Prasiolacece 



99 



in form. The cell-wall is strong, rigid and hyaline. The single 

 chloroplast is central, star-shaped, and contains one pyrenoid. 

 Sometimes the thallus is fixed by rhizoids and sometimes not. In 

 the broader, flat expansions the cell-walls are thick and confluent, 



Fig. 36. A C, Prasiola pariettna (Vauch.) Wille, from Bradford, W. Yorkshire 

 ( x 500). I) G, Prasiola crispa (Lightf.) Menegh. ; D, examples from Bradford, 

 W. Yorks. (nat. size); E, simple filament from Helvellyn, Westmoreland; 



F, portion of irregular filament from Wimbledon Common, Surrey ( x 500) ; 



G, basal portion of broader thallus, from Bradford, W. Yorks. (x400). 



and the cells have the appearance of being separated by consider- 

 able spaces. Reproduction is by a process of gemmation, by 

 akinetes liberated at the margins of the thallus, and by tetraspores 1 . 

 Gay and Chodat separate the genera Schizogonium and Prasiola, 

 but I am inclined to agree with Wille 2 in uniting them under 

 Prasiola Ag. 



Wille and Bb'rgesen 3 have each described some interesting 

 marine forms of this genus in which the plants are more amply 

 supplied with rhizoids. The expanded thallus of these forms does 

 not reach such a large size as the thallus of the land forms. 



Two species are abundant, P. crispa (Lightf.) Menegh. [which includes 

 Hormidium murale Kiitz. ; Schizogonium crispum (L.) Gay ; and Ulothrix 

 radicans Kiitz.], the cells of which are 7 14 p in diameter (fig. 36 D G), 

 and P. parietina (Vauch.) Wille [which includes Schizogonium murale Kiitz. 

 and Hormidium parietinum Kiitz.] with cells 9 18 p in diameter (fig. 36 



1 Lagerheim, ' Ueber die Fortpflanzung von Prasiola,' Ber. Deutsch. Botan. 

 Gesellsch. 1892, Bd x, Heft 7. 



2 Wille, ' Studien iiber Chloropkyceen I- VII,' Vidensk. Skrifter, I math.- 

 naturv. Klasse, 1900, no. 6, p. 13. 



3 Borgesen, ' Marine Alg. of the Faeroes,' Bot. of Faeroes, Part II, 1902. 



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