92 



BIOLOGY OF PLANTS. 



only exhibits the skill and benevolence of Him who estab- 

 lished and upholds the laws by which it is governed ; but 

 challenges the constant gratitude of those, who can offer no 

 other return for having been placed under a constitution so 

 wisely ordered for their good. 



Sect. 2. Agency of Water upon tlie Vital Functions of Plants. 



Water is a compound body, and may easily be decomposed 

 by galvanism, or by adding to it sulphuric acid and iron turn- 

 ings. By these processes it is shown to be composed of eight 

 parts, by weight, of oxygen to one of hydrogen, or of one 

 volume of the former to two of the latter ; its constituents, 

 therefore, hydrogen and oxygen, enter into the composition 

 of vegetables in large quantities. 



Water is found under three different forms, solid, liquid 

 and gaseous. 



I. In the solid form, as in ice and snow, water exerts con- 

 siderable agency upon the living functions of plants. This 

 it does, either by its influence upon the soil, or upon the seeds 

 and roots of plants. 



1. When water freezes in the soil, it tends to expand it, 

 and to break down its coarser parts ; when the surface thaws 

 in the spring, the top melts first, and there is produced small 

 apj^ertures through which the mellowing and ameliorating in- 

 fluence of the atmosphere is exerted. Heavy clay lands are 

 thus often highly benefited. 



*2. Snow is supposed to be beneficial to winter wheat and 

 other crops. This it does by protecting the crop and the 

 soil from the influence of severe cold. It forms a light, po- 

 rous covering, which is an excellent non-conductor of heat, 

 and hence prevents the warmth of the earth from escapftig. On 

 the same principle, by covering the young shoots of plants, it 

 defends them from the influence of sudden varieties of tem- 

 perature. When the rays of the sun fall suddenly upon a 



