SECT. ii. DESMIDIACESE. 195 



and has been described under the name of the ' swarm- 

 ing of the granules,' from the resemblance of the 

 moving mass to a swarm of bees. Their subsequent 

 history is unknown. 



In the Pediastrum, a plant consisting of a cluster 

 of cells, the zoospores are not emitted separately, but 

 those formed by the subdivision of the endochrome of 

 one cell into 4, 8, 16, 32, or 64 parts, escape from the 

 parent plant still enclosed in the inner tunic of the 

 cell, and it is within this that they develop them- 

 selves into a cluster resembling that in which they 

 originated. 



Mr. Thwaites discovered that the Desmidiacese are also 

 propagated by conjugation, which would be impossible 

 if the hard coat of the adjacent cells about to unite 

 did not split open; then the whole endochrome in 

 one cell passes into and blends with that in the other 

 cell, so as to form one mass, which soon acquires a 

 delicate membranaceous envelope. At first the mass 

 consists of granular green matter, but when the mem- 

 brane becomes thicker, it changes to brown or red. 

 This body, which is called a sporangium, is sometimes 

 smooth, sometimes granular, covered with tubercles 

 or rough with spines, according to the nature of the 

 original plants. The filamental species are propagated 

 by conjugation, but the subsequent history of the pro- 

 duce is still obscure, though there is reason to believe 

 that they give rise to plants of different forms, while 

 all the other modes of increase only reproduce a fac- 

 simile of the parent. 



Desmidiacese exist in America, but their distribution 

 is little known. In Europe, their maximum seems to be 

 in the south of England. They abound in small shallow 

 pools that do not dry up in summer, and also on boggy 

 nioors. The larger kinds are spread out as a thin gela- 

 tinous stratum at the bottom of water, or collected in 



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