ASH. II 95 



mentioned give a blue reaction with congo-red. [Prianischnikow has shown 

 that Czapek's conclusions are by no means above criticism (Ber. d. bot. Gesell. 

 1904, 22, 184). At the present moment we have no certain knowledge as to 

 what acids are given off by the root. More recently, Prianischnikow (Ber. d. 

 bot. Gesell. 1905, 23, 8) has proved that phosphates which are soluble with 

 difficulty in presence of salts of ammonia are available for the nutrition of plants, 

 in proportions quite different from those available when potassium nitrate only 

 is present.] 



It will be shown later that all plant cells produce abundant carbonic acid, 

 and its presence in sufficient quantity in this relation cannot be doubted. 

 Again, from the sharpness of the outlines of the corrosion figures it has been 

 concluded that they must have been produced by a non-volatile acid ; 

 'carbonic acid,' as Sachs says, 'penetrates freely into the interspaces in the 

 soil, and hence one might expect to find corrosion in regions at some distance 

 from the root.' But Sachs's conclusions cannot invalidate Czapek's view, viz. 

 that the carbonic acid alone produced corrosion in the substances referred to ; 

 for one must remember, in the first place, that carbon-dioxide does not occur 

 as a gas, but in solution in water, and in the second, that the water with carbon- 

 dioxide in solution is markedly present in the cell-walls of the root epidermis, and 

 does not readily exude therefrom. If the solution be in intimate contact with 

 the minerals, and if the dissolved particles be at once absorbed into the interior 

 of the cell, in course of time a quite observable effect will be produced. 



Although we may look on the carbonic acid as the factor concerned in the 

 formation of the corrosion figures on certain kinds of rock, and although, further, 

 we have already ascribed to this substance, apart from the plant, an important role 

 in the disintegration of rock, we are not entitled to affirm that the root is un- 

 able to excrete other substances which may be instrumental in opening the soil 

 up. As we have already said, it has been generally accepted that the root 

 gives off organic acids, and this view was held to be supported by the fact 

 that organic acids could be demonstrated in the cells of the root and by the red 

 colour often given to litmus paper when roots were pressed against it. To Czapek 

 we owe a reinvestigation of this problem. He has confirmed the observation 

 previously made of the excretion of minute drops from roots grown in spaces 

 saturated with moisture, and has further shown that, as in the case of subaerial 

 hydathodes, this phenomenon takes place only when the plant as a whole is 

 turgid. Czapek also found that these drops gave a neutral reaction. Again, 

 Czapek has grown roots in a minimum amount of water or on small pieces of filter 

 paper, and after a time has submitted the water or filter paper to microchemical 

 analysis. He finds potassium and phosphoric acid not infrequently present in 

 quantity, smaller amounts of magnesium and chloride, and traces of calcium. 

 The reaction of these fluids is on the whole acid, owing to the presence of acid- 

 potassium phosphate, in other cases of acid salts of formic acid and (only in one 

 case) of oxalic acid. [According to Prianischnikow (1904, Ber. d. bot. Gesell. 22, 

 184), the excretion of phosphates is quite possible in seedlings, since these bodies 

 are plentifully produced in the process of decomposition of proteids ; this, how- 

 ever, is not the case in mature plants.] These bodies must have been excreted 

 from the uninjured root-cells. Looked at scientifically these investigations are not 

 above criticism, for it may be assumed that cell-sap from dead roothairs and 

 from the dead cells of the root-caps was present in the fluid examined, and that in 

 the experiments with filter paper injury was done to the roothairs in the process of 

 cutting off of the rootlets, so that, in both cases, the occurrence of the substances 

 referred to above might he explained without assuming that they were excreted 

 from living cells. At any rate it is important to note that the reaction of this 

 sap is acid as opposed to the result of Czapek's investigations on the drops ex- 

 creted from roothairs. « 



It may be further noted that the acids arising from the dead roothair 



