22 PLANT STRUCTURES 



being of this simple kind. It is evident, therefore, that the 

 group Chlorophyceae begins with forms just as simple as 

 are to be found among the Cyanophyceae. 



Pleurococcus is used to represent the group of Protococ- 

 cus forms, one-celled forms which constitute one of the 

 subdivisions of the Green Algag. It should be said that 

 Pleurococcus is possibly not a Protococcus form, but may 

 be a reduced member of some higher group ; but it is so 

 common, and represents so well a typical one-celled green 

 alga, that it is used in this connection. It should be 

 known, also, that while the simplest Protococcus forms re- 

 produce only by fission, others add to this the other meth- 

 ods of reproduction. 



23. THothrix. This form was described in 13. It 

 is very common in fresh waters, being recognized easily by 

 its simple filaments composed of short squarish cells, each 

 cell containing a single conspicuous cylindrical chloroplast 

 (Fig. 2). This plant uses cell division to multiply the cells 

 of a filament, and to develop new filaments from fragments 

 of old ones ; but it also produces asexual spores in the form 

 of zoospores, and gametes which conjugate and form zygotes. 

 Both zoospores and zygotes have the power of germination 

 that is, the power to begin the development of a new plant. 

 In the germination of the zygote a new filament is not pro- 

 duced directly, but there are formed within it zoospores, 

 each of which produces a new filament (Fig. 2, F, G). All 

 three kinds of reproduction are represented, therefore, but 

 the sexual method is the low type called isogamy, the pair- 

 ing gametes being alike. 



Ulothrix is taken as a representative of the Conferva 

 forms, the most characteristic group of Chlorophyceae. 

 All the Conferva forms, however, are not isogamous, as will 

 be illustrated by the next example. 



24. Edogonimn. This is a very common green alga, 

 found in fresh waters (Fig. 7). The filaments are long and 

 simple, the lowest cell acting as a holdfast, as in Ulothrix 



