182 PHYLUM III. ZYGOPHYCEAE 



Microspora and Ulothrix could very well serve as the 

 originals which have been modified successively into 

 the Pond Scums, the Desmids and the Diatoms. The 

 limited fragmentation of the filament in Ulothrix is so 

 much increased in the Conjugate Algae as to render the 

 production of zoospores unnecessary. In like manner 

 the sluggish protoplasm of the Conjugate Algae is corre- 

 lated with the disappearance of the freely motile gametes 

 and the degeneration of the sexual process into a sluggish 

 conjugation, which in some Desmids and Diatoms results 

 in the partial (if not complete) suppression of the sexual 

 act. According to this view "conjugation" is the result 

 of degeneration. It is sexual reproduction on its way 

 toward disappearance. Instead of affording an example 

 of the beginning of sexuality, as has so often been sug- 

 gested, these plants show sexuality on its way to disap- 

 pearance. Furthermore, it is obvious that the Conjugate 

 Algae constitute a lateral phylum which is related to 

 other phyla only in its lower members, and that its higher 

 members depart more and more widely from all other 

 forms of plants. 



Laboratory Studies, (a) Collect a quantity of bright green 

 pond scum, which always abounds in shallow ponds and pools in 

 the spring, summer and autumn, and preserve in a dish of 

 water. Collect, also, some which has begun to turn yellow and 

 brown. Upon mounting a little of the first in water and exam- 

 ining with a high power it will be found to consist of threads 

 of cylindrical cells, each containing one or more spiral chloro- 

 plasts (Spirogyra) or star-shaped chloroplasts (Zygnema). 

 Upon mounting some of the second collection, here and there 

 the formation of resting spores may be observed. In all cases 

 care must be taken not to mount too great a quantity of the 

 material, nor to injure the plants by rough handling. 



(b) Collect a quantity of pond scum and other aquatic 

 vegetation. Mount portions of this material and search for 

 desmids, using a low power objective. Two-lobed desmids 



