Chap. V] THE SOIL 91 



of the commonest cultivated trees in the tropics grow under natural 

 conditions only on the saline soil of the sea-shore ; such are Cocos nucifera, 

 Cycas circinalis, Casuarina equisetifolia. Terminalia Catappa, Erythrina 

 indica, Calophyllum Inophyllum, and many others. There can be no 

 doubt that by means of the wind, of animals, of currents of water, seeds of 

 halophytes frequently reach non-saline soil. They would there find con- 

 genial conditions, if their competitors did not hinder them from establishing 

 themselves l . The competition of more vigorous plants, however, excludes 

 halophytes from all localities, except those that are rich in salt. 



It is evident that the struggle for space has always been most severe on 

 soils that offer favourable conditions for the majority of plants. In the 

 course of time many forms have been driven out of specially favoured 

 localities by competitors that have become stronger than they. Many of 

 these conquered forms have perished, while others have owed their persistence 

 to certain characters by means of which they were enabled to colonize 

 unhealthy territories. Thus, of the expelled plants, those were able to find 

 a refuge on saline soil that had already on ordinary soil become accustomed 

 to store up plenty of common salt and had thus been rendered immune 

 from its poisonous action. The reduced competition on saline soil permitted 

 them to establish themselves permanently there. 



The property of storing salt and existing intact on saline soil does not 

 in itself of course render it impossible to continue the struggle in more 

 favourable habitats. There are actually a number of species of plants that 

 occur equally in saline and in non-saline habitats, such as Asparagus 

 officinalis and Samolus Valerandi. 



4. OTHER EASILY SOLUBLE SALTS. 



Sodium chloride is the only easily soluble salt which saturates the soil in 

 concentrated solutions over extensive areas. Other salts of similar solubility 

 appear only locally in large quantities and their action on vegetation is 

 therefore less known. The presence of larg'e quantities of alum in the 

 warm swampy soil of the solfataras of Java and Japan causes the appearance, 

 in the centre of hygrophytic regions, of xerophilous plants, which are not, 

 as in ordinary saline soil, in part peculiar to the habitat, but are individuals 

 that have emigrated from the nearest habitat of xerophilous plants. Some 

 of them are plants that elsewhere grow as epiphytes on dry bark, some are 

 immigrants from cool dry alpine regions. The factors which render xero- 

 philous structure a condition vital to these plants, are evidently the same as 

 those in the case of common salt, namely, difficulty in absorbing water, and 

 injurious action of the salt in the assimilating cells 2 . 



1 On page So has been described the appearance of elsewhere exclusive littoral halo- 

 phytes in the interior of Krakatoa, where there is not yet any competition. 



2 See Schimper, I. 



