32 



BIOLOGY OF PLANTS. 



In Fig. 2, c represents the pith, b the heart wood, a the al- 

 burnum, II m the bark. The annual layers of wood, the medul- 

 lary rays and the tubes are also represented in tliat figure, which 

 is a representation of a section of the branch of a tree. 



10. The process of converting tlie cambium into alburnum 

 and other vegetable substances, such as sugar, gum, starch, etc. 

 is called assimilation ; that of rejecting matter by the roots, ex- 

 cretion and by the leaves, transpiration. When substances are 

 thrown off from the leaves, they are also said to be exhaled ; and 

 when they are taken in by the leaves, they are said to be inhaled 

 or absorbed. 



Agricultural Chemistry is a term which has been generally used 

 to denote the ai)plication of science to Agriculture. It attempts 

 to explain the ijifluence of earth, air and water upon plants. 



That branch of the subject, which relates to the soil, as formed 

 of simple minerals and rocks, is sometimes called Agricultural 

 Geology. But as the chemical and geological agents are, in the 

 processes of vegetation, subjected to the principle of life, it seems 

 more appropriate to include the vital principle; the conditions 

 of its action ; the influence of other agents upon it ; its produc- 

 tions, with their composition and source, under the term Biol- 

 ogy of Plants ; while the subject of soils and manures is placed 

 under the term Geology and Chemistry of Soils. 



Plants and animals differ in many respects from each other, 

 as in their structure, in the nature of their food and in the 

 mode and time of taking and digesting it, as well as in being 

 governed by many different laws, but yet they both agree in 

 possessing a living principle. This power is probably the 

 same in both, and is characterized by its operations, and by 

 its pervading every part of organized bodies. Mechanical 

 and chemical agents are subordinated to it, in the living sys- 

 tem, and would be wholly inefl^cient without it. 



In the stomach of animals, for example, this power is the 

 principal agent in elaborating the juices required to digest the 

 food. It enables the lacteal^ or small tubes which open their 

 mouths into the alimentary canal, to select, from the general 

 mass, whatever is fitted for nourishment, while it permits what 

 is injurious or useless to pass by. It sends this to the heart, 



