THALLOPHYTES 15 



(i) CHLOROPHYCEAE 



General character. The green algae usually contain no pigment in 

 addition to the chlorophyll, and their appearance justifies the name. 

 They include the simplest algae, and are generally supposed to be the 

 forms from which the higher groups of plants have been derived. On 

 this account green algae may be regarded as the beginning of our 

 present vegetation. The protoplast always has a distinct nucleus and 

 one or more chloroplasts, and this mode of organization is continued 

 throughout all the higher green plants. As presented here, the Chloro- 

 phyceae contain six distinct groups, as follows: Volvocales, Protococ- 

 cales, Confervales, Siphonales, Conjugates, and Charales. It is recognized 

 that some of these groups are very artificial, and that some of them per- 

 haps should be set apart from the Chlorophyceae; but in this elemen- 

 tary presentation of the forms, it is more convenient and less confusing 

 to use this grouping. The doubtful situations will be indicated in 

 connection with the different groups. 



(a) Volvocales 



General character. These aquatic forms are distinguished from 

 other green algae by the fact that the vegetative cells have cilia and 

 therefore are motile. They are sometimes regarded as animals, 

 for they grade plainly into the Flagellates, a 

 group of organisms of mixed plant and animal 

 affinities (see p. 20). A few representative 

 forms will indicate the structure and tendencies 

 of the group. 



Chlamydomonas and Sphaerella. Chlamydv- 

 monas consists of a single cell bearing two cilia, 



the protoplast being closely 



invested by a thin membrane 



(fig. 21). The structure of 



Sphaerella is in general the FIG. 21. Chlamydo- 



same except that the cell showing the large 



r cuplike chloroplast, the 



FIG. 22. Sphaerella: has a loose membrane, embedded pyrenoid, the 

 showing the protoplast which is connected with the centrally placed nucleus, 



.avested by a loose mem- prot oplast by strands and is the two contractil e vacu - 



brane, which is pierced ... oles, the red pigment 



by the two cilia.- After P ierced b Y the tw Clha spot, and the two cilia. 

 WEST. (fig. 22). After WEST. 



