CH. ll] DIFFUSION. 49 



oxygen. Thus all that is necessary is to place the 

 preparation in the dark for two or three hours, then to 

 expose it to light, and to watch the swarming of the 

 bacteria round the green plant. The bacteria will be in 

 violent movemenf within half a minute after exposure to 

 light. If not kept long in the light they may be brought 

 to rest by a quarter of an hour's darkness. 



By means of Engelmann's Microspectral Objective 

 it is possible to cast a spectrum on the filament of 

 Spirogyra and to observe the distribution of the swjirming 

 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 Hush 

 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 or of Nerium) is placed with the 

 stomatal surface uppermost, and firmly cemented with 

 wax -mixture to the cork. The tube is filled with COo, 

 and its lower end plunged into mercury. As the CO2 

 diffuses out through the leaf, the mercury rises in the 



1 Praktikum, p. 107. 

 D. A. 4 



