318 A TEXTBOOK OF GENERAL BOTANY 
ones that indicate most clearly that they are the ancestors of 
the higher plants. Movement by means of flagella is found in 
the simplest plants, the bacteria, and is 
characteristic of the simpler alge, while 
similar types of movement are found in 
sexual cells of specialized plants even as 
high in the evolutionary scale as the sim- 
plest of the seed plants. Likewise, the 
presence of an eyespot is characteristic 
of many of the unicellular plants and 
of the sexual cells of some of the mul- 
ticellular ones. A consideration of the 
Reiter een uehe flagellates shows very clearly that there 
clear area are two contrac. 18 no absolute distinction between plants 
tile vacuoles. Most of the and animals. 
nc ost ecg s cblore. It seems reasonable to suppose that 
plastid. In the upper right : 
part of the cellisa pigment the flagellates were derived from some 
spot, eyespot. (x 1300) simpler chlorophyll-bearing plants. The 
Cyanophyceae, however, are the only 
known chlorophyll-bearing plants which are considered as more 
primitive than the Flagellata, and the way in which these two 
groups may be related is entirely uncertain. 

Fic. 320. Chlamydomonas 
CLASS CHLOROPHYCEAE (GREEN ALG) 
Green alge are more varied in form than any other group 
of plants, and it is evident that they have undergone evolution 
along a number of different lines. To acquire any clear concep- 
tion of the different subdivisions of the green alge would be 
impossible in a short course. For this reason, in the following 
discussion no attempt is made to deal with the classification of 
the green alge or to do more than indicate a few lines of evo- 
lution. The forms described have been selected because they 
are common and widely distributed; and show something of 
the diversity of green alge and the evolution of sex 
