GROWTH, ETC., IX THE DESMIDIACE^. 295 



of Xan. tetracentrotum, WoUe. Compare Cushman, Rhodora, Yp\. 

 7, No. 84, 1905. On PI. 64, figs. 8, 9, 10 show plainly the develop- 

 ment of Xan. tetracentrotum (fig. 8) into fig. 9, and thence into Xan. 

 Bengalicum, Tm-n., f. 10. With these figures cf. Lagerheim, 

 " Amer. Desm.," figs. 18, 19, 20. 24, which all belong to the same 

 species. Fig. 18, Ar. incrassatiis, Lag., is Xan. tetracentrotum, WoUe, 

 and fig. 24 is the same with two other pairs of spines developing. 

 When this form is grown, what difference will there be between it 

 and Xan. hastiferum var. Javanicum (Nord.), Turn. ? Note also 

 W^ & G. S. West, North Amer. Desm., PI. 15, f. 24. 



Life-histories. — The investigation of the life-histories of the 

 Desmidiaceae is urgently needed. Turner as long ago as 1885 in 

 " Some New and Rare Desmids," observed that " the life-histories 

 of all but a very small number are yet unknown, the latter remark 

 applying to very common forms." This is well within the mark 

 even to-day ; it would be interesting to know the names of " the 

 very small number." As a matter of fact not a single life-history 

 has yet been elucidated. It will not be supposed, of course, that 

 the life-history of a Desmid species is of precisely similar nature to 

 that, say, of one of the higher cryptogamic species. In its progress 

 to maturity every specimen of the latter passes through the same 

 well-defined series of stages. But the Desmid is the sport of the 

 conflicting forces of cell-division and cell-development, and its 

 life-history is the outcome of the resultant of these forces in any 

 particular locality. In other habitats the balance of these forces 

 is different ; the hfe-history, therefore, varies, and other forms of 

 the species result. The life-history of a Desmid species, then, is 

 the history of the degeneration and development of an ideal cell 

 which is supposed to pass through all the stages (polymorphic 

 forms) which are comprised within the species, arranged in proper 

 biological sequence as far as it can be ascertained. The cell must 

 be an ideal one, as owing to the peculiarities of Desmid life and the 

 exigences of microscopical examination it is impossible to trace 

 the process in any single individual. The biological sequence, 

 however, is correct, and the life-history (or histories, for there are 

 generally many alternative lines) a real one, although compounded 

 from many individuals. 



It is hardly necessary to make afty remarks about the maturity 

 or immaturity of specimens. As soon as it is recognised that fully 

 90 per cent, of the forms one meets with either in nature or in 

 print are growth forms of the other ten, it must be abundantly 

 evident that, though a semi-cell may be termed mature when it 

 resembles its mate, the Desmid-cell itself is not entitled to be called 

 so until it has acquired its fully-developed form. 



Biological or Sciagraphical. — In the preceding paragraphs 

 I have in most cases used the word " sub-species " in the sense 

 in which I suppose it is generally used. Under a system of classi- 

 fication by outline the only meaning it can properly have is, it 

 seems to me, " a distinctly new shape manifesting itself among 



