234 



PLANT STrT)IES 



/^ 



two hoterocysts. The fragments wriggle out of the jelly 

 matrix and start new colonies of chains, each cell dividing 

 to increase the length of the cliain. This cell division, 

 to form new cells, is the cliaracteristic nietliod of repro- 

 duction. 



At the approach of unfavorahle conditions certain cells 

 of the chain become thick-walled and well-protected. These 

 cells which endure the cold or other hardships, and upon 

 the return of favorable conditions produce new chains of 

 cells, are often called spores, but they are better called 

 " resting cells." 



IGl. Oscillaria. — 'I'hese forms are found as bluish-green 

 slippery masses on wet rocks, or on damj) soil, or fi'eely 

 floating. They are simple filaments, composed of very short 

 flattened cells (Fig. 203), and the name 

 Ofn'illaria refers to tlie fact that they 

 exhibit a peculiar oscillating move- 

 ment. These motile filaments are is- 

 olated, not being held together in a 

 jelly-like matrix as are the chains of 

 Kuxtoc, but the wall develops a cer- 

 tain amount of mucilage, wliich gives 

 the slippery feeling and sometimes 

 forms a thin mucilaginous sheatli 

 about the row of cells. 



The cells of a filament are all alike, 

 except that the terminal cell has its 

 free surface rounded. If a filament 

 breaks, and a new cell surface ex- 

 posed, it at once becomes rounded. 

 If a single cell of the filament is 

 freed from all the rest, both flattened ends liecome rounded, 

 and the cell becomes splierical or nearly so. Tlu>se facts 

 indicate at least two important things: (1) that the cell 

 wall is elastic, so that it can bo made to change its form, 

 and (3) that it is pressed upon from within, so that if free 



')i </ illtii'iit. a blne- 

 ^i ( 11 ili,'a. shnwing a 

 j.^riaip of lilamciits l^-l), 

 and a single lilanu'iit 

 mnri' oii!aii;cd U'l. — 

 Cai, DWELL. 



