30 GENEEAL HISTORT OF THE rN'FUSOEIA. 



common envelope, 'witliin whicli they moved actively about for a quarter of 

 an hour before coming to a state of rest, and arranging themselves regularly 

 in a frond (I. 64). This sac is described in the author's work on Rejuvenes- 

 cence {Bay Soc. p. 184) as the vesicular inner layer of the mother-cell. 

 He witnessed this production of gonidia from many, but not from all the 

 cells of the fronds, for it seems to take place in them in succession, and pro- 

 bably in some definite manner, according to theii' position in the frond. He 

 further describes the development of microgonidia to follow the same plan as 

 that of macrogonidia, but to differ from the latter in nimiber, size and form, 

 and in duration and cessation of motion. Macrogonidia have a subglobose 

 figure, a diameter of y^th of a millimetre ; one side hyaline, and scarcely 

 elongated, the other tui^ned towards the periphery of the frond, green, and 

 by-and-by extended and emarginate : no vibratile ciHa discoverable on them. 

 They never leave the sac in which they are produced ; and the yoimg fi'ond is 

 seen, until the close of the second day, loosely enveloped by a gelatinous 

 layer, which ultimately disappears by deliquescence (I. 64). 



On the other hand, microgonidia are at fii'st densely aggregated and 

 closely invested by the sac in which they are generated ; like macrogonidia 

 they are subglobose (I. 60, 61, 68, 69). After a while the sac is gradually 

 dilated, and, growing more and more in length, forms an acute hyaline beak 

 {rostrum) as long as the green portion, which constitutes the rest, or the 

 body, of the gonidium. This rostrum, moreover, is furnished either with a 

 pair of cUia, longer than the body, or with a single cilium. The length of 

 these developed microgonidia is nearly y^-o-th millimetre ; the thickness -g-J-o th 

 (I. 61, 68, 69). The movement within the sac is at first slow; but when 

 this is fully expanded, it is very active, and continued for an hour and 

 upwards, until the sac is ruptured, and the Avhole heap of microgonidia escape. 

 The number of microgonidia cannot, by reason of their aggregation and their 

 swarming movement, be easily determined ; at least 64 occur in a sac, and 

 most commonly many more, for instance, 128. Of theii' subsequent history, 

 Braun can give no satisfactory account. 



A reference to the same able writer's book on Eejuvenescence {R. S.) 

 informs us, at p. 200, that before the formation of the gonidia of Pediastrum, 

 the single starch-grain, the nuclear character of which has been above re- 

 marked, disappears. From the same source we also obtain a series of illus- 

 trations of the development of macrogonidia, of their arrangement in the 

 characteristic stellate frondose form, and of the varieties in the number and 

 arrangement of the component cells, which may be seen in examples of the 

 same species of Pediastrum. 



The development both of Coelastrum and Sorastrum is unknown. 



The Pediastrese are of freshwater habit, living in ponds, on which they 

 frequently form, in conjimction with other small plants, a coloured film or 

 scum. They are also common in tm-fy pools on moors, and invest the sui'face 

 of various aquatic plants. 



Systematic Position op Pediastreje. — Mr. Ealfs followed Ehrenberg, 

 Meneghini, and others in placing the Pediastrese among Desmidiese ; but 

 Corda, Nageli, and Braim have separated the two as distinct tribes. Indeed, 

 Mr. Ralfs has modified his views since the publication of his monograph, and 

 would treat the Pediastreae as a subfamily of Desmidiese. Nageli {Einzell. Alg) 

 arranges them with the PalmeUese as a distinct group, and in this has the 

 support of Braun {Gen. Nova, p. 69). These natui'alists point out that the 

 distinctive features between the Pediastreoe and the Desmidiese, are that the 

 former neither conjugate nor miiltij)ly by continued transverse division of their 

 cells in the same direction, each newly-formed segment acquiring aU the 



