26 DARK-GREEN SCUM. 



c. The partition-walls, which in optical section are seen as 

 delicate lines crossing the filament and dividing it into 

 very small cells. 



d. The comparative length and breadth of the cells. 



e. The granular contents, and their distribution in the 



cell. 1 



/. The delicate colorless sheath to be seen extending beyond 

 the green cells at some torn end of a filament, and on 

 which sometimes may be detected transverse lines indi- 

 cating the position of the end walls of the cells. 



2. Theiurgidity of the cells: notice that 



a. The transverse walls in an uninjured filament are plane, 

 while 



b. The last cells of an injured filament are bulged outward, 

 making the outer transverse walls convex, the pressure 

 from within not being counterbalanced from without. 



3. Draw parts of two or more plants. 



ANNOTATIONS. 



If the structure of Oscillatoria be carefully compared 

 with that of Pleurococcus and Nostoc more points of 

 resemblance will be found than appear at first sight. 

 New cells are formed by the process of division, as in 

 Pleurococcus and Nostoc, but the partition-walls are 

 always parallel and in one direction, which disposes the 

 cell families in filaments. The individual cells have 

 thin partition-walls, the office of protection being rele- 

 gated to the sheath. The sheath, which is formed from 

 the outside walls of the cells by a modification of the 

 outer portion, is a structure that is mostly confined to 

 certain groups of the lower plants, although it has some 



1 In some species the granules are collected along the partition-walls. 



