100 OF THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



term of life, and completed the preparation of these germs, it bursts and sets 

 them free, every one of these being capable, in its turn, of going through 

 the same set of operations. In the highest forms of Vegetable life, we find 

 but a multiplication of similar cells ; amongst which these operations are dis- 

 tributed, as it were, by a division of labour; so that, by the concurrent labours 

 of all, a more complete and permanent effect may be produced. If we ana- 

 lyze the structure of a forest tree, for example, we find that all the soft and 

 growing parts are composed of similar cells ; whose office it is, to absorb and 

 prepare the nutriment, which is afterwards to be applied to the extension of 

 the solid internal skeleton of the trunk and branches. This latter part is not 

 concerned in the functions of vegetation, in any other way than as supporting 

 and connecting the different groups of cells, which form the operative -part of 

 the fabric; and it is composed of two forms of tissue, woody-fibre, and vas- 

 cular tissue, each of which may be regarded as originating in the metamor- 

 phosis of cells ( 120). 



107. 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 nutri- 

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

 to the leaves, is there subjected to the action of the cells which make up the 

 parenchyma of those organs. The crude watery ascending sap is thus con- 

 verted, by a variety of chemical and vital operations, into the thick glutin- 

 ous latex; which, like the blood of animals, contains the materials for the 

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

 This process of conversion includes the Exhalation of superfluous liquid ; 

 and also that interchange of gaseous ingredients between the sap and the air, 

 which may be termed Aeration ; but it involves, beside these obvious chemi- 

 cal alterations, a new molecular arrangement of the particles of the sap, by 

 which a variety of new products are generated, some of them possessing 

 such a tendency to pass into the form of solid organized tissue, as to present 

 a sort of sketch of this, by a process of coagulation, when withdrawn from 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. As the elaborated sap or latex 

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

 parts the nutrient materials they respectively require. These growing parts 

 may be either the ordinary tissues, of which the chief part of the fabric is 

 composed, and which are destined to a comparative permanency of duration ; 

 and in the growth and extension of these, the process of Nutrition is com- 

 monly regarded as consisting. 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 ; and these are said to perform the act of 

 Secretion. In both cases, however, the act is essentially the same ; for the 

 process of Secretion, like that of Nutrition, consists in the growth of a cellular 

 tissue, and the difference consists only in the destination of the contents of the 

 cells ; which, in the one case, are adapted merely to give firmness and solidity 

 to their walls ; whilst, in the other, they are set apart for some other purpose, 

 to be given up again when required. 



108. It is very important to remark, in regard to all the cells thus actively 

 concerned in the Vegetative functions, by which the development and exten- 

 sion of the permanent fabric is provided for, that they have but a very transi- 

 tory life as individuals. The Absorbent cells at the extremities of the rootlets 

 are continually being renewed ; some of the old ones dying and decaying 

 away, whilst others are converted into the solid texture of the root, and thus 



