EUTHALLEPHYTA EUPHYCEAE 



189 



of the greenish-brown coloring- matter they contain, which stains articles coming in contact 

 with it. Whipple has observed that the growth of diatoms seems to depend upon certain 

 definite conditions of the water that is, they do not develop when the bottom of the pond 

 or reservoir is quiet; but in spring and fall, when the rising or lowering temperature causes 

 the water to circulate and a good supply of air and nitrates is obtained, the growth is most 

 luxuriant. Thus, it is seen that temperature is only an indirect cause, and not one that need 

 be taken into account Dy itself. 



c 



Fig. 32. Diatoms. Pinnitlaria i-iridis. 1. 

 View showing markings in wall, c central nodule; 

 e polar nodules; r raphe. 2. Girdle view, s 

 silicified cell walls; g girdle bands. 3. In process 

 of division. Fig. 1 and 2 after Pfitzer; Fig. 3 

 after Wettstein. 



CONJUGATAE 



Chlorophyll-green algae, membrane without silica ; reproduction, swarm 

 cells absent; sexual reproduction by forming zygospores through the union of 

 aplanogametes. 



DESMIOIACEAE 



The desmids are green unicellular organisms represented by such 

 genera as Cosmarium, and are found mostly with other algae. They are not 

 especially troublesome. Common genera are Closterium, Cosmarium, and Des- 

 midium. 



The filaments of Zygnema are small consisting of a single series of cylin- 

 drical cells placed end to end, occasionally with a slight constriction at the 

 points of junction. Each cell has two star-shaped chloroplasts, each one con- 

 taining a large pyrenoid. 



Spirogyra, Link 



Spirogyra is a common alga everywhere in our fresh waters, especially in 

 quiet waters as in ponds and ditches. The filaments are simple, occur in bright 

 green masses, often several feet long: cells cylindrical, variable in diameter and 

 relative length; wall smooth and slightly gelatinous; chlorophyll arranged in 



