CHAKACE.E; DESMIDIACE^E 579 



1 ransvcrse septa into from 100 to 200 small disc-shaped cells, which 

 number, therefore, from 20.000 to 40,000 in each antherid. In 

 every one of these cells there is formed, by a, gradual change in its 

 contents (the successive stages of which are seen at D, E, F), an 

 antherozoid, a spiral thread of protoplasm consisting of two or three 

 coils, which, at first motionless, after a time begins to move and 

 revolve within the cell, and at last the cell-wall gives way, and the 

 spiral thread makes its escape (G), partially straightens itself, and 

 moves actively through the water for some time (H) in a- tolerably 

 determinate direction, by the lashing action of two long and very 

 delicate cilia with which it is furnished. The exterior of the nucule 

 (A. ]>) is formed by five or ten spirally twisted tubes that give it a 

 very peculiar aspect ; and these enclose a central sac containing 

 protoplasm, oil, and starch grains. Each of these tubes consists, in 

 its lower part, of a very long unsegmented cell; while at its upper 

 part two small cells are segmented off; and these small cells of all 

 the tubes form together the crown' of the nucule. When ready 

 for fertilisation the branches of the crown part slightly, forming an 

 open passage or 'neck ' down to the central germ-cell or oosphere ; 

 and through this canal the antherozoids make their way down to 

 pc i -form the act of fertilisation by becoming absorbed into the 

 substance of the oosphere. Ultimately the nucule, which has now 

 become a hard black body, falls off, and the fertilised germ-cell, or 

 obspore, gives origin to a new plant after the nucule has remained 



dormant through the winter. 1 



. 



Among those simple Alga? whose generative process consists in 

 the 'conjugation' of two similar cells, there are two groups of such 

 peculiar interest to the microscopist as to need a special notice; 

 these are the DesmtdiacecK and the Diatomacece. Both of them 

 were ranked by Ehrenberg and .some other naturalists as animal- 

 cules ; but the fuller knowledge of their life-history and the more 

 extended acquaintance with the parallel histories of other simple 

 forms of vegetation which have been gained during the last twenty 

 years, are now generally accepted as decisive of their vegetable 

 nature. 



The Desmidiacese 2 are minute plants of a bright green colour 

 growing in fresh water ; generally speaking, the cells are inde- 

 pendent of each other (figs. 436-439) ; hut sometimes those which 



1 A full account of the CJinracctf will lie found in Prof. Sachs's Text-Bonk <>j' 

 Botiuty, '2nd English edition, p. '2'.l'2. Various observers have asserted that particles 

 of the protoplasmic contents of the cells of the Characew, when set free by the 

 rupture of their cells, may continue to live, move, and grow as independent rhizopods. 

 But the writer is disposed to think that the phenomena thus represented are rather 

 to be regarded as cases of parasitism, the decaying cells of Nitclln having been found 

 by Cienkowski (Beitrfigf zur Kenntniss tier Monaden,in Arch.f. Mikr.Anat. Bd. i. 

 18(3"), p. '208) to be inhabited by minute, spindle-shaped, ciliated bodies, which seem 

 to correspond with the ' spores ' of the Myxomycetes,goiog through an ameboid stage, 

 and then producing a plasinode which, after undergoing a sort of encysting process, 

 finally breaks up into spindle-shaped particles resembling those found in the Niti'lhi 

 cells. 



- Our first accurate knowledge of this group dates from the publication of Mr. 

 Ralfs's admirable monograph of the British Desmids in 1.S4.S. Later information in 

 regard to it will be found iu the section contributed by Mr. W. Archer to the fourth 

 edition of Pritchard's Infu.wrin, and in Cooke's British Desiiiidy, 1HM7. 



r p 2 



