EDGAR TULuo 



151 



pendent on the water of capillarity. This becomes scantier and scantier, the 

 sandier and coarser grained the soih ^\'ithout the supplemental ground 

 water tree growth cannot persist. 



If, in the course of years, the level of the ground water fluctuates a 

 half metre in average height the plant growth will adjust itself to the change 

 when an equilibrium has been reached. Both the water content and the 

 water requirement of plants are correlated with the soil moisture, as Hedg- 

 cock's^ comparative cultures in quartz sand, loam, salt soil, humus, etc., 

 show. 



Root activity depends also on the water content of soil and plant and 

 this activity is by no means passive but, as Sachs- and more especially 

 Molisch^ have shown, is essentially active because the secretions of the roots 

 decompose the inorganic and organic substances in the soil. The last named 

 investigator calls attention, in this connection, to the circumstance that un- 

 injured roots, in contact with a dilute solution of j^otassium permanganate, 

 become covered with a precipitate of brown stone, removing the oxygen 

 from the solution. The experiment is unsuccessful with stems and leaves. 

 With easily oxidizable bodies, as, for instance, guaiacum, pyrogallic acid and 

 humus, the root secretion acts as an oxidizer. A guaiac emulsion is turned 

 blue by it. Molisch considered the root secretion to be a self-oxidizer by 

 passive molecular oxygen, thereby making the oxygen active and bringing 

 about the oxidation of substances which are readily oxidized. In the 

 presence of tannic substances (pyrogallic acid, gallic acid, tannin,) which 

 are more easily oxidized than the guaiacum, the blue color does not appear. 

 In the same way it is absent in the presence of rapidly oxidized humus sub- 

 stances. When absolutely uninjured roots were dipped into dilute cane 

 sugar solutions, a reducing sugar became evident after some hours — probably 

 this transversion is caused by a root-ferment. Starch paste, put on the 

 growing roots of seedlings, did not give the starch reaction after a few hours, 

 but was turned a reddish violet by iodine. The starch on touching the 

 roots had been changed to erythro-dextrin and was soluble, passing over 

 into the reducing sugar. 



The root secretions, perceptible on the tips of the root hairs, not only 

 impregnate the membranes of the cells but can pass in the form of drops 

 into the deeper tissue of the roots when much water is supplied and trans- 

 piration reduced. They can erode minerals with their acids (they turn blue 

 litmus red) and decompose organic substances. This action of the roots 

 becomes less with increasing dryness. Roots, accustomed to a wet place, 

 when brought into a dry one, do not absorb as energetically even after water 

 has been supplied, if the plant has been wilted, as if it had not been wilted. 

 Hedgocock thinks that the root hairs actually die. 



1 Hedg-cock, G. G., The relation of the Water Content of the Soil to certain 

 Plants, etc. Botanical Survey of Nebraska. VI. Studies in the Vegetation of the 

 State. 1902. 



2 Experimentalphysiolog-ie p. 189. Bot. Zeit. 1860, p. 188. 



3 Molisch, H., Ueber Wurzelausscheidungen und deren Einwirkungen auf 

 organische Substanzen. Sitzb. Kais. Akad. d. Wiss., Wien, Section I, October, 1887. 



