STIMULI AND THEIR ACTIONS 



433 



in an ingenious manner. Engelmann observed that certain forms 

 of Bacteria that live in decomposing infusions accumulate in great 

 numbers in the neighbourhood of the sources of oxygen. Thus, in 

 an exposed drop under the microscope a dense accumulation of 

 these microbes takes place at the edges of the drop, where the 

 oxygen of the air has the freest access. Under the cover-glass, 

 likewise, the Bacteria congregate in the neighbourhood of the edge, 

 and form a dense wall parallel to it. Bubbles of air, as well as 

 plant-cells whose chlorophyll sets free oxygen in the light, act in 

 the same way, especially when lack of oxygen is produced to a 

 certain extent by covering the edges of the cover-glass by a layer 

 of oil. Engelmann employed this 

 extraordinary irritability of Bacteria 

 toward oxygen as the basis of a 

 method for the microscopic demon- 

 stration of very small quantities of 

 oxygen, and this has become very 

 important in our knowledge of the 

 assimilatory action of various kinds 

 of light upon the green plant-cell. 1 

 By the external exclusion of air from 

 a drop containing Bacteria, the place 

 may be found where only the slight- 

 est traces of oxygen are present, and 

 these are situated where there is a 

 dense accumulation of the microbes. 

 A beautiful example of such is af- 

 forded by the following observation. 2 

 In a drop under the cover-glass a 

 large diatom (Pinnularia) was in the 

 field of sight, and, since in the light 

 it gave off oxygen by reason of the 

 activity of its chromophyll, it was closely surrounded by a wall of 

 motionless Spirochcetoe. In the other parts of the field almost no 

 Spirochcetce were visible. The diatom suddenly moved a little 

 distance, and then lay still. The Bacteria, thus separated from 

 their source of oxygen, lay quiet for a few moments ; but soon an 

 active movement began among them, and they swam in dense 

 crowds again to the diatom. After one or two minutes almost all 

 were congregated again about it in a dense mass, and motionless 

 as before (Fig. 211, 1). Engelmann has recently figured similar 

 observations (Fig. 211, 11 and III). 



The striking and systematic investigations of Pfeffer ('84/88) upon 



chemotaxis had their starting-point in observations on the sper- 



matozoids of ferns, in which chemotactic relations to the egg-cell 



were found. It is now known that analogies to this exist in almost 



1 Of. p. 217. 2 Cf. Verworn ('89, 1). 



F F 



FIG. 210. Leucocyte devouring a bac- 

 terium of splenic fever. (After 

 Metschnikoff.) 



