::-M 



BOTANY 



orconnected together by fine threads (Dactylococcus stage, Tig. 239, g). By the longi- 

 tudinal division of the cells of these forms the four-celled Scenedesmus family may 

 again be produced (/, h, i, k). No formation of swarm-spores occurs in this Alga. 

 One of the simplest forms of this order is represented by the genus Chlorella, which 

 multiplies solely by cell division. This genus is particularly interesting also from a 

 biological standpoint, as its small round cells live symbiotically in the plasma of Infu- 

 soria, in the cells of Hydra viridis, Spongilla fluviatilis, and other lower animals. 



Pediastrum (Fig. 240) maybe cited as an example of a genus which gives rise to 

 cell-families. Each cell- family forms a free-swimming plate, composed internally of 

 polygonal cells, and on the margin it consists of cells more or less acutely crenated. 



The formation of asexual swarm-spores is 

 effected in Pediastrum by the division 

 of the contents of a cell into a number (in 

 the case of the species illustrated, P. 

 granulatum, into 16) of naked swarm- 

 spores, each with two cilia. The swarm- 

 spores, on escaping through the ruptured 

 cell wall (Fig. 240, A, I), are enclosed in 

 a common envelope. After first moving 

 vigorously about within this envelope, 

 they eventually collect together and form 

 a new cell-family. Pediastrum possesses 

 also an asexual mode of reproduction. 

 The gametes are all of equal size, and, 

 except that they are smaller and are pro- 

 duced in greater numbers, they are other- 

 wise similar to the swarm-spores. They 

 move freely about in the water, and in 

 conjugating fuse in pairs to form zygotes. 

 The further development of the zygotes 

 into cell-families is not yet fully known. 

 In the spring the cell - families develop 

 from peculiar, thick-walled, spiniferous resting-cells or polyhedha, the contents 

 of which separate into swarm-spores, which escape enclosed in a common envelope, 

 and give rise to a new family. The polyhedra are probably formed from swarm- 

 spores developed in the zygotes. 



The Volvoeaceae include also forms whose cells live either isolated or united into 



Fig. 1240. — Pediastrum granulatum. A 



cell-family : a, cells containing spores ; b, 

 spores in process of extrusion (the other 

 cells have already discharged their spores) ; 

 B, cell-family shortly after extrusion of 

 the spores ; C, cell-family 44. hours later. 

 (After Al. Braon, x 300.) 



Pig. 241.— A, B, Spliaerella pluvialis (X3G0): A, swarming cell; B, formation of swarm-spores. 

 C-G, Spha&rclla Biitsclilli : C, formation of gametes ( x 400) ; D, gamete ; E, conjugation of two 

 gametes ; F, G, zygotes (x800). (C-G after Blochmann.) 



colonies, but which, unlike the types of the Protococcoideae heretofore considered 



