72 



ON THE DESMIDIACE^E — By Dr. Horatio C. Wood, 



JlJN.* 



Of all the fresh water Algse, with the exception of the diatoms, 

 this family has attracted most attention, owing, not only to the 

 beauty and variety of its forms, but also to their universal presence 

 and abundance, and the ease with which their most wonderful life- 

 histories are observed. They are exclusively, as far as known, 

 denizens of fresh water, and preferably that which is pure and 

 limpid. Although Mr. Ralfs states that they never grow in 

 stagnant water, I have often found them in great abundance in 

 such, yet never in that which was actually putrid. The same 

 authority is also too sweeping, at least as far as this country is 

 concerned, in stating they are never found in woods, although they 

 are really most abundant in the open country. My experience has 

 taught me to look for them in brick-ponds, small mountain lakes, 

 springy fens, ditches, and, in the fall, growing among mosses and 

 in the thick jelly composed of unicellular algae on the face of 

 dripping rocks ; or to sum up in a word, they dwell in quiet shallow 

 waters, for I have never found them in rapidly moving or very 

 deep water. 



The single cell of which a Desmid is composed is mostly divided 

 into two very marked similar portions, the exact counterparts one 

 of the other, which by some have been asserted to be distinct cells. 

 Their close union and connection, and their inherent oneness are, 

 however, so apparent that it is needless here to spend time in 

 demonstrating that they really are halves of one individual cell. 

 They contain together all the parts found in the typical vegetable 

 cell ; an outer cellulose wall, chlorophyllous protoplasm, a nucleus, 

 starch granules and semi-liquid contents. The cell wall, or 

 cytioderm, as it is called in this memoir, varies in thickness and 

 firmness. During life it is mostly, if not always, colourless; but 

 in certain species in the dead empty frond is of a reddish yellow. 

 The markings upon it are various, and are not unfrequently al- 

 together absent ; they are such as fine or coarse punctations, 

 granulations of various size, stria?, furrows or elevated ribs, 

 tubercles, obtuse or sharp, simple or forked spines, hair-like 

 processes, umbonations, &c. These markings are within narrow 

 limits constant in each species, and more or less peculiar, so. that 

 they afford valuable characters to the systematist. The cytioderm 

 itself is mostly composed of cellulose, free from appreciable 

 inorganic matters : but in certain species contains a large amount 

 of silex. Thus, according to De Bary, if Closterium lunula be 

 carefully burnt upon a slide, a perfect hyaline silex cast of the 

 cells is left. 



The chlorophyl is variously placed in the cell, sometimes it is 

 arranged in lamina, sometimes in spirals, sometimes in the form of 



* Extracted from " A Contribution to the Natural History of the Fresh Water 

 Algae of North America," 1873. 



