62 



employs aerobic bacteria as living reagents, has given us what has hitherto 

 been unattainable, a micro-chemical test for the gas. It is a well-known 

 fact that there is in green plants, going on side by side with the ordinary 

 respiration of the cells, a second process of gaseous exchange accompany- 

 ing the assimilation of carbon dioxide and also often incorrectly called 

 ' respiration.' The carbon dioxide is split up in the green parts of the 

 plant by the energy of the sunlight, the carbon being retained, and the 

 oxygen thrown off into the atmosphere. Now it is not all rays that are 

 equally powerful to cause this chemical change. The absorption spectrum 

 of chlorophyll shows that the red rays between Frauenhofer's lines B and 

 C are the most strongly absorbed by green plants, and after them that 

 part of the spectrum just beyond F. It is just these regions that are most 

 active in causing the evolution of oxygen, and Engelmann demonstrates 



this in the following way. By means 

 of an arrangement of prisms in the 

 substage of a microscope he pro- 

 jects a spectrum upon green algal 

 filaments or moss-leaves lying in 

 the field of view, the water in which 

 they are mounted containing aerobic 

 bacteria. If, now, the illumination 

 be strong, and all extraneous light 

 be carefully excluded, it can be 

 seen that the aerobic bacteria crowd 



* 



FIG. 16. Detection of oxygen by means of bacteria. rnnnH trip filpmpntQ iimt 



The vertical lines are the Frauenhofer lines of a spectrum rouna lne niameniS JUSt 



thrown on the field of the microscope. In the spectrum o^ttwo r-3\7C fall tVip rVi*f ciira rm 



lies a filament of the alga Cladophora, and around it, at attlVC rayS Tail, Uie CniCI SWai HI 



B, C, and F, the bactena swarm (see text). Magn. 200. l y j ng between B and C, and an- 



other smaller one near F (Fig. 16). 



Around these parts of the alga only a few scattered organisms occur, show- 

 ing that the evolution of oxygen is at a minimum. The application of 

 this bacterial method for the detection of minute traces of free oxygen is 

 an operation of great delicacy, and the results which it gives must be 

 interpreted with great care because the movements of bacteria are very 

 often rendered more active by other substances beside oxygen, particularly 

 by food-stuffs, as will be explained in the section on chemotaxis. The 

 attraction by oxygen is in fact only a special instance of chemotaxis. 



The great amount of energy derived from the combustion of food-stuffs, 

 and from the oxidation of the tissues in higher organisms, is not all expended 

 in the form of mechanical work, but applied in part to raising the tempera- 

 ture of the tissues. In animals this is conspicuous in the case of ' warm- 

 blooded ' creatures, and in plants we have instances in the germinating grains 

 of cereals and in the flowers of the Aroideae, which are often sensibly warm to 

 the touch. Fermenting and putrefying substances also (hay, manure, cotton- 



