Scientific Agrculture. 



528 



December, 1909, 



growth in the deserts of Australia, 

 where the dried algae form a crust re- 

 sembling elastic bitumen on the surface.* 



In all these cases the algae and lichens 

 do not merely cling to the rock surface ; 

 they 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 

 auimals of the soil is practically un- 

 touched. Miiller found Difflugia, a large 

 freshwater rhizopod, in bog humus t 

 and I have mentioned the case of Nitro- 

 somonas. 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 

 Fuhgo varians '^Bthalium sepfcicum), 

 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 or- 

 ganisms in the soil do when all decompo- 

 sition possible has been accomplished " ? 

 We have seen what teeming myraids 

 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 in- 

 ference 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 

 rhabdolths, the blue-green algse, Chroo- 

 coccus 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 oxida- 

 tion of the carbon in their protoplasm 

 carbon dioxide is produced, which acts 

 on calcium carbonate and forms a 

 soluble compound. The formation of 

 oolite grains is another instance : the 



* L. A. Boodl , Bull. Miscellaneous Inf. Keio- 

 No. 5, 1907, p. 145 ; see also W. T. Thiselton 

 Dyer, Australian Caoutchouc, Journ. Bot. New 

 Ser. Vol. i. 1872, p. 103. 



f P. E. Miiller, Nalurliche Humusformcn, 

 Berlin, p. 27. 



collection of the carbonate of lime is 

 supposed to be brought about by the 

 thalius of an alga, which encrusts the 

 central grain, depositing calcium carbon- 

 ate in concentric layers as it grows. 

 Certainly the encrusting red algae act 

 in the same way, but living organism has 

 never been observed on the oolite grains, 

 though nodules of carbonate of lime in 

 fresh-water lakes are usually covered 

 with blue-green algae, Glceocapsa, etc. 

 The same process is believed to give rise 

 to the pisolites which separate in pea- 

 liko granules as deposits from hot 

 springs. The blue-green algae can live 

 in hot water, but the actual organisms 

 on the pisolites have not been seen, 

 though Cohn asserts their presence. 



On the other hand, the destruction of 

 oolite grains and shells generally is 

 accomplished by boring algse, such as 

 Hyella, 15 which send their microscopic 

 filaments through aud through the hard 

 calcite similarly to the mycelia of a 

 fungus peuetrating rotten wood ; Lind.t 

 in fact, found that fungi were actually 

 capable of sending their hyphae through 

 marble. Boring sponges like Oliona 

 do the same, J 



The separation of silica cannot be so 

 easily explained. There are countless 

 plants and animals which absorb and 

 secrete silica, and the lower forms are 

 usually closely allied to the lime-secret- 

 ing genera. Among the silicious plants 

 there are the hosts of diatoms, and among 

 animals the radiolaria and sponges. 



In the case of iron again, there are 

 differences of opinion. The bog-iron 

 which forms at the bottom of lakes and 

 under the soil in marshy places, where it 

 is kmwn as moor-bed-stone, ortstein 

 or oude klip, ig thought to be the result 

 of chemical deposition. Organic acids 

 certainly dissolve the iron, and when the 

 solution is oxygenated, carbon dioxide is 

 given off and the iron is deposited either 

 as a carbonate or as a hydrate. This is 

 what happens in the laboratory ; in 

 Nature, however, the precipitation goes 

 on in the bottoms of lakes and in soil 

 which is not properly aerated. Ehrenberg 

 attributed the deposition to a diatom 

 which he called Gallionable ferruginea.$ 



* B. Bornet et Flahault, Note sue duex aouveaux 

 genres d' algues perforantes. Jour. Bot. vol. ii. 1888, 

 p. 161; see also J. E. Duerden, Boring Al^<a, Bull. 

 ANN. Mus. Nat. Hist. New York, Vol. xvi. 1902, 

 p. 323. 



t K. Lind, Jahrblicb.. Wiss. Bot Bd. xxxii. 1898, 

 p. 603. 



t Topsent, Arch. Zool. Exner. (3), Vol, viii 

 1900, p,226, 



§ C. G. Ehi^nberg, Mikrogeologie, Leipzig, 1854. 



