566 MICROSCOPIC FOKMS OF VEGETABLE LIFE THALLOPHYTES 



zob'spores, each of them possessing two flagella at its anterior semi- 

 transparent extremity. Their motile condition, however, does not 

 last long, often giving place to the motionless stage before they have 

 quite freed themselves from the parent-cell ; they then project long 

 angular processes, so as to assume the form of irregular polyhedra, 

 at the same time augmenting in size ; and the endochrome contained 

 within each of these breaks up into a multitude of zoospores, which 

 are at first quite independent and move actively within the cell- 

 cavity, but soon unite into a network that becomes invested with 

 a gelatinous envelope, and speedily increases so much in size as to 

 rupture the containing cell-wall, on escaping from which it presents 

 all the essential characters of a young Hydrodictyon. The rapidity 

 of the growth of this curious organism is not one of the least 

 remarkable parts of its history. The individual cells of which the 

 net is composed, at the time of their emission as zoospores, measure 



FIG. 428. Various phases of development of Pediastrum granulatum. 



no more than ^^ th of an inch in length ; but in the course of a 

 few hours they grow to a length of from ^th to ^rd of an inch. 



The members of the family Pediastreae were formerly included 

 in the Desmidiacece ; but, though doubtless related to them in 

 certain particulars, they present too many points of difference to be 

 properly associated with them. Their chief point of resemblance 

 consists in the firmness of the outer covering, and in the frequent 

 interruption of its margin either by the protrusion of ' horns ' (fig. 

 428, A), or by a notching more or less deep (fig. 429, B) ; but they 

 differ in these two important particulars that the cells are not 

 made up of two symmetrical halves, and that they are always found 

 in aggregation, which is not, except in such genera as Scenedesmus 

 which connect this group with the Desmids, in linear series, but in 

 the form of discoidal fronds. In this tribe we meet with a form of 

 multiplication by motile ' megazoospores ' which reminds us of the 

 formation of the motile spheres of Volvox, and which takes place in 



