A manual of botany / by J. Reynolds Green.

  • Green, J. Reynolds (Joseph Reynolds), 1848-1914.
Date:
1897-1902
Catalogue details

Licence: In copyright

Credit: A manual of botany / by J. Reynolds Green. Source: Wellcome Collection.

    437/540 (page 417)
    the raw materials in the way described in the last chapter. The complex products of the anabolic process are at first mainly, if not entirely, confined to two classes, carbohydrates, and some nitrogen-containing compound, probably proteid. Many other forms of organic substance appear in plants, such as fats or oils, glucosides, &c., but these are probably all produced by processes of catabolism, and are not directly constructed from the simple bodies absorbed. We will examine first the process of formation of carbo- hydrates. These consist of the elements carbon, hydrogen, and oxygen, the latter two existing in the combination in the same proportion as they do in water. The source of the carbon and the oxygen is the carbon dioxide absorbed from the air, while the hydrogen is derived from the water of the cell-sap. The process can only take place under the influence of light and while the cells are at an adequate temperature. It has generally been spoken of as the assimilation of carbon dioxide ; but there are objections to this term, as the word assimilation is often used to indicate something entirely different, viz. the actual appro- priation of prepared food stuffs by the living substance. As the process under consideration is one of building up complex substances from very simple ones, and takes place only under the influence of light, it has recently been termed photo- synthesis. When carbon dioxide is exposed to the influence of the chloro- plast in the presence of light and moderate warmth, it is thought to undergo a decomposition which may be expressed by the equation 2CO„ = 2CO + O.,, splitting up into carbon mon- oxide and oxygen. At the same time, water is decomposed, probably in the way denoted by the equation 2H.,0 = 2H„ + 0„. The two molecules of oxygen unite and are exhaled, the quantity being the same as the quantity of carbon dioxide undergoing decomposition. The two residues, 2CO and 2H„, are then thought to unite to form 2CH..O, a body known as formic aldehyde. It is a property of the aldehydes to undergo readily what is known as polymerisation or condensation of several molecules together. Such a condensation of formic aldehyde would lead to the formation of sugar thus,—6CH,0 = C,.HjaO(.; and it is suggested that this process takes place. At any rate, sugar is present in the mesophyll cells very speedily after the absorption of the carbon dioxide and the beginning of the exhalation of oxygen. These processes are carried out by the chloroplast under