100 Hydration and Growth. 



Later, when the tubers had begun to sprout, a set was arranged to 

 obtain another series of tests, extended over a period of 22 days, with 

 many renewals of the solutions. The swellings were as follows at 

 20° C: 



Table 79. 



p. ct. 



Distilled water 34 . 4 



Citric acid. 0.01 N 24.4 



Sodium hydroxid, 0.01 M 13.8 



Potassium chloride, hydrochloric acid, 0.01 M 20 



All of the tubers, with two exceptions, were turgid and firm at the 

 end of the tests, even when extended over 22 days. Immersion in 

 any of the solutions for a day or two usually prevented sprouting by 

 killing the buds, although the remainder of the tuber kept alive. One 

 of the three tubers in water in the last test sprouted in the dish at the 

 end of a week and the excessive increase shown in this lot may be 

 attributed in part to the action of the continued hydrolysis of starch 

 as the growth of the etiolated stem proceeded. 



Stiles and J0rgensen have made an extensive series of tests of the 

 swelling of the potato, in which a wholly different technique was used. 

 Plugs were cut from tubers, from which sUces 2 mm. in thickness were 

 taken, and these were immersed in solutions in bottles. Variations 

 were taken by weight and the unavoidable difficulties of the method 

 were dealt with in an exact manner. The interpretations of the action 

 of the sections in the different solutions by Stiles and J0rgensen are 

 all based on the assumption that the hydration is one based entirely 

 on osmosis and that swelling ceases when the parenchymatous walls 

 become permeable.^ 



The conditions presented by my experiments included the action of 

 the external coat of the tuber, which is composed of non-Uving elements 

 the permeabiUty of which to water has been tested in a careful manner 

 in certain structures by Denny. The fact that the swelling was 

 greater in water than in any of the solutions might lead to the con- 

 clusion that absorption was largely by osmosis. Such an explanation 

 can not be accepted as an adequate one, however. The cell colloids 

 of the potato, being high in carbohydrates, would show the greatest 

 swelling in water, and hydration would be retarded and limited by 

 any contained acid or salt, except the amino compounds. In addi- 

 tion to this action of the living cells, the retarding action of the outer 

 coat, with its possible differential action with respect to the acid, salt, 

 and hydroxid, are to be taken into account.^ 



* Stilea, Walter, and Ingvar J0rgensen. Studies in permeability. The swelling of plant tissue 

 in water and its relation to temperature and various dissolved substances. Annals of Bot., 31 : 

 415. 1917. See also Stiles and J0rgensen. Quantitative measurement of permeability, Bot. 

 Gazette. 65: 526. 1918. 



' Denny, F. E. Permeability of certain plant membranes to water. Bot. Gazette, 63 ; 373-397. 

 1917. Permeability of membranes as related to their composition. Bot. Gazette, 63: 468- 

 486. 1917. 



