74 



A PRIMER OF BIOLOGY 



themselves to the margin of the cover-glass, since, 

 near the edges, oxygen will have been absorbed by 

 the water from the air. Engelmann has made use 

 of this fact in a very ingenious manner to prove that 

 the rays absorbed by chlorophyll are those chiefly 

 concerned in the processes which result in photo- 

 synthesis with its accompanying evolution of oxygen. 

 1'or if a filament of an alga be placed on the field of 

 the microscope and illuminated from below by the 

 solar^speetrum, obviouslyrsome cells will be affected 



FIG. 31. Muscle- nerve preparation and recording drum. 

 (After Waller.) 



by red, some by orange, some by blue rays, and so 

 on. Since the rays which are absorbed by chloro- 

 phyll, viz., the red and the violet, are believed to be 

 those chiefly concerned in photosynthesis, the Bacteria 

 will congregate near these regions, for there oxygen 

 will be given off during photosynthesis. 



The general conclusion we arrive at, then, is that 

 plants as well as animals are sensitive to external 

 stimuli, and that the protoplasm alone is the sensitive 

 substance. 



Sensitivity Perhaps we may most easily gain some ele- 

 animais. mentary acquaintance with the general mechanism 



