36 BEHAVIOR OF THE LOWER ORGANISMS 



region into the dark before reacting; the reaction then carries them 

 back into the light. With the smaller bacteria the reaction usually 

 occurs in this manner, while in larger species (Monas okeni; Ophido- 

 monas sanguined) the reversal of movement occurs when only one end 

 has passed into the dark. A sudden increase of light merely causes the 

 organisms to swim forward a little more rapidly. 



The purple bacteria are sensitive in different degrees to lights of dif- 

 ferent colors, tending to gather in certain colors more than in others. 

 This is shown in a most striking way when a spectrum is thrown on a 

 preparation of Chromatium photometricum (Fig. 30). The largest num- 



1 



&BC 



D 



g 



FIG. 30. Distribution of bacteria in a microscopic spectrum. The largest group is in the 

 ultra-red, to the left; the next largest group in the yellow-orange, close to the line D. After 

 Engelmann. 



ber of the bacteria collect in the ultra-red rays, which do not affect the 

 human eye at all. There is another collection in yellow-orange, while 

 a few are scattered through the green and blue. None are found in the 

 red, the violet, or ultra-violet. These collections arise in the same man- 

 ner as those in the white light. Bacteria swimming from blue toward 

 yellow-orange, or from red toward ultra-red, do not react at all, but con- 

 tinue their course. But specimens swimming in the opposite direction 

 react in the usual way, by leaping back, when they come to the outer 

 boundary of the ultra-red or the orange-yellow. Hence, in the course of 

 time, if the bacteria continue moving, almost all of them will be found 

 in the two regions last named. 



It is a most interesting fact that the colors in which the bacteria col- 

 lect are exactly those which are most absorbed by them, and are also 

 those which are most favorable to their metabolic processes. Engel- 

 mann showed that most oxygen is given off, and hence that most carbon 

 dioxide is assimilated, in the ultra-red rays, while next to the ultra-red 

 the orange-yellow are most favorable to these processes. The reactions 

 of these bacteria to light are therefore adapted with remarkable preci- 

 sion to bringing them into regions which offer the best conditions for 

 their development. This is the more remarkable when we consider that 



