CHLOROPHYCE^ 177 



PROTOCOCCACE.E 



General Characters. This is a very large order, 

 very sparingly represented in the sea. Except in 

 European seas, however, the marine forms have been 

 little studied, and it is probable that many more 

 types will be added in time, especially after a more 

 extended examination of the floating Algse of the 

 open ocean. The thallus is unicellular, and the cells 

 are never associated intimately to form a tissue or 

 definite group obeying a common law of growth. 

 There are no cell-divisions during vegetative life. 

 The plants are either stipitate or free, and never 

 motile, unless of course in the zoospore stage. The 

 conjugation of equal gametes has been observed, and 

 also non-sexual reproduction by zoospores. 



The Thallus. Chlorochytrium and Chlorocystis in- 

 habit the tissues of other plants, the former mainly 

 flowering plants in fresh-water or on land, but also 

 seaweeds, and the latter exclusively seaweeds. The 

 cells are globular, oval, or irregular in shape, and 

 the main vegetative difference between the genera 

 appears to be in the shape of the chromatophores, 

 and the fact that Chlorochytrium has many pyrenoids 

 while Chlorocystis has but one in each chromatophore. 

 Chlorocystis Cohnii inhabits the tissues of many sea- 

 weeds, and C. Sarcophyci causes deformities of the 

 thallus of Sarcophycus. Halosphmra is a beautiful 

 globular form, living free in the ocean, and forming 

 a considerable constituent of the plankton or floating 



