Chap. V] THE SOIL 87 



Besides the relatively simple and direct physical and chemical actions 

 mentioned above, salts exercise a more or less visible but indirect influence 

 on the structure of vegetable organisms. Easily soluble salts evoke 

 protective means against transpiration identical with those accompanying 

 life in a dry soil, and these for the most part are to be ascribed oecologically 

 to the impeded absorption of water *. Such protective means are met 

 with both when the soluble salt is nutritive, as for instance saltpetre, and 

 when it is useless and not assimilable, like sodium chloride. Yet in the 

 latter case the salts commence to act in less concentrated solutions and 

 with greater intensity. From this we may learn that protective means 

 against transpiration oppose the increasing concentration of a salt that soon 

 becomes poisonous, and consequently in the case of injurious salts they step 

 in earlier than would be necessary if the salts in the substratum merely 

 rendered difficult the absorption of sufficient water to cover the loss due to 

 transpiration. Whilst the protective means against transpiration appear to 

 be influenced only as to their earlier or later occurrence by the chemical 

 differences in the substances absorbed, these act in a definite specific way, 

 which perhaps resembles the changes induced in their host-plant by 

 certain fungi. Many of the modifications thus caused in the structure of 

 plants have a decidedly pathological character, and rarely, or never, occur 

 under natural conditions. Others, on the contrary, in no way impair the 

 vitality of the plant, and are extremely important in rendering com- 

 prehensible the diversities in the floras of soils that differ chemically. To 

 this latter class belong, independently of the protective means against 

 transpiration mentioned above, the modifications caused in the structure of 

 plants by sodium chloride, salts of zinc, serpentine (a silicate of magnesium), 

 and calcium carbonate. 



3. SODIUM CHLORIDE. 



i. PRESENCE AND FUNCTION OF SODIUM CHLORIDE IN PLANTS. 



It has been proved by means of cultures in artificial nutritive substrata, 

 especially in culture-solutions, that sodium chloride is of no significance as 

 a nutritive material in the case of most plants. For the normal develop- 

 ment of Fagopyrum esculentum this salt has, however, been found necessary, 

 and it is probably necessary for some other plants also, although the 

 number of species with which experiments have been conducted is rela- 

 tively small. 



Plants that require sodium chloride can obtain it in all natural soils, for 

 only a small amount is probably needed in each case. All plants, how- 

 ever, to which chlorides, especially sodium chloride, are offered, actually 



1 See p. 4. 



