ENTRY INTO THE BIOSPHERE 



361 



(d) The Pigmentary System of the Greefi, Purple, and Brown Bacteria 

 This is very similar to that in the green algae and green plants. The 

 brown or purple colours are due to the presence of various carotenoids. 

 It is likely that as in diatoms and the blue algae, the carotenoids play a part 

 in the absorption of light, and transfer the energy thus accumulated to 

 chlorophyll. The chlorophyll in the Thiorhodaceae and the Athiorhodaceae 

 is bacteriochlorophyll-«, whilst the green bacteria contain a different 

 chlorophyll. 



Fig. 96 (Errera and Laurent) — Nodules on pea roots. 



1 . A young plant which has remained stunted in sterilized sand containing neither 

 combined nitrogen nor thinned with soil on which legumes have been grown; there 

 are no nodules on the roots. 



2. A young plant of the same age which has flourished in sterilized sand not containing 

 combined nitrogen but thinned with soil in which peas had formerly grown; nodules 

 are present on the roots. 



3. A root bearing nodules. 



II. PHOSPHORUS 

 The incorporation of phosphorus by green plants is the main means of 

 entry of this element into the biosphere. If we consider a plant, the 

 surroundings in which the roots live are very complex. Phosphorus is 

 present in the form of sparingly, soluble calcium, iron and aluminium salts, 

 as adsorbed anions on the soil particles and as organic compounds derived 

 from the corpses of plants, animals and micro-organisms (iron, calcium or 

 aluminium salts of phytin, and of nucleoproteins). In neutral or calcareous 

 soils, the calcium salts of organic forms of phosphorus are more soluble 



