VaucJieriaceas 



249 



Species of Vaucheria are widely distributed in temperate and tropical countries, 

 occurring more especially in streams, cataracts, and boggy springs, where there is 

 abundant aeration. The most ubiquitous species is undoubtedly V. sessilis (Vauch.) DC. 

 Some species occur in brackish waters and some in the sea, but the majority live in fresh 

 water or on damp ground. The terrestrial and some of the aquatic species show con- 

 siderable variation, and there appears to be experimental evidence in support of the 

 suggestion that V. terrestris and V. geminata are adaptational forms of the same Alga 

 (Desroche, '10). The largest species of Vaucheria is V. dichotoma, in which the filaments 

 attain a diameter of more than 200 p.. This species is also dioecious. Dichotomosiphon 

 occurs in fresh water in Europe and North America. 



Bohlin ('01), and following him, Blackman & Tansley ('02), transferred the family 

 Vaucheriacese to the Heterokontse, but the reasons for this change appear to be quite 



Fig. 161. Dichotomosiphon tuberosus (A. Br.) Ernst. A and B, part of thallus showing tubers 

 (t) ; A, x33; B, x 30. C, part of thallus with oogouium and antheridia, x 33. D, oogonia 

 and antheridia, x about 70. oo, oogonium ; an, antheridium. (A, C, and D, after Ernst; 

 B, after Virieux.) 



inadequate. There is but one Heterokontan character in the Vaucheriacese, and that is 

 the storage of oil instead of starch. Moreover, this occurs only in the genus Vaucheria, 

 the closely allied genus Dichotomosiphon storing starch. The intense green colour of 

 Vaucheria in the mass is never exhibited by any of the Heterokontae, whereas such a 

 depth of colour is quite common among the Siphonales ; and the equal length of the 

 paired cilia of the ' synzoospore ' is also against the inclusion of the Vaucheriacese in the 

 Heterokontse. The nature of the thallus and of the chloroplasts points to a very close 

 relationship with other families of the Siphonales, such as the Bryopsidacese and Codiacese. 

 The genera are : Vaucheria DC., 1803 [inclus. Woronina Solms, 1867] ; Dichotomo- 

 siphon Ernst, 1902. 



