THE ADAPTATION OF THE PLANT TO THE SOIL. 



17 



factors wiiich I have just indicated. For example, if aii uukiiown soil 

 is subjected to analysis one is able at once to say if the proportion of 

 sand and clay are such as brmg it within the limits which have been 

 found by experience suitable to the growing of ordinary mixed fruit. We 

 can further say that the soil might, on the one hand, be of a character 

 that would not permit of the long duration of fruit-trees; or, on the 

 other hand, of a character that would result in very slow growth. We 

 can also indicate the methods which might be adopted to ameliorate 

 the soil. Again, we might begin by the analysis of a number of soils 

 on which roses are known to grow well, and so obtain a general idea 

 of the composition of a rose soil, from which we may predict the 

 behaviour towards roses of any unknown soil by its analysis alone. 

 It should be noticed, however, that we are forced to proceed empiri- 

 cally ; we have to analyse a number of soils which experience has shown 

 to be suited to the plant in question before we can arrive at our type. 

 In the present state of our knowledge it would be impossible to predict 

 d priori the requirements of a plant which has not been under trial. 

 This impossibility is particularly due to the fact that very little experi- 

 mental work has yet been done on the nutrition requirements of the 

 plant under natural conditions. To take an example, we know that 

 all leaves are constantly transpiring water, and that the amount of 

 water evaporated through the plant will be roughly proportionate to 

 the amount of growth it makes ; certain measurements have been made 

 which indicate that for every pound of dry matter grown by the plant 

 from 250 to 500 pounds of water will be evaporated thi^ough the leaves. 

 The measurements also indicate that this factor must be a variable 

 one, depending upon the dryness of the air, the amount of sunshine, 

 wind, and similar external conditions ; but no exact experimental know- 

 ledge is available. Again, we are accustomed to regard a waxy coating 

 to the leaves, coverings of hair, &c., as devices for checking trans- 

 piration, though how far they do so has not, I believe, been experi- 

 mentally demonstrated. Our knowledge, then, of the associations of 

 given plants with given types of soil as determined by mechanical 

 analysis will remain empirical for some time to come, and in our 

 empirical method we must beware of being deceived through the effects 

 of competition in nature. To return to our old illustrations, the 

 mechanical analysis of a shingle bank would not reveal the kind of 

 material the horned poppy likes best, although it is only to be found* 

 in such a habitat. In order to find the best type we must make our 

 comparisons under strictly similar conditions, such as is attained in 

 land under cultivation. 



Turning now to the aspect of the soil which we have hitherto set 

 aside— the chemical factors determining its association with given plants 

 — the most important is probably the acidity or otherwise of the soil. 

 As a rule our cultivated soils are kept neutral or in a very faintly alkaline 

 condition by the presence of carbonate of lime, but in nature many soils 

 occur which have developed a faintly acid reaction because they started 

 with little or no carbonate of lime and the decay of vegetable matter 

 VOL. xxxvi. c 



