MODES OP MULTIPLICATION OF DBSMIDIACBA 273 



posed of sixteen cells ; whilst at a is figured a form which is 

 designated as P. tetras, but which may be strongly suspected to 

 be merely a 4-celled variety of b and c. 



168. Many similar cases might be cited ; and the Author would 

 strongly urge those Microscopists who have the requisite time 

 and opportunities, to apply themselves to the determination of 

 the real species of this group, by studying the entire life-history of 

 whatever forms may happen to lie within their reach, noting all 

 the varieties which present themselves among the offsets from 

 any one stock. It must not be forgotten that this process of 

 multiplication is analogous to the propagation of the higher Plants 

 by budding, and to the subsequent separation of the buds, either 

 spontaneously, or by the artificial operations of grafting, layer- 

 ing, &c. ; and just as in all these cases, the particular variety is 

 propagated, whilst only the characters of the species are trans- 

 mitted by the true generative operation to the descendants raised 

 from seed, so does it come to pass that the characters of any par- 

 ticular variety which may arise among these unicellular Plants, 

 are diffused by the process of duplicative subdivision, amongst 

 vast multitudes of so-called individuals. Thus it happens that, 

 as Mr. Ralfs has remarked, " one pool may abound with indi- 

 viduals of Staurastrum dejectum or Arthrodesmus incus, having 

 the mucro curved outwards ; in a neighboring pool, every speci- 

 men may have it curved inwards ; and in another, it may be 

 straight. The cause of the similarity in each pool no doubt is, 

 that all its plants are oflsets from a few primary fronds." Hence 

 the universality of any particular character, in all the plants of 

 one gathering, is by no means sufficient to entitle these to take 

 rank as a distinct species ; since they are, properly speaking, but 

 repetitions of the same form by a process of simple multiplication, 

 really representing, in their entire aggregate, the one plant or 

 tree that grows from a single seed.^ In the genus Ccelastrum, the 

 frond of which, like that of Pediastrum, is composed of clusters 

 of cells, the endochrome subdivides into segments, as if for the 

 formation of zoospores ; but no motion takes place. These seg- 

 ments acquire cellulose coats, and arrange themselves within the 

 parent-cells according to the typical pattern ; and then the wall 

 of the parent-cell splits and peels off, leaving them as the founda- 

 tion of a new cluster (Pringsheim). A somewhat parallel phe- 

 nomenon has been observed and figured in Olosterium by Focke ; 

 the entire endochrome being retracted from the walls, and break- 

 ing up into a number of globules, every one of which acquires a 

 very firm envelope, resembling that of the " resting-spores" or 

 " winter- spores" of many other Protophytes. Probably the fol- 

 lowing observation of Mr. Jenner's refers to a more limited pro- 

 duction of these " resting-spores :" — " In all the Desmidiacese, 



' For a discussion of the question what really constitutes indimduality in this and 

 similar cases, see the Author's " Principles of Comparative Physiology," Chap. XI, Sec- 

 tion 2. — Jm. Ed. 



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