32 PLANT MORPHOLOGY 



the colony and there fertilization occurs. The zygote becomes heavy- 

 walled and remains dormant for several months (Fig. 2()E). In some 

 species it then gives rise to a single biciliate zoospore, while in others it 

 forms a new colony directly. In connection with the germination of the 

 zygote, the number of chromosomes is reduced one-half. As is Eudorina, 

 some species of Volvox are monoecious, others dioecious. 



Summary. The Volvocales are distinguished from the other Chlo- 

 rophyceae by the fact that their vegetative cells are cihated and motile. 

 They exhibit a range of development from single isolated cells to com- 

 plex globular colonies. Each cell has one nucleus and generally one 

 chloroplast. Asexual reproduction occurs by zoospores and by the 

 formation of a new colony from a single parent cell. The number of 

 cells in the colony is definite and is determined during early development. 

 It is not subsequently increased by vegetative cell divisions. Sexual 

 reproduction shows an advance from isogamy to heterogamy, while 

 dioecism is attained by some species of Eudorina and Volvox. 



2. Chlorococcales 



The Chlorococcales constitute a large order of diverse forms that are 

 probably not closely related. They are chiefly fresh- water algae, only a 

 few occurring in the ocean. Some live in moist places on land. Some 

 are endophytic in the intercellular spaces of certain seed plants, while 

 others live symbiotically in the lower animals. Others are lichen formers. 

 The order contains 90 genera and approximately 700 species. Some 

 characteristic genera are Chlorococcum, ChloreUa, Scenedesmus, Pedias- 

 trum, Hydrodictyon, and Protosiphon. 



Chlorococcum. This simple alga grows on damp soil or rocks. It is 

 unicellular, spherical, and nonmotile. At first it has a single nucleus 

 and a large cup-hke chloroplast with one or more pyrenoids (Fig. 21). 

 Later the cell becomes multinucleate and the protoplast divides to form 

 a variable number of bicihate zoospores that escape. After coming to 

 rest, a zoospore loses its ciha, secretes a wall, and becomes a vegetative 

 cell. Asexual reproduction may also occur by aplanospores. These 

 arise in the same way as zoospores but have no ciha and develop a cell 

 wah before being freed. As in Volvox, a "palmella" stage may develop 

 by gelatinization of the cell walls in a group of cells. Sexual reproduction 

 is accomplished by the production of a large number of bicihate isoga- 

 metes by a vegetative cell. These escape and fuse in pairs. In general, 

 Chlorococcum is like Chlamydomonas except that the vegetative cells have 

 lost their motility. 



ChloreUa. ChloreUa lives on the bark of trees, damp walls, and soil; 

 also in various infusoria, the fresh-water sponge, and the green hydra. 

 It can be grown easily in water cultures and is much used in experiments 



