Ulotrichales 281 



Letts ('13) has found a remarkable amount of nitrogen in specimens of P. crispa, 

 amounting in some instances to 8'94 per cent, of the dried material. This percentage is 

 almost double that which is found in ordinary vegetable substances, higher than that 

 contained in dried cheese, and almost as high as that contained in dried meat. 



Order 6. ULOTRICHALES. 



This order, which is identical with the ' Chaetophorales ' of Wille, includes 

 a relatively large number of Green Algae which are mostly inhabitants of 

 fresh waters. The thallus is filamentous, sometimes simple, but more 

 frequently branched. The branches are generally attenuated and often 

 piliferous. The cells are uninucleate (except in the peculiar genus Wit- 

 trockiella), and in all the families of the order, with the exception of the 

 Trentepohliacese, they possess a single parietal chloroplast with one or more 

 pyrenoids. 



The only families in which simple, unbranched filaments occur are the 

 Ulotrichacese, Microsporaceae and Cylindrocapsaceae ; in fact, only one-fifth 

 of the known genera are unbranched types. The majority of the unbranched 

 forms are free-floating, whereas practically all the branched forms are attached, 

 many of them being epiphytes. In the latter the branches are sometimes 

 concrescent on the surface of the host, so that discoidal growths are formed, 

 and the cells are frequently furnished with setae or bristles. 



Reproduction by biciliated, or more rarely quadriciliated, zoogonidia is 

 general throughout the order, although it has not yet been observed in 

 every genus. Aplanospores or akinetes, or both, are known in most genera. 

 Isoplanogametes occur in very many of the genera, and in the Cylindro- 

 capsaceae, Aphanochaetaceae and Coleochaetacese there are well-differentiated 

 egg-cells and antherozoids, Coleocheete being, as regards its sexual reproduction, 

 probably the highest type among the Green Algae. 



A few genera (Arthroch&te, Endophyton} are endophytic in larger red and 

 brown sea-weeds, and others have epizootic (Trichophilus) and endozootic 

 (Endoderma) representatives. Tellamia perforates the shells of marine and 

 freshwater bivalves, and some species of Cephaleuros are destructive parasites 

 on phanerogams. 



The Ulotrichales are a natural and extensive group which appear to 

 have been derived from certain of the Tetrasporine Protococcales by the 

 development of the filamentous and ultimately of the branched habit. 

 Most of them are aquatic, but the Trentepohliacese are a conspicuous 

 family of subaerial Algae. The Microsporacese and Trentepohliaceae are 

 rather anomalous groups, the peculiarities of the latter family being partly 

 the result of complete adaptation to subaerial conditions. 



