OF THE DESMIDIE^. ' 15 



jugation-cell, in the strongly expanded isthmus of this. The delicate internal 

 membrane, mth the contents enclosed by it, draT\ang itself out of the extre- 

 mities of the double cell, forms a seed- cell, at first cniciate, four-lobed, then 

 bluntly quadi^angular, and finally globular, clothed by a many-layered 

 thickened membrane "within the persistent four-horned conjugation-cell. 

 From Ralfs's representation, this is most probably the way in which the pro- 

 cess is to be understood in Cylindrocystis (Penium) Brehissonii.''^ 



The second mode, when the union of the isolated cells is also lateral and 

 parallel, is exemplified in CJosterium Lunula, in which, according to Morren's 

 express statement, thi-ee different membranes take part in ^ the formation of 

 the canal of union, — an inner and an outer cell-membrane, and a membrane 

 (the primordial utricle) immediately enclosing the green mass. The glo- 

 bular reproductive cell formed in the connecting canal is an active gonidium, 

 which begins to revolve even while in the canal, and soon breaks through the 

 gelatinously-swollen membrane of the latter. Very often two approximated 

 individuals divide again and conjugate before they have completely separated, 

 whence result conjugated double pairs. 



The third scheme of conjugation, the most widely extended, is itself 

 reduced by Braun to two principal secondaiy varieties, and to several sub- 

 sidiary ones. Thus conjugation takes place either in a parallel position or in 

 a crossed (decussate) manner. The former is peculiar to the Closterina ; the 

 latter is met with in Euastrum and allied forms, and also in many genera 

 formerly united with Desmidium. The modifications, in various species, of 

 these plans are well explained in Braun's work, to which we would refer 

 for particulars, as well as for an elucidation of the production of a " really 

 double spore (not two-lobed, as Ealfs terms it) " in Closterium lineatum. 



The next question which presents itself is, whether the product of con- 

 jugation is to be esteemed a spore or a spore-case, i. e. a sporanyium. That 

 the latter is its natiu-e appears pretty clear, and is assumed as a fact by Mr. 

 Ralfs. This authority observes: "The sporangia I consider capsules, and 

 this view seems to be confirmed by the experience of Mr. Jenner, who states 

 that the coveiing of the sporangium swells, and a mucus is secreted, in 

 which minute fronds ai)pear, and by their increase at length rupture the 

 attenuated covering." In this opinion Siebold coincides ; and the Rev. W, 

 Smith {A. N. H. 1850, p. 4) represents, on the authority of Mr. Jenner, the 

 biu-sting of a sporangium of Closterium acerosum, and the development of 

 young fronds from its contents. 



Braun, in his philosophical treatise (oj). cit. B. S. p. 133), remarks of the 

 products of conjugation in the Desmidieae, that " they do not pass, Hke the 

 swarming-cells of the PalmeUaceae and the reproductive cells of the Dia- 

 tomaceae, directly and by uninterrupted growth into the primaiy generation 

 of the new vegetative series, but persist for a long time in a condition of rest, 

 during Avhich, excepting as regards imperceptible internal processes, they 

 remain wholly unchanged. To distinguish these from the direct germ- cells 

 (gonidia), I shaU call them seed-cells (spores). The development of these 

 spores has not yet been observed ; but it may be assumed as certain, that 

 they do not pass as such into the primary generation, but produce this at the 

 period of germination, by an internal transformation of their contents, and 

 bring these to light as a new generation with a dehiscence of the old en- 

 velope. Certain early conditions observed in Closterium and Euastrum, namely 

 fanuhes of unusually small individuals, enclosed in transparent colourless 

 vesicles, render it even probable that in certain genera of Desmidie«, a 

 number of individuals are produced from one spore, by a formation of transi- 

 toiy generations occiUTing already within the spore. The enclosing vesicle 



