Osmotic Pressure in Pi, ants. 163 



gradient corresponding to that in a non-saline soil, ' ' seems to 

 apply only to saline soils whose solutes are not to tic to the plants. 

 Of course a chemically poisonous substance existing in the soil 

 in comparatively low concentration may produce apparently 

 the same effect upon plants growing therein (dwarfing, loss of 

 leaves with increasing eva orating power of the air or with 

 increasing sunshine intensitv, etc.,) as does a nontoxic salt at 

 a much higher concentration. * 



As 1 have long since pointed out, j the stimulating effect 

 produced by the toxic materials of an unproductive soil upon 

 plants growing therein seems to be primarily due to a more or 

 less complete inhibition of the formation of laterals and an 

 early senescence of the primaiy roots, rendering them very 

 poor absorbers of water. Such phenomena probably occur 

 also in the acid humus soils t which, since Schimper's time, 

 have been called ''physiologically dry."' These considerations, 

 while not of the quantitative nature which Fitting quite right- 

 ly desires as a basis for our theory of physiological aridity, do, 

 it seems to me, render the general problem somewhat clearer 

 than our author's sentences would indicate. There are appar- 

 ently two very distinct kinds of physiologically dry substrata. 

 In one of these the resistance to water absorption by plants 

 is due simph to a high osmotic concentration of the soil solution,^ 

 With this form Fitting has had to deal in his work upon the 

 Biskra salt-spots. In the second kind, the osmotic pressure of 

 the soil solution is not markedly higher (indeed it is sometimes 

 actually somewhat lower* •=)than that in non-arid soils, but there 

 are here present small amounts of chemically toxic materials. 

 In the presence of these substances root development is mark- 

 edly abnormal and the abnormal root structures may hinder the en- 

 trance of water from the soil. Thus, we may conclude that in the 

 first variety of physiologically arid soils the hindrance to water 



*The reader may refer, for examples, to the interest'ng papers of Kand^ and of Jen- 

 sen upon the toxir limit'; for plant activity in regard to salts of copper, zi"c, nickel, etc 

 in the soil: — Kanda M .Stiidieniiber die Reizwirkung einiger Metallsalze auf dasWachatum 

 hjherer Pflanzen. Jour. Coll Sci Imo. Univ Tokyo 11: Article 1.?, pp. 1-37 1904 Jen- 

 sen. G. H , Toxic limits and sti.mulation effects of some salts and poisons on wheat Bot 

 Gaz. 43: 11-44. 1907. 



tLivingston, B. E., Note on the relation between growth of roots and tops in wheat 

 Bot. Gaz 41 139-143. 1906. See also Schreiner, O , Some effects of a harmful organic 

 soil constituent. Bot. Gaz. 50: 161-181. 1910. 



tTivingston. B. E., Physiological propreties of bog water, Bot. Gaz, 39 :348-3 55. 1905. 

 Dachnowski A , Phvsiologi-^a'.ly arid habitats and drougth resistance in plants. Bot Gaz 

 325-339. 1910. 



* *Livini{ston. B. E.. Physical properties of bog water. Bot. Gaz., 37: 383-385. 1904 



