346 A TEXTBOOK OF THEORETICAL BOTANY 



Most Bacteria are colourless or only faintly coloured when seen in bulk, 

 but some contain brightly coloured carotinoid pigments ; yellow, orange 

 and red being the commonest tints. It has been suggested that these colours 

 may serve as light filters, protecting the cells from the lethal effects of the 

 shorter wave-lengths present in sunlight, to which the cells are sensitive. 

 Most uncoloured forms are killed by even short exposure to sunlight, which 

 has therefore a most important disinfecting influence. 



A very interesting group, the Purple or Red Bacteria, which are common 

 in natural waters, especially in the sea, possess two pigments, bacterio- 

 purpurin, a porphyrin, and bacteriochlorophyll, which is related to 

 chlorophyll. These pigments enable the organism to assimilate Carbon 



•^ ■ V 



Fig. 335. — Streptococcus salharius. Cells in chains. Gram 



stained. 



dioxide in the presence of light, with the liberation of Oxygen. They are 

 unique in being the only organisms known to carry out photosynthesis 

 without true chlorophyll. 



Classification 



Bacteria were classified in the past chiefly on their microscopical appear- 

 ance, and many of the names thus given have been retained as the names 

 of genera, although we now know that a true classification must take account 

 of their biochemical properties as well as their structure. 



The simplest form is that of a minute sphere, known as a coccus. When 

 these occur separately they are put in the genus Micrococcus ; if in pairs 

 they form the genus Diplococcus ; if in chains, the genus Streptococcus (Fig. 

 335) ; and, lastly, if in clusters, the genus Staphylococcus (Fig. 336). A 

 peculiar type of aggregation is that into a cubical packet, which characterizes 

 the genus Sarcina, one of the oldest names in Bacteriology, since Sarcina 

 ventriculi was recognized in the human stomach as early as 1842. 



