3 6 COMMON POND SCUM. 



2. The uniform diameter. 



3. The cell contents ; colorless, except the conspicuous 

 green chlorophyll bands. 



B. THE INDIVIDUAL FILAMENT. Using a high 

 power, notice 



1. The shape of the cells. 



2. Their relative length and breadth. 



3. 'Thz cell wall : 



a. The lateral walls j parallel and without markings 

 of any sort. 



b. The end walls ; at right angles to the longitudinal 

 axis, and plain (unless slightly nodulated or 

 infolded, which occurs in a few species). 



4. The absence of any visible sheath, although the pres- 

 ence of at least a thin one has been demonstrated by 

 the slippery feel. 



5. The cell contents. 



a. The chlorophyll bands, taking a spiral course from 

 one end of the cell to the other, passing near the 

 periphery. Note 

 i. The number in each cell. 3 



3 'When a cell is crowded with chlorophyll, the following method may 

 be used to advantage in determining the number of bands : count the 

 number appearing to cross the band ab, between the point a, the upper 

 profile view, and the point b, the lower profile view ; this number plus 



one will be the 

 number re- 

 quired. The 

 diagram shows 

 a cell with four 

 bands of chlo- 

 rophyll. From 

 Bot. Gazette, ix., 



