42 DIFFUSION. [CH. II 



slip, the bacteria ultimately become sluggisb and come to 

 rest. It is of this fact that Engelmann's method takes 

 advantage. If a filament of Spirogyra or the leaf of a 

 submerged plant be included with the sealed bacteria we 

 have it in our power, by the exposure of the preparation 

 to light, — to produce free oxygen. Thus all that is 

 necessary is to place the preparation in the dark until 

 the bacteria are at rest, then to expose it to light, and to 

 watch the swarming of the bacteria round the green plant. 

 By means of Engelmann's Micro-spectral Objective 

 it is possible to cast a spectrum on the filament of 

 Spirogyra and to observe the distribution of the swarming 

 bacteria in the different colours. We do not propose to 

 enter into Engelmann's method of " successive observa- 

 tions" for which the student may consult Engelmann's 

 papers in the Botanische Zeitung from 1881 onwards. 



(54) Diffusion. 



In connection with assimilation the diffusion of gas 

 through the cuticularised epidermis should be studied. 

 Detmer's method^, may be used, 



A pierced rubber cork is fitted over a glass tube 

 (3 cm. diameter) so that the surface of the cork is flush 

 with the upper rim of the tube. On the aperture in the 

 cork a piece of fine wire gauze is laid and on this a leaf 

 (e.g. that of Platanus) is placed with the stomatal surface 

 uppermost, and firmly cemented with wax-mixture to 

 the cork. The tube is filled with CO2, and its lower end 

 plunged into mercury. As the CO2 diffuses out through 



' Praktikum, p. 107. 



