1 ULSll-VVATEK ALUJi OF THE UMTKD STATK8. 101 



libene vel in fascia* filifrrnies nut tseniiformps arete conjunrtie aut in mnco matrirali nidulantcs et 

 in latnilius iudcliuitus coii.MH-iaiu.-. Cvtimleriua non silici-um, plus minus fmauiii, lieve ant varie 

 a.<|'iuiuin (striutum, costatum, ai-uleuiiiin, \c.). Mausa cbloropuyllacea iu lauiiuulfs axiles vel 

 parictulcs, .-a'pe e crutro railiantc.-, di>tribulu. 



1'rupugatio nun sexuulis JUT divisioue tranaverea in eaudcm directionem rcpetiu ; sexualis per 

 cygosporati, qua; per ccllularum binaruiu conjugaliouem uriuutar. 



I'nicellular alga-, without 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 tttiiiforiu fascia, or involved in the maternal jelly so as to form indefinite fami- 

 lies. Cytiixli-rm not silii -eous, more or less firm, smooth, or variously roughened (striatc, costate, 

 aculeate, <kc.) Chlurophyl iua*ses in axillary or parietal lamina, which often radiate from the 

 centre. 



Non-sexual propagation by repented transverse division in one direction; sexual by zygosprires 

 which are formed \>y the coiijugalioii of two cells. 



Remark*. Of all the fresh-water alga?, 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 

 fur as known, denizens of fresh-water, and preferably that which is pure and limpid. 

 Although Mr. Rail's 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- 

 ]><>n<ls, 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 drip- 

 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 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 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 arc not infrequently altogether absent ; they are such as fine or coarse 

 pnnrtations, 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 in certain species contains a large amount of silex. Thus, 



