NOVEMBEB 6, 1908] 



SCIENCE 



625 



tially different from that of plants leading 

 an independent existence, so that nothing 

 is thereby proved as to the source of the 

 etching in Sachs's experiments. 



Czapek instituted a fresh series of ex- 

 periments with smooth slabs prepared by 

 floating on to glass plates mixtures of 

 plaster of paris and various phosphates of 

 calcium, iron and aluminium; since only 

 the calcium phosphate was etched, most of 

 the possible acids were excluded and the 

 etching action of the plant's roots was re- 

 stricted to carbon dioxide or acetic acid. 

 This latter was again excluded by a 

 further experiment in which the slab was 

 colored with congo red, and as this was 

 not affected, the sole remaining solvent 

 body the plant could have excreted was 

 carbon dioxide. Again it has already been 

 shown that water cultures containing 

 nitrates, where the plant is growing in 

 such solutions as exist under normal soil 

 conditions, tend to become alkaline instead 

 of acid, so that the balance of evidence is 

 against the idea that plant roots excrete 

 any fixed acids exerting a solvent action 

 upon the soil particles. The carbon di- 

 oxide, however, probably exerts a consider- 

 able action, especially in the immediate 

 vicinity of the root from which it is given 

 off, for as it passes through the cell wall it 

 must momentarily form a solution of con- 

 siderable concentration, possessing a pro- 

 portionally increased solvent power, and it 

 is to this supersaturated solution that may 

 be attributed the highly localized attack of 

 the roots upon the soil particles. An ex- 

 periment of Kossowitseh 's illustrates the 

 part played by the roots in attacking the 

 insoluble materials in the soil: two pots of 

 sand were prepared, each mixed with the 

 same quantity of calcium phosphate in the 

 form of ground rock phosphate; a third 

 pot contained sand only. In this latter, 

 and in one of the pots containing the 

 calcium phosphate, seeds of mustard, peas 



and flax were sown. The growing plants 

 were then furnished with a slow continu- 

 ous supply of water containing appro- 

 priate amounts of nitrates, potash and 

 other nutrient salts except phosphates. 

 Before, however, this nutrient solution 

 reached the pot containing the sand only 

 it was made to percolate through the 

 second pot containing sand and calcium 

 phosphate, but it was applied directly 

 to the pot containing calcium phosphate. 

 In the pot containing calcium phosphate 

 the growth was much greater than in the 

 other pot, where the nutrient solution only 

 contained what phosphoric acid it could 

 dissolve in its passage over the calcium 

 phosphate in the pot where nothing was 

 growing, although this solution was con- 

 tinually renewed. The only factor de- 

 termining the supply of phosphoric acid 

 and the consequent difference in growth 

 was the solvent action of the roots where 

 they were actually in contact with the 

 calcium phosphate, and this solvent action, 

 as has already been shown, may most prob- 

 ably be attributed to the carbon dioxide 

 excreted by the roots. 



Following up their conclusions that the 

 soil water possesses an approximately con- 

 stant composition under all circumstances 

 and always contains more of the constitu- 

 ents of plant food than would be required 

 for the nutrition of the plant, Whitney 

 and his colleagues have suggested another 

 theory of fertilizer action. According to 

 this point of view a soil falls off in fertility 

 and ceases to yield normal crops, not be- 

 cause of any lack of plant food brought 

 about by the continuous withdrawal of the 

 original stock in the soil, but because of 

 the accumulation of injurious substances 

 excreted from the plant itself. These 

 toxins are specific to each plant, but are 

 gradually removed from the soil by pro- 

 cesses of decay, so that if a proper rota- 

 tion of crops be practised, to ensure that 



