84 BRITISH PLANTS 



The Relations between Soil and Climate. 



In Chapter I. we considered the factors determining 

 climate, and we learned that differences in the vegetation 

 are associated with differences in climate. But climatic 

 conditions are the same over wide areas, and yet marked 

 and profomid differences may be found in the flora. We 

 cannot go far from our doors without seeing this. In 

 some localities we may, in a few hours' walk, pass by cul- 

 tivated fields, wet meadows, rough fields, dry pastures, 

 reedy swamps, and several kinds of woods and thickets. 

 The climate is the same for all ; it could hardly vary in 

 the course of a short walk. The same is true when we 

 examine the vegetation in an upward direction. Hills 

 of the same height may carry very different kinds of 

 plants. One may be covered with reedy bogs, another 

 with gorse and heather, and another with grassy pastures. 

 Wood differs from wood, and field from field. Even in 

 the same field, when we come to details, many differences 

 are apparent ; the vegetation on the dry rising knolls is 

 not the same as in the damper hollows, and disturbed 

 patches can be detected by their distinctive plants, even 

 from a distance. What is the cause of all this variation 

 within the limits of practically the same climatic condi- 

 tions ? It is the soU. All soils do not react in the same 

 way to climate. The sun's rays will warm one kind of 

 soU much more than another ; with the same rainfall some 

 soils become wetter and remain wetter than others. In 

 a short journey we may tread many kinds of soil, each 

 bearing its own characteristic plants. Climate deter- 

 mines the broad features of the vegetation, but the minor 

 differences in the flora are due to the character of the soil 

 and the way it behaves towards the external sources of 

 heat and moisture. 



The relations between soil and climate must therefore 

 be considered under two aspects : 



I. The reaction of the soil to water. 

 II. The reaction of the soil to heat. 



I. Soil and Water. — The most important soil or edaphic 

 factor is the amount of available water present. This 

 clearly depends upon — 



