52 SCIENCE PROGRESS 



is passing outwards and would possess a correspondingly active 

 solvent action. Sachs's experiments were extended by Czapek 

 {Brings. Jahr. f. wiss. Bot. 1896, 29, 321), who contrived an 

 excellent method of making plates for the study of these 

 corrosion figures by floating a mixture of equal weights of 

 plaster of Paris and the substance to be examined upon a 

 smooth surface of plate glass. In this way could be obtained 

 delicately polished surfaces containing various carbonates and 

 phosphates of known solubility. On exposing them to the 

 action of the roots of a growing plant it was found that while 

 the phosphates of calcium, magnesium, and iron were attacked, 

 aluminium phosphate remained untouched. All the inorganic 

 acids, and also oxalic, malic, citric, lactic, and tartaric acids, 

 will attack precipitated aluminium phosphate, just as they will 

 the phosphates of calcium and the other bases specified. Ex- 

 cluding carbon dioxide, there remain among the possible plant 

 acids in the sap only two which have but little solvent action 

 upon aluminium phosphate — acetic and propionic acids, and 

 these give an intense blue colour with Congo red. But when 

 a plaster plate stained with Congo red was exposed to the 

 attack of the roots, there was no blueing to mark the line of 

 contact. Czapek thus confirmed Sachs's original conclusion 

 that the carbon dioxide secretion of the root is the seat of its 

 solvent action, and will sufficiently account for its etching effect. 



Some experiments of Schloesing {Ann. Set. Agron. 1899, 316), 

 in which he showed that plants could perfect their growth in 

 the extremely dilute solutions of phosphoric acid that can be 

 extracted from soils or result from the action upon them of slightly 

 carbonated water, provided the solution were continually re- 

 newed, also indicate the sufficiency of the carbon dioxide 

 secreted from roots to effect the necessary attack upon the soil 

 without invoking the intervention of any more permanent acid. 



Kossowitsch {Ann. Sci. Agron. 1903, 220) showed that plants 

 could not succeed in a nutritive solution made up with no 

 phosphoric acid, but which was first allowed to flow gently 

 through a pot containing sand and certain ground mineral 

 phosphates, although the same plants grew well in a similar 

 pot when the roots were in contact with the ground phosphate. 

 But in these experiments the solvent power of the carbon 

 dioxide and of any permanent acid excreted from the root 

 are equally eliminated ; they only prove that the aqueous 



