OOCARDIUM. 205 



slightly constricted, unequally depressed on the two 

 sides so that the cell is longer on the one side than the 

 other ; lateral view oblong-elliptic, slightly longer than 

 broad, with the faintest indication of a constriction ; 

 vertical view oblong-elliptic, ratio of axes 1 : 1|. Chloro- 

 plasts axile, one in each semicell, concentrated in the 

 front view towards the broader side of the cell, with a 

 central pyrenoid in each chloroplast. Cells embedded in 

 the ultimate ends of branched gelatinous strings, each 

 strand of which is surrounded by a hollow cvlinder of 



/ J 



lime, from the apex of which the cell just protrudes; cell 

 inserted in the tube so that the broader end of the cell, 

 containing the greater part of the chloroplast, is directed 

 towards the periphery, and the narrow end within the 

 tube, with the constriction in a vertical direction. 



Zygospores rectangular or polyhedral, , with several 

 large mamillate projections ; membrane smooth. 



Length 13-20 a ; breadth, 18-24fi, ; thickness 17 a ; 

 cliam. zygosp. (of var. minor with cells 7-8 [i long, 

 12-14 pi wide) 15-20 a. 



ENGLAND. Gordale and Austwick, W. Yorks ! 



IRELAND.- -Leinster Province (Adams). 



Geogr. Distribution.- -France. Germany. Austria. 

 India (form). 



This remarkable Desmid is very rare and only occurs in 

 streams in limestone districts, usually in waterfalls or in swiftly 

 flowing water, where it forms a calcareous deposit on rocks, 

 stones or even twigs. 



Its preference for situations in which an abundance of lime is 

 available is rather peculiar, since the majority of Desmids strictly 

 avoid such localities. 



The gelatinous threads, containing at their extremity the algal 

 cells, become enclosed in cylinders of lime by the metabolic 

 activities of the living cell, whereby carbon dioxide is with- 

 drawn by the algae for the purpose of photosynthesis from 

 the water in which much calcium carbonate is also dissolved. 

 As a result of the removal of this carbon dioxide, the calcium 

 carbonate is no longer as soluble as before, and consequently 

 comes out of solution and is deposited round the algal cells. 

 In order that it shall not thus be completely encased in lime, the 



