32 . DARK GREEN SCUM. 



herbarium sheets on which specimens have been 

 dried. 



The cells are assisted in keeping together by the 

 investing sheath, into which they are packed like a roll 

 of lozenges in their case. This structure, together with 

 the community of action exhibited in producing the 

 peculiar oscillating and nutating movements, makes it 

 evident that the cells of each filament have a certain 

 dependence upon each other, although at the same 

 time each is capable of independent existence. It may 

 be that the smallness of the cells and the thinness of 

 their walls is in some way correlated to this unity of 

 function. It is not yet definitely known how the move- 

 ments in Oscillaria are produced. 3 



Turgidity is a characteristic of living cells. It is the 

 means by which the soft parts of plants are enabled to 

 keep their form, and otherwise to serve their purpose. 

 It is brought about by the strong imbibition power. of 

 the protoplasm, causing water to be taken up until 

 a considerable internal pressure is created. 4 



3 Engelmann has discussed several theories, and suggested that the 

 movements are brought about by vibratile thread-like extensions of the 

 protoplasm through the cell walls. Bot. Zeit. 1879, p. 49 According 

 to Hansgirg it is due to an osmotic action of the protoplasm. Bot. 

 Zeit. 1883, p. 831. 



4 Cf. Bessey, Botany, p. 166. 



