266 COLLEGE BOTANY 



larged, passes into tiie center of the sphere and develops into 

 a new colony. In the latter case certain cells become enlarged 

 and develop into female' gametes or oospheres ; while certain 

 other cells prodnce nnmerotis motile, male gametes or sperms. 

 Both female and male gamLet.es escape into the inner part of 

 the sphere, fuse and form oospores, which are capable of pro- 

 ducing new colonies. 



Therefore, it will be readily seen that the unicellular fo'rms 

 belonging to this order are fully developed plants performing 

 all the functions of a plant, regardless of size or differentiation. 

 In some of the colonial forms each cell retains all its functions, 

 while in others there is a division of labor in which some of 

 the cells perform the duties of reproduction. It will also be 

 readily seen that we iind in this order several stages in 

 the evolution of reproduction: (1) thei formation of new indi- 

 viduals by a sexual cell division, ( 2 ) the formation of new indi- 

 viduals by the union of like sex cells or gametes, (3) the forma- 

 tion of new individuals by the union of unlike cells or gametes, 

 (4) the formation of colonies, and (5) the division of labor. 



The order Protococcales is composed of unicellular and mul- 

 ticellular species in which the vegetative cells do not possess 

 cilia and are not motile, but the reproduction is practically the 

 same as in the Volvocales. They are mostlv aquatic, but some of 

 them live on damp soil, stones and the bark of trees. Some are 

 endophytic, living in the intercelhilar spaces of other plantsi; 

 while others are parasitized by fungi, thus forming the plants 

 known as lichens (see page 2sl). So^me of the most common 

 genera are: Pleurococcus, Scenedesmus, Pediastrum, Hydro- 

 dictyon and Euglena. 



Pleurococcus is a veiw simple, unicellular form, which 

 forms a green coating over wet flower pots, walls, soil and trees. 

 The cells are more or less spherical and non-motile. The proto- 

 plasmic content is highly developed. It reproduces by simple 

 fission only. 



Scenedesmus (Fig. 120, a) is a fresh-water form, consisting 



