276 University of California Puhlications in Botany [Vol. 8 



of the frond which attach themselves and grow independently; (2) 

 by akinetes, formed from individual cells assuming thick walls, and 

 developing either directly into a filament or a membrane, or indirectly 

 by aplanospores, several in each akinete; (3) by aplanospores, formed 

 4-512 in a cell, by walls in 2 or 3 directions ; sexual reproduction and 

 zoospores unknown. 



Meneghini, Cenni Organo. e Fisiol., 1838, p. 36. Tribe 4, Ulvae 

 (Prasiola) Agardh, Sp. Alg., 1822, p. 416. 



The genus Prasiola was first described as a tribe of the genus 

 TJlva by C. A. Agardh {loc. cit.) and later raised by Meneghini {loc. 

 cit.) to independence. There are three monographs of Prasiola, viz., 

 by Jessen (1848), hy Lagerstedt (1869) and by Imhauser (1889). 

 De-Toni (1899, p. 140 et seq.) enumerates twelve species, of which 

 three are reckoned doubtful. Three of the remaining nine species 

 are marine and the rest terrestrial or inhabitants of fresh waters. 

 Several additional species have been described, one of which is a 

 marine species from our own coast. 



"We find four species along our coast which have claims to be 

 considered marine and there are probably as many more terrestrial 

 or fresh-water species known from western North America. Much 

 more study of seasonal forms and developmental stages is necessary, 

 and the number of aplanospores produced by the aplanosporangia 

 needs more careful determination. The formation of the "aplano- 

 spores" resembles very closely the formation of the male cells or 

 sperms in Porphyra and may, according as the mother cell separates 

 completely or incompletely into two or four at the first two divisions, 

 be a certain number or four times that number as Hus has shown 

 takes place in antheridial formation in Porphyra (cf. Hus, 1902, 

 p. 190). Complete aplanospore formation has been followed in only 

 one species, viz., Prasiola delicata. 



Key to the Species 



1. Cells in distinct areoles, intercellular lines more or less distinct 2 



1. Cells in indistinct areoles, intercellular lines absent or indistinct 3 



2. Frond slender, long, stipitate, stipe of a single row of cells towards the base 



1. P. calophylla (p. 277) 



2. Frond broad, when stipitate, stipe broad, of more than one row of cells 



towards the base 2. P. borealis (p. 277) 



3. Membrane up to 17-20^ thick 3. P. delicata (p. 278) 



3. Membrane up to 40-45m thick 4. P. meridionalis (p. 278) 



