340 DESMIDEJS. 



multiplication of cells; and while it effects the increase of 

 the individuals of a species, does not provide against the ex- 

 tinction of that species when it shall have reached the ter- 

 mination of the brief existence allowed to it. Moreover, the 

 cells resulting from the bisection of other primary cells have 

 no periods of juvenescence and growth ; they are produced 

 at once fully developed and perfect in size and organization. 

 It is the nature, on the contrary, of a true reproduction, that 

 the bodies or organs by which it is effected should be at first 

 minute, and subsequently pass through successive stages of 

 devclopement. The second method is assuredly the usual and 

 legitimate mode of reproduction, viz. that by bodies analogous 

 to zoospores, while in the third the organs resulting from the 

 union of two individuals are probably to be regarded in the 

 same light in which Agardh viewed the similar bodies of the 

 Conjugates, viz. as receptacles in which the zoospores are 

 stored, and destined, as I think, not for immediate but for 

 future use, that they are in fact hibernacula, designed to 

 preserve the contained propagules until the vicissitudes and 

 rigour of winter shall have passed away. 



The formation of sporangia has been noticed to occur in 

 nearly all the genera of the family of Desmidea, and it is 

 probable that it occasionally occurs in all of them. By 

 Ehrenberg it has been noticed in different species of the 

 genus Closterium, by Brebisson in Desmidium, by Ralfs in 

 his genus Tetmemorus, and in Staurastrum. In the Cylin- 

 drocystis Brebissoni, a production placed by Meneghini 

 amongst the Nostochinece, but which seems to me to belong 

 to Desmidece, union of the cells has been observed, and it is 

 most probable that this union is followed by the formation of 

 sporangia. 



So much for the reproduction of the Desmidea: a few 

 words may be added upon their growth. It has already 

 been stated in the definition of the family that certain species 

 of Desmidece are filamentous. The filaments of these in- 

 crease in length though not in number by the continual 

 division of the cells, as do other filamentous Alga; but 

 in the Desmidea, which are not formed of filaments, the 



