THE ORGANISMS OF THE SOIL 155 



vigorous growth in the deserts of Australia, where the dried 

 algae form a crust resembling elastic bitumen on the surface. 1 



In all these cases the algae and lichens do not merely cling 

 to the rock surface ; the}' definitely eat and digest the rocks on 

 which they grow, as may be seen by the corrosion of the surface, 

 and also by the presence of the substances in the rock in the 

 cells of the plants in the form of crystals of oxalate of lime 

 and so forth. 



The investigation of the microscopic animals of the soil is 

 practically untouched. Miiller found Diffhigia, a large fresh- 

 water rhizopod, in bog humus 2 and I have mentioned the case 

 of Nitrosomonas. The intestines of earthworms swarm with 

 gregarines, which seem to play the same part in them as 

 bacteria do in the case of plants. In tan pits the Faligo varians 

 (iEthalium septicum), commonly known as " flowers of tan," 

 spreads out in colonies a foot or more in diameter ; the germs 

 of this organism must exist in the natural bark and in the soil 

 of forests. 



It is a legitimate question to ask, "What do the bacteria and 

 other organisms in the soil do when all decomposition possible 

 has been accomplished ? " We have seen what teeming myriads 

 live in the soil. We know that if the soil is treated with weak 

 solutions of carbolic acid and mercury chloride, which kill 

 bacteria, the soil is rendered sterile. We shall now proceed to 

 show that bacteria are known to act directly on inorganic 

 substances and the inference seems to follow naturally that a 

 large part of the activities of the micro-organisms in the soil is 

 concerned with the breaking down of rock substances. 



The absorption of carbonate of lime by the lower organisms 

 is well known. In plants the minute coccoliths and rhabdoliths, 

 the blue-green algae, Chroococciis and Glceocapsa, the larger red 

 or calcareous seaweeds, are examples, whilst among animals all 

 the protozoa and some sponges absorb and secrete carbonate of 

 lime as one of the functions of the activities of their cells. The 

 action is perfectly simple : by the oxidation of the carbon in 

 their protoplasm carbon dioxide is produced, which acts on 

 calcium carbonate and forms a soluble compound. The for- 



1 L. A. Boodle, Bull. Miscellaneous Inf. Kew, No. 5, 1907, p. 145 ; see also 

 W. T. Thiselton-Dyer, Australian Caoutchouc, Jottrn. Bot. New Ser. vol. i. 

 1872, p. 103. 



2 P. E. Miiller, Natiirliche Hwm/sfoi-Jiien, Berlin, p. 27. 



