REJUVENESCENCE IN NATURE. 133 



In the cases hitherto examined we could observe, in 

 the series of successive generations of cells, only one 

 different from the rest, the transitional generation, which 

 is at once the last of the old and the first of the new 

 cycle of generations. In the Desmidiaceae, the Zygne- 

 macese, and in Palmogltea, the transitional generation 

 is divided into a double one, since the last generation 

 does not pass directly into the first, but the first 

 generation of the succeeding cycle is produced, as a 

 new structure, in the germination ; so that we have 

 here to distinguish three kinds of generation of 

 cells, the commencing generation, the concluding 

 generation, and the intermediate vegetative generations. 

 In the Desmidiaceae, after a long series of vegetative 

 generations of cells, which sometimes remain connected 

 in rows (Desmidium, Didymoprium^ Hyalotheca), some- 

 times vegetate as completely separate cells (Micrasterias, 

 Euastrum, Closterium, &c.), there occurs a conjugation of 

 two cells and union of their contents in the point of 

 connection, whence is formed one,* or, more rarely (by 

 double conjugation), twot reproductive cells, wholly 

 different from the preceding vegetative generations in the 

 form and aspect of the cells. They are always globular 

 and thick- coated, sometimes smooth, j sometimes rough, 

 like the ancient spiked mace-heads, with simple or many- 

 pointed, forked or hooked spines ; and they do not 

 pass, like the swarming-cells of the Palmellaceae and 

 the reproductive cells of the Diatomaceae, directly and by 

 uninterrupted growth into the primary generation of the 

 new vegetative series, but persist for a long time in a 

 condition of rest, during which, excepting as regards 



* Vide the illustrations in Rolfs' excellent Monograph of the British 

 Desmidiacese, (London, 1848), for instance, t. iii, (Didymoprium) ; t. xvi, 

 (Cosmarium) ; t. xxiv, ( Tetmemorus) ; t. xxv, (Penium) ; t. xxvi, (Docidium); 

 t. xxvii, xxviii, (Closterium); also Nageli, 'Einzell. Algeii.,' t. vii, f. 6, 

 (Cosmarium rupestre). 



t Ralfs, 1. c., t. xxx, (Closterium lineatum). 



% Ralfs, 1. e., t. iv, (Desmidium} ; t. xxv, (Penium) ; t. xxix, (Closterium), &c. 



Ralfs, 1. c., t. vii, (Micrasterias) ; t. xii, (Euastrum) ; t. xvi, (Cosmarium) ; 

 t. \xii, (Phi/casti-urn), &c. 



