302 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



the soil water, which frequently constitutes a determining factor in 

 plant geography. Examples occur in the cases of plants which seem 

 to require ' lime ' {calcicoles or calciphytes) and those which appear to 

 avoid it [calcijuges or oxylophytes), though frequently such prefer- 

 ences are bound up rather with questions of basicity or acidity, 

 respectively. For the soil ' reaction ' (hydrogen-ion concentration) 

 also can determine the presence or absence of particular species 

 locally. 



Thus can the varying tolerances of many plant species, etc., to 

 different environmental factors come into play as particularly 

 important phytogeographically, though it should be remembered 

 that away from their optimum sphere plants tend to be more and 

 more susceptible to competition and other influences, and hence to 

 show less and less ability to persist. Consequently it is especially 

 towards the limits of their ranges that they are liable to be most 

 narrowly restricted to special habitats involving particular environ- 

 mental conditions. 



(3) Soil atmosphere mainly occupies the interstices between the 

 soil particles or crumbs, with their covering films of water. It 

 tends to contain a slightly lower proportion of oxygen and a much 

 higher one of carbon dioxide than ordinary air, and to be normally 

 saturated with water-vapour. This may not be the case in the 

 surface layers of very dry soils, while at the other extreme water- 

 logged ones are liable to be deficient in oxygen, their lack of aeration 

 making them unsuitable for most forms of plant and animal life. 

 Normally, plentiful oxygen in the soil is necessary for the life of 

 most of the microorganisms and other inhabitants and for the 

 respiration of the underground parts of higher plants growing in 

 it, the differences in composition from the free air being reflective 

 of the gaseous exchanges involved, namely, absorption of oxygen 

 and giving out of carbon dioxide. However, interchange with 

 the free air by diffusion and other agencies appears to be fairly 

 rapid, 



(4) Organic matter, arising from the death of plants or parts of 

 plants, or of animals or added as manure, forms another highly 

 important constituent of most soils. Included are roots that decom- 

 pose m situ. Indeed all soils which bear vegetation (and according 

 to some authorities all true soils) contain dead organic matter, 

 usually more or less broken down to humus, though the amount 

 may vary from very little in fresh ' new ' soils to virtually 100 per 

 cent, of dry weight in peat and leaf-mould. In the dissemination 



