rKESH-WATER ALG^ OF THE UNITED STATES. 101 



libersB vel in fascias filifcrmes aut tseniiformes arete conjunctae aut in muco matricali nidulaiites et 

 in lamilias indeliuitas cousociatie. Cytioderma non siliceuin, plus minus firmum, liuve aut varie 

 asperatum (striatum, costatum, aculeatum, &c.). Massa uldorophjllacea iu laminales axiles vel 

 parietales, sajpe e centre radiaiiles, distributa. 



Propagatio non sexualis per divi.sione transversa iu eaudeiu directionem repetita ; sexualis per 

 zygosporas, quse per cellularura Ijiuaruui coujugationem oriuutur. 



Unicellular algaj, wilbout branches or terminal growth. Cells of very various forms, mostly more 

 or less profoundly constricted in the middle and divided into two symmetrical semicells, free or con- 

 joined in filiform or taaniform fascia, or involved in the maternal jelly so as to form indefinite fami- 

 lies. Cytioderm not siliceous, more or less firm, smooth, or variously roughened (striate, costate 

 aculeate, &c.) C'hloropliyl masses iu axillary or parietal lamina, which often radiate from the 

 centre. 



Non-sexual propagation by repeated transverse division in one direction ; sexual by zygospores 

 which are formed by the conjugation of two cells. 



Remarhs. — 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 case 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. 

 Altliough 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 dri[> 

 ping 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 counterj)arts 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 semiliquid 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, colorless ; but in certain 

 species in the dead empty frond is of a reddish-yellow. The markings upon it are 

 various, and are not infrequently altogether absent; they are such as fine or coarse 

 punctations, granulations of various size, striae, furrows or elevated ribs, tuber- 

 cles, obtuse or sharp simple or forked spines, hair-like processes, umbonations, &c. 

 &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 inor- 

 ganic matters, but iu certain species contains a large amount of silex. Thus, 



