39. Chromatium Okenii. 

 Optical longitudinal section. 



fin are) Immediately adjoins the 

 outer layer of protoplasm, which 

 carries the colouring matter 

 shown as fine dots. L:irge sul- 

 phur ijT.'nul'-s ar present iu the 

 interior o h C . M;m'ii. h.nt 

 1600. (After F. Forsttr.) 



ASSIMILATION AM) OXYCiKN KLI.MIN ATION 113 



It has already been stated in chapter iii. that the purple bacteria exhibit 

 great avidity for light, and therefore always strive towards the sunlight. 

 Closer observation shows this behaviour to be intimately connected with the 

 presence of bacterio-j>ur|tiirin. It was a liappy 

 thought on the part of Engelmann to investigate 

 the nature and extent of the influence exerted on 

 the vitality of the purple bacteria by the several 

 colours of the spectrum. 



If a preparation rich in such organisms be 

 placed in a drop of water, and a micro-spectrum of 

 a few millimetres in length be projected thereon, 

 a rapid movement towards certain parts of the 

 spectrum will be observed under the microscope, 

 the organisms collecting and resting there in micro- 

 scopically visible bands. By quickly killing the 

 cells they will remain in position and constitute a 

 permanent preparation, or, as Engelmann appro- 

 priately named it, bacteriospectrogram. This, when 



submitted to examination, is found to correspond T heceii-waiuthethicknessof which 

 with the absorption spectrum of bacterio-purpurin, is somewhat exaggerated in the 

 showing a sharply defined band in the ultra-red 

 (wave length X = 0.9 to 0.8 /*) ; a second, less power- 

 ful, small band in the orange (X = 0.6 1 to 0.58/1); 

 and, finally, a pale washed-out band in the green 

 (X = o.55 to 0.52 /A). Eiigelmann then determined 

 by an accurate quantitative photometric examina- 

 tion, with the aid of the bolometric method, that a remarkable ratio prevails 

 between the intensity of the physiological action and the extent of the absorp- 

 tion, i.e. the attractive force of a given colour of the spectrum is greater in 

 proportion as the latter is retained by the colouring matter. From this is 

 deducible the further conclusion, that the purple bacteria have great need, not 

 merely of light in general, but of certain components thereof in particular, 

 and especially those corresponding to the lines A , D, E, of the spectrum. 



93. Assimilation and Oxygen Elimination. 



This behaviour of the purple bacteria, unique in the bacterial kingdom, 

 reminds one so much of the connection between light and the activity of chloro- 

 phyll in the higher plants, that not only does the question naturally arise as 

 to the nature of the reactions occurring under the influence of the selected 

 rays, but also the idea that here likewise there is assimilation accompanied by 

 the elimination of oxygen. This is, in fact, the case, the purple bacteria 

 excreting oxygen in the presence of light. 



ENGELMANN (VI.) proved this in a variety of ways. One of his experiments, 

 which demonstrates it in a very elegant manner, is based upon that already 

 given in 41. Quiescent forms, resembling a zoogloea, of one or other of these 

 species are employed, a portion about 2 sq. mm. in size being placed in a drop 

 of water. To this are added a number of aerobic organisms (e.g. Spirillum 

 tenue, Sp. undula, various infusoria, &c.), capable of reacting on even small 

 quantities of oxygen, and the cover-glass is surrounded with vaseline to prevent 

 the admission of air. The oxygen dissolved in the water is very quickly 

 exhausted, and the organisms come to a standstill. If, now, the preparation be 

 illuminated, it will be seen directly that the wanderers scattered about ri*e and 

 i 11 



