

 produced from them. The plants are dioecious, the males being 

 somewhat smaller, paler and fewer in number than the females. 

 The carpogonial branches of D. coccinea arise from the lower cells 

 of short side branches and when fully developed are composed of 

 seven to nine cells : they are branched once or twice and may have 

 short sterile side branches arising from the lowest cell. In the middle 

 of the mature carpogonial branch there are two to three larger cells 

 which function in a purely nutritive capacity, whilst the auxiliary 

 cells develop in similar positions on neighbouring branches that are 

 homologous with the carpogonial branches. After fertilization the 

 carpogonium sends down a protuberance containing the diploid 

 nucleus and this cuts off two cells when it is near to the nutrient 

 cells of the carpogonial branch. These all fuse together and sporo- 



