GENERAL SIGNIFICANCE OF RESULTS. 17 



The increased thickness of the cuticle and of the deposit of wax on 

 the leaves observed in the experiments described in these pages can 

 safely be attributed to the influence of an excess of soluble salts in 

 the soil, since the presence of these salts in different concentrations 

 or their absence was the only variant introduced. Pfeffer a is of 

 the opinion that as a rule the cuticle is more strongly developed 

 when there is a scarcity of soil moisture. But even under extremely 

 arid climatic conditions, soils containing a large amount of soluble 

 salts are usually in a moist condition. It is generally believed, how- 

 ever, that an excess of soluble salts in the soil will check the absorb- 

 ing activity of the roots, thus creating a condition of " physiological 

 drought." 



In all the experiments, the results of which are summarized in 

 Tables I, II, and III, the plants in saline soils and the controls grown 

 in nonsaline soils were given an equal amount of water, yet the plants 

 grown in saline soils modified their structure by depositing bloom 

 on the leaf surface, by tliickening the cuticle, and by reducing the 

 size of the epidermal cells. 6 It would seem then that the plants in 

 saline soils, although furnished an amount of soil moisture that 

 was sufficient to produce a normal growth in the nonsaline soil, 

 were actually subjected to xerophytic conditions. The explana- 

 tion appears to be that the roots of the plants in saline soils were 

 unable to take up moisture as readily as those in nonsaline soils, 

 and the plants were therefore forced to modify their structure in the 

 manner above described in order to reduce their transpiration. 



So far as the results of these experiments can be regarded as con- 

 clusive, it may be said that when wheat plants are grown in a soil 

 containing 0.7 to 1.4 per cent of sodium chlorid in addition to other 

 salts the plants begin almost immediately after germination to take 

 on xerophytic characters. 



An indirect influence of the salt on transpiration is also shown 

 (see Table IV), since the leaves of wheat plants grown in a soil con- 

 taining 1.5 per cent total salts (1 per cent of sodium chlorid) lost 

 considerably less water than the leaves of plants grown in nonsaline 

 soils. c The decrease of transpiration from leaves of plants grown 

 in saline soils when compared with those of the controls may be 

 attributed to two factors: (1) The deposit of wax or bloom on the 



a Physiology of Plants (Ewart's Translation), I: 239. 



h Kissel (Der Ban des Gramineenhalmes unter dem Einflus verschiedener Diingung. 

 Inaug. Diss. Giessen, 1906. Review in Bot. Centralbl., 109:403, 1908) found that 

 phosphoric acid caused a thickening of the cell walls, and a diminution of the cell 

 cavities in the stems of grasses. On the other hand nitrogen arid lime induced a con- 

 trary effect. Results with potash were inconclusive as regards oats, but in the case 

 of other grasses the effects were similar to those of nitrogen and lime. 



c These results are in accord with those of Reynolds previously referred to. 

 134 



