yy BIOLOGY OF PLANTS. 



but that the quantity which mimially falls is, upon an average, 

 thirty-three inches in depth. Hence the water, which so con- 

 stantly ministers to vegetation, must be distilled five or six 

 times during every year, 



6. The process of evaporation of water produces cold, hence 

 we may explain the reason that wet soils and clayey lands are 

 called cold soils. The water which is everywhere, and constant- 

 ly passing into the state of vapor, absorbs large quantities of 

 heat, so as to produce a difference of temperature in the air 

 of two adjacent fields. The only remedy is thorough drain- 

 ing. 



Such is the indispensable agency of water in the vegetable 

 economy. It is the vital fluid of plants. Upon its proper 

 regulation depends the quantity, quality and perfection of 

 most of the products of the earth, especially of those intend- 

 ed for the use of man. 



Its constitution and properties, its abundance and univer- 

 sality, illustrate the beneficent provisions of the Creator for 

 the preservation and sustenance of animal and vegetable life; 

 so that here, as in all his other operations, and throughout all 

 his works, has he shown the same skill and goodness. In 

 the midst of this unbounded profusion, nothing is wasted, 

 nothing is supplied without a purpose. Dead matter and 

 material agencies are all made subservient to the demands 

 oflife. 



Sect. 3. Influence of the Imponderable Agents upon the vital 

 Fhinctions of Plants. 



These agents are gravity, cohesion, affinity, caloric, light 

 and electricity. They are the great natural forces or causes 

 of change in the materiiU world. I shall consider them here, 

 only in their relations to vegetation, or in their influence up- 

 on the vital power. 



I. Gravity. Gravity or the attraction of gravitation is that 



