REJUVENESCENCE IN NATURE. 131 



generation leading from one generation-cycle to another, 

 are alike. As long as mere vegetative multiplication 

 lasts, similar generations are formed ; with the commence- 

 ment of fructification is formed a last generation of cells, 

 different from the preceding, constituting at the same 

 time the first generation of the new cycle, and as such, 

 although originally different, exhibiting in its final develop- 

 ment the same characters as the succeeding vegetative 

 generations. The last generation (that of the germ-cells 

 or spores,) is either imperceptibly or perceptibly different 

 from the foregoing, which circumstance gives room for 

 a series of subordinate gradations . Among the instances of 

 the former condition are the Chroococcacese,* to which are 

 allied (with more intimate connection of the vegetative 

 generations of cells) the Oscillarieae, and a part of the 

 Palmellacese,! to which are related the Hormidia,\ with 

 Prasiola. In all these the propagative cells agree with 

 the vegetative in their origin, and are only distinguished 

 from them by the circumstance of becoming free at last, 

 while the vegetative cells of all or at least of several 

 immediately succeeding generations remain combined 



* Vide Nageli, ' Emzellige Algen,' p. 44, 1. 1 . In Synechococcus, Glteothece 

 and Aphanothece the cells of all the generations become elongated and 

 divided in the same direction, and would, if they did not separate from each 

 other, form filaments, like Oscillaria; in Merismopaedia- the generations are 

 divided alternately in two directions of a plane, whence flat, single-layered 

 plates are formed ; in Chroocowis, Gleeocapsa, and Aphanocapsa the division 

 takes place alternately in three directions of space, whence arise globular or 

 finally shapeless families. Directing our attention to the difference in the 

 position of the axes and the planes of section of the cells, in the last-named 

 cases, we may distinguish in the succession of the otherwise similar genera- 

 tions, subordinate cycles, in one case with two members, in the others with 

 three. There is also another respect in which the succeeding generations 

 are not always exactly alike, the last generations frequently remaining smaller 

 than the earlier, as, for example, is ordinarily the case in Gleeocapsa, where 

 the size of the cells diminishes with the increasing magnitude of the family- 

 stock (phytodom). 



f Ex.gr. Stiyhococcus, (Nag., 1. c., t. iii, G); Hormospora, (Nag., t. iii, n) ; 

 Pleurococcus, (Nag., t. iv, E) ; Glffocystis, (Nag., t. iv, F) ; PalmeUa, (Nag., 

 t. iv, D) ; Porphyridium (Nag., t. iv, il) ; the first two forming rows of cells, 

 the last four solid groups of cells. 



J In the genuine Hormidia, not living in water, I have found no other 

 propagation but a breaking up of the filaments into single cells. Hence 

 Hormidiitm seems to me most closely allied to Stichocorcns and Hormospora. 



