100 BOTANY 



oblong, thick-walled, and the cytoplasm forms a thick layer next the wall, 

 leaving a large central sap-cavity. Imbedded in the cytoplasm are numerous 

 nuclei and many pyrenoids. The chromatophore forms an irregularly broken 

 thin plate, which finally becomes separated into many small chromatophores. 



Reproduction. When the cells have reached a certain size, the protoplasmic 

 contents may divide into a great many (sometimes several thousand) minute 

 cells, each of which has a nucleus and a small piece of the chromatophore. 

 These cells do not escape, but assume the form of biciliate swarm-spores, which 

 move about for a short time within the mother-cell, where they soon come to 

 rest, having arranged themselves end to end, so as to enclose small polygonal 

 areas (Fig. 69, C). In short, they form a very small network, fitted within the 

 mother-cell as in a mould. The wall of the mother-cell slowly softens and dis- 

 solves, as the young net grows, and finally it is set free in the water, and finally 

 grows to its full size, but no further divisions occur in the cells. In the young 

 net (Fig. 69, E) each cell has a single nucleus and a girdle-shaped chromatophore 

 containing a single pyrenoid. As the cells grow, the nucleus divides repeatedly, 

 and the chromatophore grows with the cell, being broken through in places, and 

 often presenting a somewhat reticulate appearance. Numerous pyrenoids also 

 arise in the chromatophore. 



Sexual Reproduction. The sexual reproduction consists in the division of 

 the contents of certain cells into a very large number (sometimes 30,000) of 

 motile cells, much like the swarm-spores, but smaller. These escape from the 

 mother-cell and unite in pairs (Fig. 69, F) to form the zygote, which, after 

 increasing in size, gives rise later to several large swarm-spores, which in turn 

 produce secondary resting-spores, known as polyedria. From these polyedria 

 are formed small nets, much as in the ordinary cells. 



It has been found that the conditions under which the plants are grown exert 

 a strong influence upon the reproduction. Grown in certain nutrient solutions 

 (e.g. a two per cent solution of maltose), the tendency to form new nets is 

 immensely increased. So if plants are grown in a solution of cane sugar, the 

 production of gametes is greatly stimulated. 



The beautiful, star-shaped colonies of Pediastrum (Fig. 68, F) are familiar to 

 all students of Algae. The development of the plant is very much like that of 

 Hydrodictyon, the main difference being that the swarm-spores escape from the 

 mother-cell, enclosed in a delicate membrane, within which they arrange them- 

 selves in the form of the colony. 



Affinities of Protococcoideae 



The simpler Protococcoideae are closely related on the one hand 

 to the simpler Volvocacese, on the other to the lower members of the 

 Confervoideee. Thus the genus Tetraspora resembles very closely 

 the genus Ulva, and many of the unicellular forms like Chlorococ- 

 cum are extremely like the early stages of many of the filamentous 

 Confervaceae. 



The Protococcoideae (Engler and Prantl, 9) may be divided as 

 follows : 



a Vegetative cell-division present. 



1. Zoospores present. Families: Tetrasporaceae, Chlorosphae- 



raceae. 



2. Zoospores absent. Family : Pleurococcaceae. 



