356 



BOTANY 



cells of which often live symbiotically in the protoplasts of lower animals (Infusoriae, 

 Hydra, Spongilla, Planariae) ; it is multiplied only by division of the cells into 

 2, 4, or 8 aplanospores which surround themselves with a wall and grow to the 



full size. 



The simplest type of cell-colony, 

 consisting of four cells, is found in the 

 genus Scenedesmus C 27 ), which is 

 widely spread in fresh water, and 

 connects on to Chlorclla. The com- 

 monest form, Sc. acutus, has spindle- 

 shaped cells, while the colonies of Sc. 

 caudatus are distinguished by four long 

 horn-like prolongations of the cell wall 

 (Fig. 287). In reproduction each cell 

 divides in the direction of its length 

 FIG. 286.-CorHa mdgafis. 1, Cell ; 5, 3, division into four daughter -cells, which, on 

 into four aplanospores U, 5, division into .eight { f h u f 



aplanospores. (After GRINTZESCO.) 



new colony. More complicated cell 



colonies are met with in Pediastrum ( M ) (Fig. 288), in which each cell-family 

 forms a free-swimming plate, composed internally of polygonal cells, while on the 

 margin it consists of cells more or less acutely crenated. The formation of asexual 

 swarm-spores is effected in Pediastrum by the divjsion 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 A. 

 ruptured cell wall (Fig. A, b), are enclosed 

 in a common envelope. After first mov- 

 ing vigorously about within this envelope, 

 they eventually collect together and form 



Flo. 287. A, Scenedesmus acutus. Jl, tin- .-.ame, 

 undergoing division; C, Scenedesmus 

 (After SENN, x 1000.) 



38. Pcdiastrum granulatum. A, An old 

 cell-family : a, cells containing sporrs ; i>. 

 spores in process of extrusion (the other 

 cells have already discharged their spon-s) : 

 B, cell-family shortly after extrusion of the 

 spores ; C, cell-family 4J hours later. (After 

 AL. BRAI:N, x 300.) 



a new cell-family. Pediastrum also possesses a sexual mode of reproduction. 

 The gametes are all of equal size, and except that they are smaller and are pro- 

 duced in greater numbers, are 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. 



The life-history of the Water-net (Hydrodictyon utriculatum ( l - 29 ) is essentially 



