552 AQUATIC PHYCOMYCETES 



culum (2-3 (j. diameter) which often remains adherent after dehiscence; 

 there is a short richly branched internal rhizoidal system. Sporangium 

 developed by direct enlargement of the zoospore. Resting spore [sub- 

 spherical to oval] (7-8 x 6.5-7.5 [jl) with a thick, smooth wall and con- 

 taining several refractive globules; formed by the fusion of two thalli 

 through a conjugation tube (to 10 u. long). Empty male thallus (2.5-4 x 

 4-4.5 tx). Rhizoidal system of male and female thallus identical with 

 that of sporangia" (Canter, in Canter and Lund, loc. cit.). 



On Synedra acus var. angustissima, Canter and Lund (1953:34), 

 Switzerland, Italy; ? Asterionella formosa (loc. cit.), Great Britain; 

 Synedra sp., Paterson (1956: 276), (Michigan), United States. 



Eighty per cent of the Swiss diatoms were infected and 50 per cent of 

 the Italian. 



Paterson's studies of this and the preceding species led him to conclude 

 that there were no striking morphological differences between the two. 

 The smaller sizes of the parts of the present species, recorded by Canter, 

 might well be due, he argues, to measuring preserved material. He found 

 the sizes of all parts in living material of both species to be essentially 

 the same. 



Zygorhizidium parvum Canter 



Ann. Bot. London (N.S.), 14: 287, figs. 14-15, PI. 8 C. 1950 



(Fig. 11 C-E, p. 110) 



"Thallus monocentric, eucarpic, consisting of a sporangium develop- 

 ed from the body of an encysted zoospore and the whole or a part of 

 the original germ tube. Sporangia pear-shaped, cylindrical, or oval, 

 varying in size from 5 to 13 jjl high by 2.5 to 8 u, broad. Zoospores from 

 4 to 40 fully formed within the sporangium, emerging after the detach- 

 ment of a lid. Zoospores, 2 \x in diameter, posteriorly uniflagellate, with 

 an anterior lateral globu)e and a minute shining granule. Rhizoid a 

 short, unbranched thread. Resting spore sexually formed, male thallus 

 (resembling an encysted zoospore and its germ-tube) makes contact with 

 a slightly larger female thallus by means of a conjugation tube which 

 arises as a lateral branch from the germ-tube of the male. Resting spore 

 spherical, 6-8 \i in diameter, or elongate-oval, 5-15 [jl x 3.5-8 u.. Wall 



