THE ADAPTATION OF THE PLANT TO THE SOIL. 



19 



As soon as the land is limed and made neutral, the hacteria come back 

 and the peaty material slowly disappears, being oxidized and burnt up 

 by the bacteria which re-establish themselves. In other respects the 

 acidity or otherwise of the soil has a very potent effect upon certain 

 special kinds of plants. 



We know that many plants, notably heaths, Coniferae, and orchids, 

 do not draw their nutriment directly from the soil, but depend 

 upon the mycelium of certain fungi which clothes their roots. Such 

 plants grow, as a rule, in acid soil, where the production of ammonia 

 and nitrates by bacteria is stopped, so that the plants depend for their 

 nitrogen supply upon the power that the fungi associated with their 

 roots (mycorhiza) possess of attacking and digesting the otherwise in- 

 soluble nitrogen compounds of the humus. The mycorhiza plants 

 may even be recognized by their external appearance. Thus, as in 

 the Rothamsted experiments, cereals fed upon ammonia are shorter in 

 the straw and broader in the leaf with a deeper green colour than 

 those supplied with nitrate, so, according to Stahl, the plants fed by 

 mycorhiza possess a peculiar sm.ooth shiny foliage and a distinctive 

 colour. Naturally enough when any of these plants dependent upon 

 mycorhiza are transferred to an alkaline soil in which their associated 

 fungi do not flourish, they in their turn lose their vitality and may 

 succumb entirely. Even if the plant is given a soil rich in humus but 

 not containing the fungus with which it is usually associated it may 

 refuse to grow. This has been found to be the case with orchids, the 

 seeds of which only develop properly if sown on a medium partly 

 derived from that on which the parent plant was growing. 



So numerous are these cases of the association of a particular plant 

 with acid or alkaline soils that it would seem to be a point worth watch- 

 ing by the practical cultivator of some of the difficult plants like 

 alpines. It is, indeed, customary to consider whether the habitat of the 

 plant is sandy, peaty, calcareous, or granitic, but two distinct factors 

 are involved and may be confused. The soil may be acid or neutral, 

 but it is not necessarily acid if it is sandy or even peaty; secondly, 

 the soil may be calcareous or not, but if calcareous it is never acid. 

 It is a fact of observation that certain plants are rarely or never 

 found upon calcareous soils; for example, the foxglove, corn marigold, 

 the sheep's sorrel, the cultivated Rhododendron, and many of the 

 Ericas seem to be intolerant of such soils. In such cases, however, 

 we have probably not positive poisoning by the lime but negative 

 action due to the suppression of the acidity which these plants require. 

 In other words, such plants are probably positively acid-humus-loving 

 rather than lime-hating, though as the presence of carbonate of lime is 

 incompatible with acidity they may appear to be negatively correlated 

 w^th lime. 



On the other hand, it can hardly be doubted that there is what we 

 might call a positive calcareous factor, so specially associated are 

 certain plants with soils rich in carbonate of lime, and so entirely are 

 they absent from other soils which are perfectly neutral, but which 



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