262 FLORA OF THE WEST YORKSHIRE HILLS. 



ponding change of vegetation take place, but at a much more 

 rapid rate. 



Dalton, the famous chemist, calculated that for every rise of 

 three hundred feet, there is a fall in the temperature of i*^ F. 

 This rate of decrease, however, is not constant, but for the com- 

 paratively low elevations of Yorkshire it is approximately correct, 

 and as such is sufficient for present purposes. By a simple 

 calculation it is easy to find out that the difference in temperature 

 between sea level and the tops of the highest hills in this county, 

 two thousand four hundred feet in height, amounts to 8° F. The 

 corresponding decrease of temperature caused by latitude is ex- 

 tended over 20°, or to a point as far North as the south of Iceland. 



(2) Moisture. — Plants are influenced in their distribution by 

 the presence or absence of moisture, in a greater or less degree, 

 just as much as they are by increased or decreased temperature. 

 Some love damp situations, and others prefer dry ones, and though 

 many plants have remarkable powers of adaptation to altered 

 circumstances, they often fail to thrive when taken from their 

 natural surroundings and exposed to the opposite conditions, 

 especially if the temperature also becomes changed. 



It is a well-known fact that mountains, lifting their heads 

 amongst the clouds, wring moisture from them, and from the 

 warm, vapour-laden currents of air which impinge against their 

 cold crests. Hence arise the numerous shallow pools, and the 

 springs bubbling out of the ground near mountain summits, and 

 the rivulets meandering down each little gill. The plants which 

 make their home in elevated regions, find this humidity essential 

 to their well-being. 



(3) Soil. — Many plants show a preference for certain soils, or, 

 to be exact, certain constituents of the soil ; and the spread of 

 this knowledge amongst agriculturalists has led to their adoption 

 of the system of rotation of crops. It is often possible to tell the 

 kind of rock (using the word in its geological sense) which appears 

 on the surface, from the flowers growing upon a particular spot, 

 and often, too, from the flowers which are " conspicuous by their 

 absence." For example, many of our common grasses which 

 abound in fields, and yellow-flowered plants, like the dandelion 

 and buttercup, are not found on a peaty soil, but, instead, heather 



