298 



ALG.E 



Order 3. — Pediastre.e. 



The Pediastrese are most nearly allied to the Hydrodictyeae in their 

 mode of reproduction. Of the typical genus Pediastrum Mey., several 

 species are very common in fresh water, whether stagnant or running, 

 attached in the form of minute (usually microscopic) discs to other algae 

 or water-plants, or swimming free. Each disc is of a regular symmetrical 



"<4 



Fig. 261. — PediastrujH integ"2iin Nag. A, younger, B, older coenobe ( x 300). C, portion 

 of older coenobe showing resting-cell, y (x 600). (From, nature.) 



form, usually elliptical, and consists most often of 8, i6, or 32 cells, or 

 some larger number which is probably always, when perfect, a power of 

 two. The coenobe is invested in a ven,^ thin gelatinous envelope, and 

 the peripheral cells have commonly horn-like or crescent-shaped appen- 

 dages. Pediastrum is multiplied either by non-sexual propagation or 



by sexual reproduction. In the former 

 case one of the ordinary vegetative cells 

 becomes a zoosporange, its protoplasm 

 breaking up into a number of nearly 

 globular megazoospores each furnished 

 with two very fine and inconspicuous 

 cilia, which, after swarming about for a 

 time, lose their cilia and arrange them- 

 selves in the form of a plate, which 

 then escapes from the zoosporange in- 

 vested in mucilage, and develops into 

 a new Pediastrum-disc. Others of the cells become gametanges, the 

 contents dividing in the same way into zoogametes of an ovoid or 

 pear-shaped form, which conjugate after escaping separately from the 

 mother-cell, but apparently only with those from other gametanges. The 

 peripheral cells of the ccenobe appear to have a tendency to develop 

 into resting-spores. 



A 



Fig. 262. — A, polj'bedra of Pediastrinn 

 ( ^ 55o)' B, formation of Pediastriini- 

 disc within polyhedra ( x 550). (After 

 Askenasy.) 



