26 PLANT PHYSIOLOGY 



96, 11. 23-35, f or What the materials are . . . mineral acids read Indeed 

 KUNZE (1906) has recently clearly established the fact that acids are excreted 

 from root-hairs, at least in many plants a fact that previously had been often 

 stated ; in some cases he was able to show that formic acid was excreted, 

 but in other cases the chemical nature of the excretion has not as yet been 

 determined. Many plants indeed give off so little acid from their root-hairs 

 that no red reaction is given by litmus. It must further be noted that it is 

 not from living cells alone that substances are produced that are of importance 

 in rendering the soil soluble. Dead root-hairs also add acid sap to the soil, and 

 we know that root-hairs are very short-lived. Again, the exuviated and decom- 

 posing cells of the root-cap have similar effects on intractable minerals. 



If substances which are soluble only with difficulty are to be absorbed 

 by the plant they must be in an extremely fine state of division, and must 

 be presented to a much more extensive absorbent surface than is the case 

 with the plates above mentioned. Thus in CRONE'S culture solution we have 

 seen that finely pulverized calcium phosphate and ferric phosphate may serve 

 as the source of iron and phosphoric acid to the plant, and similarly mica 

 (PRIANISCHNIKOW, 1905 b) may be employed if the plant be permitted to 

 come in contact with it at many points. It should also be noted that a dis- 

 solution of nutrients may be effected by the root taking up certain kations 

 from the medium, and thus rendering it acid, which acidity in its turn has 

 a solvent effect. Thus PRIANISCHNIKOW (1901) showed that grasses could 

 employ that very intractable mineral apatite as a source of phosphoric acid, 

 only if the nitrogen was presented to them in the form of ammonium sulphate 

 (instead of potassium nitrate). He explained this by supposing that the 

 PH 4 -ions were rapidly absorbed, thus releasing the sulphuric acid, which in 

 its turn had a solvent action on the calcium phosphate. His more recent 

 researches (1905 a) have established the same solvent action for ammonium 

 nitrate, but whether this salt operates in the same way must remain for the 

 present undecided. It is very probable that under natural conditions Bacteria 

 also frequently play a great part in the dissolution of soil (KocH, 1906), but 

 it cannot be at present estimated how far Bacteria co-operated in PRIANI- 

 SCHNIKOW'S experiments. 



11. 49-50, for possess none. Plants which . . . \vater-cultures read possess 

 none, or, under certain circumstances, e.g. in water-cultures, do not develop any. 



97, 1. 7, after humus, read Statements are not wanting (e.g. MOEBIUS, 

 1904) according to which roots grown in a sandy soil poor in nutrients are 

 more extensively branched than when grown in good soil, and this fact also 

 can be readily explained biologically. Further research must, however, show 

 whether or not the difference between these divergent observations is due 

 to the fact that in one case the whole root, in the other only parts of it, occurred 

 in bad soil. It must also be noted in this relation that an acceleration of growth 

 has often been observed in roots (compare BENECKE, 1903) when grown in 

 media which were deficient in nutrients more especially in nitrogen. 



98, 11. 9-10, for for no soil . . . from it. read for it is very improbable that 

 ordinary sandy soils are so poor in calcium that some plants cannot obtain 

 all that they require from them. 



1. 31 P. 99, 1. 21, for very quickly ; solutions ... II, p. 124).] read very 

 quickly. PAUL (1906) has shown that quite a number of species of Sphagnum 

 cannot tolerate a solution of 0-03 to 0-008 per cent, of calcium carbonate, whilst 

 they are practically indifferent to gypsum. OEHLMANN (1898) has obtained 

 similar results, and the statements to the contrary made by WEBER (cited by 

 SOLMS-LAUBACH, 1905) are not perfectly intelligible. 



