16 GENERAL HISTORY OF THE INFUSORIA. 



is probably the dissolved and swollen-iip internal cell-coat of the spore, 

 which holds the young individuals combined for some time after the outer 

 coat of the spore has been throvrn off." 



Although Braun has, in the preceding account, made use of the teiTn ''spore'* 

 to express the conjugation-product, yet, in the veiy admission that, in those 

 Desmidieae in which only we have any clue to the subsequent history, it 

 produces, not a single indi\idual, as does a spore commonly so called, but a 

 multitude, he essentially agrees with Mr. Ralfs, who prefers to call the body 

 a capsule. We may quote Mrs. Thomas in support of the same view ; for she 

 considers the s]X)rangium a capsule, or {T. M. S. 1855, pp. 36, 37) " the 

 winter casing of a large nimiber of j^oung plants which escape fi'om it by 

 rapidly knocking against its walls, when these have been loosened by spring- 

 warmth, or which grow up as the waUs gradually decay in the midst of slimy 

 gelatinous masses." In proof of this oj)inion this lady appeals to the immense 

 increase in the number of plants seen in the spring beyond what can be ex- 

 plained as the result of self-fission. 



In her opinion the sporangium is a capsule (I. 8, 9) filled with zoospores 

 similar to those moving granules, supposed to be such, seen mthin the fuU- 

 grown plant, capable, when their fitting time comes, of filling the waters 

 with their countless progeny. 



In these accounts there is a pervading harmony ; and the truth seems to 

 be that, by the formation of a sporangium, provision is made for the per- 

 petuation of the species through the "wdnter, when the large majority, at 

 least of adult plants, have ceased to exist. The phenomenon is clearly 

 analogous to that of the foimation of seeds by herbaceous plants, or of ova 

 by insects and other animals, when the cycle of existence of the parent being 

 is complete, or is put an end to by unfavoiu-able external circumstances. 



Braun has expressed the sequence in the phases of existence in the follow- 

 ing technical language {R.S. p. 133): '' In the Desmidiaceae, the Zygnemaceae, 

 and in Pahnoglcea, the transitional generation is divided into a double one, 

 since the last generation does not pass directly into the fii^st, but the first 

 generation of the succeeding cycle is produced as a new structure in the ger- 

 mination ; so that we have here to distinguish three kinds of generation of 

 cells, — the commencing generation, the concluding generation, and the 

 intermediate vegetative generations." The last-named is represented by the 

 process of self-fission, which takes place in the perfect plant, and is con- 

 tinued through a long series of individuals. 



Between its firet appearance and its ultimate development, the sporangium 

 of Desmidieae undergoes a progressive series of changes ; at first it is pale 

 and homogeneous, but soon gets granular, acquires a gradually deepening 

 green colour, and presents vesicles and globules in large number. The enve- 

 lope, at first very dehcate, augments in thickness, and becomes lined by 

 others, whilst its surface either remains smooth or becomes granular, tuber- 

 culated, or spinous, and the spines themselves in many instances forked or 

 branched (II. 15, 22, 25, 30, 34). Simultaneously with these changes the 

 integument increases in density, and together with its processes acquires 

 considerable firmness and toughness. Moreover, as it advances in age it 

 usually assumes a reddish-brown colour; when this has happened, the 

 sporangium and contents may be presumed to have reached maturity. 



Mrs. Thomas (op. cit. p. 35) thinks she encountered a mature sporangium 

 of Cosinarium margaritiferum in the shape of a many- coated ball filled with 

 granules in the same rapid motion as observed in the full-grown Cosmarium 

 (I. 10, 11). " The similarity of the movement (she says) attracted my 

 attention; and I also saw that in one part the enclosing membrane appeared 



