ZOOSPORES. 221 



include the greater part of the green algae of our ponds 

 and streams. Those which have no chlorophyll are sapro- 

 phytes, and live upon dead organic matter. They are doubt- 

 less to be regarded as modified forms of some of the types 

 of the chlorophyll-bearing portion of the group. 



I. CLASS ZOOSPOREJS. 



202. This class is a somewhat doubtful one ; it is com- 

 posed of plants which, while differing in many other re- 

 spects, agree in having locomotive sexual cells (zoospores). 

 In this they agree, however, with the Volvocinece, and bear 

 a close resemblance to Protococcus and its allies. It is prob- 

 able that a fuller knowledge of some of the plants of this 

 class will result in their being distributed elsewhere. 



The general structure of the plants referred to this class 

 may be understood from the examples which follow. No at- 

 tempt will be made here to indicate the orders to which 

 they belong. 



293. Pandorina is a unicellular alga, which is united into 

 colonies (called cwnobia), which swim about freely in the 

 water (A, Fig. 149). Each colony consists of sixteen rounded 

 or pointed cells, each provided with two cilia (called zoogo- 

 nidia), and united into a spherical mass by a gelatinous enve- 

 lope, through which the cilia project. Each zoogonidium 

 breaks itself up into sixteen new zoogonidia, forming sixteen 

 small and new colonies (B, Fig. 149), which are soon set free 

 by the absorption of the common envelope of the colonies. 

 The process of colony-formation just described is repeated 

 again and again, thus giving rise, asexually, to a large num- 

 ber of colonies. 



294. The sexual process begins in the same way ; but the 

 zoogonidia of the new colony separate by the softening of 

 the colony-envelope ( and D, Fig. 149), becoming zoospores, 

 which are naked protoplasm-masses, which swim about by 

 means of their cilia. After a time two zoospores meet, their 

 points coming in contact, and their bodies soon fusing into 

 one common body (E, F, G, Fig. 149). The result of this 

 union, which is regarded as a very simple kind of sexual 



