Pithophoracece 107 



growth is apical. The thallus is almost always branched, the 

 branches arising a little below the top of their supporting cells, 

 and it is attached below by well-developed haptera put out from a 

 basal cell. It is doubtful if there is sufficient evidence to warrant 

 the separation of the Pithophoracese as a distinct family from the 

 Cladophoracese. 



Asexual resting-spores are produced here and there in the 

 thallus, either intercalary and cask-shaped, or terminal and ovoidal 

 or fusiform. These spores, which were termed by Wittrock 'agamo- 

 spores,' are richly filled with chlorophyll and the spore-wall increases 

 considerably in thickness. Formation of spores may take place in 

 all the cells of the cauloid part of the thallus. After a short period 

 of rest the spores develop in opposite directions from the two 

 apices. There is another method of asexual reproduction by ' pro- 

 lific cells.' 



Genus Pithophora Wittr., 1877. This striking genus of Algse 

 is almost exclusively tropical in its distribution. It is the only 

 representative of the family, and few Algas are more characteristic 

 than a plant of Pithophora with ripe spores. 



P. (Edogonia (Mont.) Wittr., var. polyspora Rendle and West f. 1 (fig. 41) 

 may be regarded as a British Alga, having been found in the Reddish Canal, 

 near Manchester. P. Keivensis Wittr. occurred in a tank in the water-lily 

 house, Kew Gardens, but was most probably introduced from tropical 

 S. America. 



Family 3. SPH^ROPLEACE^. 



This family includes only one Alga, which occurs in extensive 

 masses on flooded plains and by the margins of lakes. The 

 filaments are cylindrical and imbranched, and consist of single 

 series of coenocytes which reach an extraordinary length as com- 

 pared with their breadth. The transverse cell-walls often become 

 of great thickness, and each segment of the filament contains a 

 considerable number of small nuclei. The chloroplasts, which arc 

 in the form of parietal rings, are very numerous and some of them 

 contain pyrenoids. 



The sexual organs are oogonia and antheridia, which may be 

 formed without change of shape from any segment of the filament. 

 Sometimes the oogonia and antheridia alternate in a filament, but 



1 A. B. Eendle and W. West, jtmr. , 'A New British Freshwater Alga,' Journ. 

 Bot. July, 1899, pp. 289291, t. 399. 



