;hap. V] THE SOIL 91 



f the commonest cultivated trees in the tropics grow under natural 

 onditions only on the saline soil of the sea-shore ; such are Cocos nucifcra, 

 yeas circinalis, Casuarina equisetifolia. Terminalia Catappa, Erythrina 

 idica, Caloph>-llum InophyUum, and many others. There can be no 

 oubt that b}- means of the wind, of animals, of currents of water, seeds of 

 alopli\"tcs frcqucntl}' reach non-saline soil. They would there find con- 

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

 lemsclves '. The competition of more vigorous plants, however, excludes 

 :»lophytes from all localities, except those that are rich in salt. 



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

 )ils that offer favourable conditions for the majority of plants. In the 

 3urse of time many forms have been driven out of specially favoured 

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

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

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

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

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



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

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

 em to establish themselves permanently there. 

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



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

 vourable habitats. There are actualh' a number of species of plants that 

 :cur equal!)- in saline and in non-saline habitats, such as Asparagus 

 ficinalis and Samolus Yalerandi. 



4. OTHER EASILY SOLUBLE SALTS. 



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

 ncentratcd solutions over extensive areas. Other salts of similar solubilit}' 

 ipear onl\- locally in large quantities and their action on vegetation is 

 erefore less known. The presence of large quantities of alum in the 

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



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



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

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



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

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

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

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



urious action of the salt in the assimilating cells-. 



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

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

 See Schimper, I. 



