40 COMMON POND -SCUM. 



3. The cell wall: 



a. The lateral walls; parallel and without markings of 

 any sort. 



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

 and plane (unless slightly nodulated or infolded, which 

 occurs in a few species). 



4. The absence of any visible sheath, although the presence 

 of at least a thin one has been demonstrated by the slip- 

 pery feel. 



5. The cell contents: 



0. The chlorophyll bands (chloroplasts), taking a spiral 

 course from one end of the cell to the other, passing 

 near the periphery. Note: 



(1) The number in each cell. 1 



(2) The number of turns of the spiral. 



(3) The surface, the crenulated and wrinkled margin, 

 and the turned-up edges of the band forming a 

 more or less flattened V in optical section. To ob- 

 tain a complete conception of these particulars, first 

 focus upon the peripheral surface of the band, i.e. 

 upon the upper (outer) surface of the part nearest 

 the eye, then focus upon the axial (inner) surface, 

 and finally examine the profile of the band seen on 

 the right or left of the cell. 



(4) The nodules at varying distances along the median 

 line of the band. Stain with iodin, and in the 

 nodule note: 



(a) An outer ring of granular material which is more 

 deeply colored, the starch grains, 2 and 



(6) A central light spot, pyrenoid. Both are best 

 seen when but faintly colored. 



1 If crowded so as to make a direct count difficult, see Bot. Gaz. 9: 13, 

 for an easy method of determining the number. 



J Unless the plants have been in sunlight for a few hours the test 

 for starch may not be fully successful. 



