124 



PHYSIOLOGY OF NUTRITION 



DeVries determined the partial osmotic pressure developed by some of the 

 constituents of the cell sap. The following table gives an idea as to what sub- 

 stances are instrumental in the production of osmotic pressure in plants. The 

 figures denote percentage of the total pressure. 



Diffusion in solution is very important in the absorption of materials by 

 plants but it cannot account for the transfer of absorbed substances within the 

 plant, for movement by diffusion alone is much too slow. 1 For example, it 

 would take 319 days for 1 mg. of sodium chloride, a rapidly diffusing substance, 

 to diffuse 1 m. out of a 10 per cent, solution of that salt. A period of fourteen 

 years would be required for the same amount of albumin to migrate the same dis- 

 tance. Since diffusion progresses rapidly in gelatine and agar as well as in water, 

 these substances may be employed in diffusion experiments, being poured into 

 a glass cylinder while hot and then covered, after cooling, with a solution of the 

 substance to be studied {e.g., indigo). Intercellular protoplasmic connections, 

 like thin threads reaching through the cell walls, are now known to be of common 

 occurrence in plants (Fig. 72). How these structures may influence exchange 

 of materials between the cells is still unknown, however. 



very dry habitats than for those growing in more moist situations. For further studies bearing 

 on this and related matters, see: Dixon, H. H., and Atkins, W. R. G., On osmotic pressures in 

 plants and on a thermo-electric method of determining freezing points. Proc. Roy. Dublin 

 Soc, n.s. 12: 275-311. 1910. Idem, Osmotic pressures in plants. I. Methods of extracting 

 sap from plant organs. Ibid, n.s 13: 422-433. 1913. (Reprinted in: Notes from Bot. Sch., 

 Trinity Coll., Dublin 2 : 154-165. 1913.) Idem, same title, II. Cryoscopic and conductivity 

 measurements on some vegetable saps. Ibid. n.s. 13: 434-440. 1913. (Reprinted in: Notes 

 from Bot. Sch., Trinity Coll., Dublin 2: 166-172. 1913.) Harris, J. Arthur, and Lawrence, 

 John V., assisted by Gortner, R. A., The cryoscopic constants of expressed vegetable saps 

 as related to local environmental conditions in the Arizona deserts. Physiol, res. 2: 1-49. 

 1916. (Other papers are there referred to.) Hibbard, R. P., and Harrington, O. E. , De- 

 pression of the freezing-point in triturated plant tissues and the magnitude of this depression 

 as related to soil moisture. Ibid. 1 : 441-454. 1916. For a general discussion of the osmotic 

 relations of cells see: Atkins, W. R. G., Some recent researches in plant physiology, xi + 

 328 p. London, 1916. — Ed. 



1 Stefan, J., Ueber die Diffusion der Fliissigkeiten. II. Berechnung der Grahamschen Versuche. 

 Sitzungsber. (math.-naturw. Kl.) K. Akad. Wiss. Wien 79II : 161-214. 1879. Vries, Hugo de, Ueber die 

 Bedeutung der Circulation und der Rotation des Protoplasma fur den Stofftransport in der Pflanze. Bot. 

 Zeitg. 43: 1-6, 17-26. 1885. 



