Chap. V] THE SOIL 89 



irrespective of the fact that the soil is more or less wet, for the quantity 

 of salt in such cases is alone the determining factor. Thus we find in 

 halophytcs the reduction of the transpiration surface that is so common in 

 xerophytes, as we have already described them, exhibited in their external 

 configuration, and also in their internal structure in the diminution of the 

 intercellular spaces. Moreover in halophytcs the following are more or 

 less common : the profile position of the leaves, abundance of hairs, thick 

 outer walls of the epidermis, storage-iracJieids in the leaves, sunken stoma ta 

 with protective mechanisms, mucilage-cells, and especially water-tissues. 

 This last is specially adapted to guard against injurious concentration of 

 salt in the assimilating cells, and consequently increases in size with the age 

 of the leaves and with the absolute increase within them of salt. All these 

 xerophilous characters of halophytes become weakened in ordinary soil 

 and to some extent completely disappear. 



Besides its osmotic effects, sodium chloride also undoubtedly exercises 

 a chemical action on metabolism. Hansteen has shown that it is probable 

 'that sodium chloride, as well as potassium chloride, stands in a certain 

 relation to the manufacture of proteids from amides and carbohydrates.' 

 This relation is not always the same, as it sometimes consists in a retarda- 

 tion and sometimes in an acceleration in the manufacture of proteids. In 

 any case concentrated solutions of chlorides cause abnormal conditions of 

 nutrition, and finally harmful and considerable disorders. The protective 

 means against transpiration oppose this injurious influence by delaying the 

 increase in concentration during sunlight ; the total quantity of salt in 

 the leaves does, it is true, increase with their age, but the water-storing 

 tissues also enlarge simultaneously and with increasing energy depress the 

 concentration of the cell-sap in green cells. 



Protective means against transpiration depend on adaptation, and during 

 the course of ages they have been gradually selected as useful devices. 

 Common salt, however, gives rise to more direct and intense structural 

 modifications, which, being exhibited in plants that do not grow naturally 

 in salt water and do not receive any benefit from them, cannot be considered 

 as adaptations. Thus Richter observed in fresh-water Algae, which he 

 cultivated in solutions of common salt of gradually increased concentration, 

 that a considerable increase in the size of the cells was quite a general 

 feature, and in many cases he noted modifications in configuration, in the 

 thickness of the cell-walls, in the cell-division, and in the structure of the 

 chromatophores. It has not yet been ascertained whether this was a case 

 of specific action of common salt, or whether other salts act in a similar way. 



At one time 1 supposed that common salt exercised a retarding influence on 

 assimilation, or at least on the manufacture of starch and glucose. This assumption 

 has become much less probable since Stahl demonstrated that non-halophilous 

 plants, such as those with which I experimented, close their stomata in the presence 



