DESMIDS. 



Order, ZYGOSPOR^ ; Family, CONJUGATE. 



The DESMIDS form a large group, nearly equal in number of 

 species, to that of all the other orders of fresh-water Algce. They 

 are microscopic plants, and are to be found floating free in pools, 

 ponds and sluggish streams in all parts of the world ; at least 

 representatives of them are to be met with in every clime, from 

 the frigid arctic latitudes to the torrid equatorial zone ; but un- 

 like the higher order of plants this wide difference in temper- 

 ature is not in these Algce always accompanied by corresponding 

 structural difference ; for in New Jersey, varieties have been 

 discovered, which previously were thought to belong exclusively 

 to the hottest parts of South America, and in the same State are 

 also found species peculiar to the region of Nova Zembla, and 

 Spitzbergen ; we may assume these latter to have been a northern 

 legacy to New Jersey upon the breaking up of what is known as 

 the Glacial period, but we have no plausible reason to give for 

 the presence in the same localities of species which are indigenous 

 to Brazil .and the East Indies. In every country, however, there 

 are varieties of Desinids that have not been found elsewhere ; a 

 remark which applies peculiarly to the United States, where 

 nearly one hundred species wholly distinct from any heretofore 

 known, have been discovered, and aie now for the first time col- 

 lectively described in this work. 



The Desmids are all more or less gelatinous ; certain genera 

 as Hyalotheca, Desmidium, Sphaerozosma, and some species of Cos- 

 marium, and of Staurastrium^ have a distinct, wide, colorless en- 

 velope ; but the majority are provided with nothing more than 

 an extremely thin mucous covering, which, though hardly per- 

 ceptible under the microscope, becomes sufficiently evident in 

 the firmness with which the Desmids adhere to the paper on 

 which they are dried. 



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