LESSON 2.] INTRODUCTION. 5 



to be applied to the extension of the solid internal skeleton of the 

 trunk and branches. 



22. At the extremities of the roots of all the more perfect plants, 

 we find a set of soft cells, making up those succulent bodies which 

 are known as the spongioles ; these are specially destined to perform 

 the absorption of nutritious fluid. 



23. This fluid, being conveyed by the vessels of the stem and 

 branches to the leaves, is here subjected to the action of the cells 

 which make up the parenchyma (pulp) of these organs. 



24. The crude, watery, ascending sap, is thus converted, by a va- 

 riety of chemical and vital operations, into the thick, glutinous 

 latex / which, like the blood of animals, contains the materials for 

 the production of new tissue, and also the elements of the various 

 secretions. 



25. This process of conversion includes the exhalation of super- 

 fluous liquid, and also that interchange of gaseous ingredients between 

 the sap and the air, which may be termed aeration ; but it involves, 

 besides these obvious chemical alterations, a new arrangement of the 

 particles of the sap, by which a variety of new products are generat- 

 ed, some of them possessing a tendency to pass into the form of solid 

 organized tissue, by a process of coagulation, when withdrawn from 

 the living vessels. To this peculiar converting process, which is such 

 an important step towards the production of perfect living tissue from 

 the crude aliments, the term Assimilation is applied. 



26. ASSIMILATION, therefore, is the peculiar process by which for- 

 eign substance is converted into the likeness of the individual. Thus 

 the food assimilated by a plant, becomes identical with itself ; and 

 on the same principle, Man, in common with other animals, assimi- 

 lates or converts his and their food. As the elaborated sap descends 

 in its proper vessels through the stem, it yields up to the growing 

 parts the nutrient materials they respectively require. These grow- 

 ing parts may be the ordinary tissues, of which the chief part of the 

 fabric is composed, and which are destined to a comparative perma- 

 nency of duration ; and in the growth and extension of these, the 

 process of Nutrition is commonly regarded as consisting. 



27. By NUTRITION is meant the food which promotes the growth 

 of plants, and promotes the growth and repairs the waste in animals. 

 On the other hand, certain groups of cells have for their office the 

 separation of peculiar products from the sap, such as oil (fixed or 

 essential), starch, resin, &c., which they store up against the time 

 when they may be demanded : these cells are said to perform the act 

 of Secretion. 



