THE ORIGIN OF LIFE 



Figure 34. Volvox. 



stantially identical. The sex cells are localized in antheridia (sperm) and 

 oogonia (eggs). The eggs of Volvox are always large, nonmotile cells, and 

 the sperm are always actively swimming flagellate cells which seek out the 

 egg. As both of these are produced by a single colony, the colony may be 

 regarded as hermaphroditic, a condition which is very much more com- 

 mon in plants and in lower organisms generally than it is among higher 

 animals. It may be, therefore, that hermaphroditism is primitive. 



Multicellular Individuals. In another line of descent, which may be 

 typified by Ulothrix, Draparnaldia, and Ulva, the green algae have devel- 

 oped not merely colonies of substantially independent organisms, but 

 multicellular individuals, the various cells of which are interdependent 

 (Figure 35). These are the filamentous algae, the typical "seaweeds" of 

 laymen, and it is from this group of algae that the bryophytes and vascular 

 plants appear to have developed. Ulothrix has the form of a simple, un- 

 branched, multicellular filament. The basal cell is specialized as a holdfast 

 to anchor the plant to a rock or other substrate. Cell divisions occur, but, 

 whereas this results in asexual reproduction in the unicellular algae, in 

 Ulothrix and its allies mitosis results in growth without reproduction. 

 Reproduction may occur either sexually or asexually. Asexual reproduc- 

 tion occurs by fragmentation of the plants with subsequent regeneration 

 by each fragment, or it occurs by the formation of zoospores from the 

 vegetative cells of the plant. These zoospores are not unlike those of 

 Chlamijdomonas, except that thev have four flagella instead of two. Each 

 zoospore develops into a plant like the parent plant. But any vegetative 

 cell of the plant may also give rise to gametes. These are smaller than the 

 zoospores, and they have only two flagella. All of the gametes are identical 

 morphologically, but physiologicallv there must be a sexual differentia- 

 tion, for zygotes are always formed by the union of two gametes from 

 different plants. After a quiescent period, the zygote undergoes the two 

 meiotic divisions, forming four zoospores which then develop into the 



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