REJUVENESCENCE IN NATURE. 125 



a new cycle ; in which, consequently, the whole vegetative 

 life is run through in the same cell where the propagation 

 also finally appears. But in a wider sense, there are 

 other Unicellular plants, if, namely, we regard those as 

 one-celled, in which the conclusion and renovation of the 

 cycle by the formation of a germ does not occur until 

 after several generations of cells, yet the vegetative life is 

 passed through, not in a rigidly combined series of cells, 

 but in separate cells.* These cases are of two kinds. 

 In cases of one kind the vegetative life is really completely 

 shut up in one cell, but the formation of germ-cells in 

 this cell is attained, not directly, but through the medi- 

 ation of one or more transitory generations of cells.f 

 Such plants are truly Unicellular, as far as regards vege- 

 tation,! since the transitory cells neither possess a special 

 and pecuhar vegetative development, nor contribute to 

 the formation of a uniform vegetative tissue, but appear 

 only as a rapidly over-passed, transitional stage previous 

 to the formation of germ-cells, — as germ-cells, as it were, 

 which divide once or more times before they arrive at the 

 point from whence a new vegetative cycle can begin 

 again with germination. § In the other cases the multipli- 



* Unicellular plants occur in the series of Fungi and Algse, which have 

 very varied correspondence in morphological respects. In regard to the 

 latter I refer to Nageli's most recent work (' Gattungen einzelliger Algen,' 

 1849), in which is laid a new and solid foundation for the knowledge of these 

 plants, so interesting in this respect, but hitherto (with the exception of the 

 JDiatomaceEB and Desmidiacese), from want of observation on the development, 

 only most superficially known, and heaped together in the most chaotic 

 manner in the Systems of Algae. 



I Vide Nageli, 'Einzelliger Algen,' p. 28. 

 In systematic respects, also, the unicellular genera are most closely 

 connected, since many genera with transitory cell-formation in the passage 

 to spore-formation exhibit great agreement with those without this (except- 

 ing in this one character). Thus Cystococcus, Nag. (1. c, p. 84, t. 3, e), 

 with transitory cell-formation, corresponds with the genus Protococcns (as 

 limited by Nageli), with immediate or simultaneous spore-formation; Chara- 

 oiiim, A. Br. (Nag., 1. c, p. 86, t. 3, d), perfectly corresponds to the genus 

 Ascidium, A. Br. ; Pediasirum, Meyen, in many respects to the genus 

 Hydrodictyon, Roth. 



§ This case is repeated in multicellular Algse, Ulothrix, JJlva, Porphp-a, 

 (Nag. ' Algensyst.,' t. 1, f. 61, 62), Eciocarpm, (ibid., t. 2, figs. 3, 4, 5); 

 similar cases occur even among the Pucoidese, since, according to Decaisne 



