NUTRITION IN CELLULAR PLANTS. 223 



the same surface performs alike the functions of absorption, exha- 

 lation, digestion, and respiration, there is no general circulation 

 of fluid ; since each of the cells composing the whole structure, 

 imbibes the materials of its nutriment for itself, and converts 

 them into the substance of its own tissue, or employs them in 

 the production of new cells. These seem to be usually developed 

 from the fluid within the parent, in the same manner as the cells 

 of the young seed are produced from the gummy matter it con- 

 tains, as just now described ; and the analogy is the more close, 

 since the membrane lining the seed may be regarded as itself a 

 single large vesicle. The increase in size of any organ, is occa- 

 sioned in part by the enlargement of each individual cell, and in 

 part by the development of new ones ; which are formed in some 

 instances between those previously existing, and in other cases 

 (especially in the root) at the extremity only. 



358. In the simplest Cellular plants, therefore, there is no 

 necessity for any general circulation of fluid; and no other move- 

 ment is seen but that which occurs in single cells. But in the 

 more highly-organised tribes, where the parts which receive the 

 different kinds of food from the elements around, are at a distance 

 from each other, and from those to which the nutritious fluid 

 must be supplied, a general circulation is required, to bring 

 them all into connection ; and this is accordingly found to exist, 

 so that every part of the structure is nourished by a fluid, that has 

 been elaborated by a system of organs, of which each is parti- 

 cularly adapted to a single object, whilst the actions of all are 

 directed to a common purpose. This elaborated sap, being sup- 

 plied to the growing parts of a plant, gives to them all the means 

 of development that they can require ; and they then only need 

 the influence of light and heat, to perform their respective actions 

 with vigour. These actions, however, are all performed, in the 

 highest plant, as in the lowest, by cells alone ; the vessels serv- 

 ing merely as conduits for the passage of fluid ; and the woody 

 fibres, where they do not assist in this office, being destined 

 merely to give mechanical support to the fabric. It is by the 

 cells of the spongioles, that the fluid taken in by the roots is 

 absorbed ; it is by the cells of the leaves and other green parts, 



