40 



COCCOPHYCE.E. 



imperfect birth of the germ cells just described is repeated at the tran- 

 sition to the third, and mostly even to the fourth generation, so that 

 little arborescent groups are produced with twice or thrice-repeated um- 

 bellate ramification, till at length the cells which form the outermost 

 umbellules scatter out their germ cells, which, after a short swarming, 

 fix themselves again to be developed into ramified stocks of new 

 families " -Braun Rejuvenescence, p. 187. 



Plate XV. a, b, young cells ; c, commencement of the first generation 

 of daughter-cells ; d, further progress of the first generation of progeny ; 

 e, second generation being evolved from the first ; /, old plant evolving a 

 third generation X 300, after Braun ; g, zoogonidia. 



Sub-Family 7. PEDIASTRE^I. 



Ccenobium discoid, plane. 



For other features of this sub-family see the characters of the genus, 

 which is the only one at present comprised within it. 



GENUS 29. Pediastrum. Meyen. (1829.) 

 Ccenobium plane, frond-like, discoid, or stellate, free swim- 

 ming, formed of cells in a single, rarely in the centre in a 

 double stratum, continuous, or with the cells here and there 

 interrupted, perforate or clathrate. Cells polygonal, central 

 entire or slightly ernarginate, those of the periphery entire or 

 two-lobed, the lobes wedge-shaped, either simple or two-toothed, 

 sometimes elongated into a horn. Cell-contents green, homo- 

 geneous at first, then granular. Rabli. Alg. Eur. iii. 69. 



Formerly this genus was included in Desmidiacese, but the knowledge 

 of its life history has shown that it has no relationship with the Con- 

 jngatae. Braun illustrated the development of one species (Rejuvene- 

 scence, PL III.), and we have reproduced some of his figures (on PI. XVI.) 

 Fig. 1 is an old disc, in great part emptied by the birth of gouidia. 

 Several of the empty cells exhibit a cross slit, through which the con- 

 tents have been discharged. The order in which this emptying took 

 place is indicated by the letters a, b, c, d, e. One cell is in the act of 

 discharging the gonidia, these having in part entered the projecting por- 

 tion of the hernia-like vesicle, formed by the swollen innermost layer of 

 the membrane of the mother-cell, in part still remaining in the internal 

 cell cavity. Three other cells still possess their perfect contents in 

 different conditions. Two of them are filled by sixteen extremely closely 

 crowded gonidia, only half of which are visible, as they form a double 

 layer. The third unemptied cell is in the actual transition to the forma- 

 tion of gonidia. It exhibits the first division of the contents into two 

 halves, one of which already appears halved again. Fig. 2 is anew-born 

 family immediately after the birth. The innermost layer of the mother- 

 cell has wholly emerged from the old cell, as an extremely thin vesicle, 

 enclosing the gonidia, the gonidia in the interior moving actively. Fig. 

 3 is the same family, as seen from the upper surface. Fig. 4 is the same 

 family, a quarter of an hour after birth. The gonidia, now at rest, have 

 arranged themselves in a plane disc. Fig. 5 is the surface of the same 

 family at the same stage. Fig. 6 the same family one hour after birth. 

 The emargination of the cells has proceeded further. Fig. 7 the same 

 again, but four hours after the gonidia ceased to move. The emargination 

 of the border-cells has passed into the formation of horns. The cells are 



