231) DESCRIPTION ov \_Polygasirira, 



eitlier to arise from an unhealthy condition of the plant, or else to 

 be parasitic. 



" All the Be&midiecb are gelatinous. In some the mucus is con- 

 densed into a distinct and well defined hyaline sheath or covering, 

 as in Bidymoprium GrevilUi, and Staurasfrum tumidum ; in others 

 it is more attenuated, and the fact that it forms a covering, is dis- 

 cerned only by its preventing the contact of the coloured cells. In 

 general, its quantity is merely sufficient to hold the fronds together 

 in a kind of filmy cloud, which is dispersed by the slightest touch. 

 Wlien they are left exposed by the evaporation of the water, this 

 mucus becomes denser, and is apparently secreted in larger quantities 

 to protect them from the effects of drought. 



" Besides the movement of the granules spoken of, a circulation, 

 or rather a rotation of the contents of the cells of Besmidiea may 

 occasionally be seen under a careftil adjustment of the microscope," 

 (See p. 180.) Mr. Ralfs has witnessed it in Clostermm Lunula, and 

 in Penium Digitus. He says, " It seems, at least in the Closterium, 

 to be restricted to the space between the mass of endochrome and the 

 integument ; for neither Mr. Bowerbank nor myself could detect it 

 in the internal parts of the endochrome. The movement, however, 

 extends over the whole surface of the endochrome. The circulation 

 being canied on between the integument, and the mass of endochrome, 

 which is usually brought full into view by the observer, explains the 

 difficult}^ experienced in detecting the circulation, except at the 

 margins. The motion was very irregular : the fluid flowed at one 

 time towards the extremities, and at another in the opposite direction, 

 and the intervals between these changes were of uncertain duration. 

 Streams,-^ also, though apparently not separated by any partitions, 

 flowed side by side in contraiy directions. The currents evidently 

 consisted of an homogeneous fluid ; but, from time to time, minute 

 granules were detached from the internal mass of endochrome, and 

 carried along in the streams for short periods, after which they either 

 returned to the quiescent portion, or passed into other ciuTonts. It 

 seems to deserve particular notice, that the circulation was not 

 interrupted at the suture. The process in Penium Digitus was some- 

 what different from that just described." 



Nageli believes in the presence of a nucleus in the Desmidiece, 



