(159) Function of Nutrition. — The first of the two general functions (art. 152.), that of nutrition, may be conveniently subdivided into about seven distinct processes or subordinate functions, which are all carried on simultaneously in different parts of the vegetable structure, more especially during those seasons of the year in which the powers of vegetation are the most active. Sometimes, only one of them is in activity, whilst the rest are either partially or entirely suspended. But as the whole of the materials which serve to nourish the plant must have been subjected to these several processes in succession, we may consider the function of nutrition to be carried on during as many successive periods, before it is completed. We will briefly mention what these successive processes are, before we enter upon the details necessary for the more accurate description of each of them. In the first place, plants absorb their nutriment by the roots; this nutriment is then conveyed through the stem into the leaves; there it is subjected to a process by which a large proportion of water is discharged; the rest is submitted to the action of the atmosphere, and carbonic acid is first generated, and then decomposed by the action of light: carbon is now fixed under the form of a nutritive material, which is conveyed back into the system; and this material is further elaborated for the development of all parts of the structure, and for the preparation of certain secreted matters, which are either retained within or ejected from the plant.