they are absent from the cells of the epidermis, though in some of
the lower plants they are met with there also. The plastids are not
rigidly embedded in the cytoplasm, but are capable of a certain
amount of movement therein. Each is a small protoplasmic
body, in the meshes of whose substance the green colouring matter
chlorophyll is contained in some form of solution. Various solvents,
such as benzene, alcohol and chloroform, will dissolve out the
pigment, leaving the plastid colourless. Chlorophyll is not soluble
in water, nor in acids or alkalies without decomposition.
These plastids are especially charged with the duty of manufacturing carbohydrates from the carbon dioxide which the air contains, and which is absorbed from it after it has entered the intercellular passages and has so reached the cells containing the plastids. This action is found to take place only in the presence of light, preferably moderate sunlight. The reason for the distribution of the chloroplasts described above is consequently seen. The relation of the chlorophyll to light has been studied by many observers. If a solution of the pigment is placed in the path of a beam of light which is then allowed to fall on a prism, the resulting spectrum will be found to be modified. Instead of presenting the appearance of a continuous band in which all the colours are represented, it is interrupted by seven vertical dark spaces. The rays which in the absence of the solution of chlorophyll would have occupied those spaces have no power to pass through it, or in other words chlorophyll absorbs those particular rays of light which are missing.
The absorption of these rays implies that the pigment absorbs radiant energy from the sun, and gives us some explanation of its power of constructing the carbohydrates which has been mentioned as the special work of the apparatus. The working of it is not at all completely understood at present, nor can we say exactly what is the part played by the pigment and what is the rôle of the protoplasm of the plastid. It is not certain either whether the action of the chlorophyll apparatus is confined to the manufacture of carbohydrates or whether it is concerned, and if so how far, with the construction of proteids also.
As the action of the chlorophyll apparatus is directly dependent upon light, and the immediate result of its activity is the building up of complex compounds, it has become usual to speak of the processes it sets up under the name of photosynthesis.
Photosynthesis.—In the presence of light and when the plant is subjected to a suitable temperature, photosynthesis commences, provided that the plant has access to air containing its normal amount of carbon dioxide, about 3 parts, or rather less, in 10,000. The process involves the inter-action of water also, and this, as we have seen, is always present in the cell. In addition, certain inorganic salts, particularly certain compounds of potassium, are apparently necessary, but they seem to take no part in the chemical changes which take place. The original hypothesis of Baeyer suggested that the course of events is the following: the carbon dioxide is decomposed into carbon monoxide and oxygen, while water is simultaneously split up into hydrogen and oxygen; the hydrogen and the carbon monoxide unite to form formaldehyde and the oxygen is exhaled. This explanation is unsatisfactory from many points of view, but till quite recently no acceptable alternative has been advanced. There is no evidence that carbon monoxide is ever produced, indeed there are strong reasons for disbelieving in its occurrence. The formation of formaldehyde has till recently not been satisfactorily proved, though it has been obtained from certain leaves by distillation. Certain Algae have been found capable of forming nutritive carbohydrates in darkness, when supplied with a compound of this body with sodium-hydrogen-sulphite. But it is certain that it can only be present in a cell in very small amount at any moment, for an extremely dilute solution acts as a poison to protoplasm. If formed, as it probably is, it is immediately changed into some more complex combination, and so rendered incapable of exerting its poisonous action.
Baeyer's hypothesis was entertained by botanists partly because it explained the gaseous interchanges accompanying photosynthesis. These show that a definite intake of carbon dioxide is always accompanied by an exhalation of an equal volume of oxygen.
Recent investigations have confirmed Baeyer's view of the formation of formaldehyde, but a different explanation has been recently advanced. The first chemical change suggested is an interaction between carbon dioxide and water, under the influence of light acting through chlorophyll, which leads to the simultaneous formation of formaldehyde and hydrogen peroxide. The formaldehyde at once undergoes a process of condensation or polymerization by the protoplasm of the plastid, while the hydrogen peroxide is said to be decomposed into water and free oxygen by another agency in the cell, of the nature of one of the enzymes of which we shall speak later.
Polymerization of the aldehyde was also a feature of Baeyer's hypothesis, so that this view does not very materially differ from those he advanced. More emphasis is, however, now laid on the action of the plastid in polymerization, while the initial stages are still not definitely explained.
The steps which lead from the appearance of formaldehyde to that of the first well-defined carbohydrate are again matters of speculation. There are many possibilities, but no definite body of simpler composition than a sugar has so far been detected. Nor is the nature of the first formed sugar certain; the general opinion has been that it is a simple hexose such as glucose or fructose, C6H12O6. Brown and Morris in 1892 advanced strong reasons for thinking that cane-sugar, C12H22O11, is the first carbohydrate synthesized, and that the hexoses found in the plant result from the decomposition of this. The whole story of the different sugars existing in the plant—their relations and their several functions—requires renewed investigation.
The first visible carbohydrate formed, one which appears so rapidly on the commencement of photosynthesis as to have been regarded as the first evidence of the setting up of the process, is starch. This is met with in the form of small granular specks in the substance of the chloroplast, specks which assume a blue colour when treated with a solution of iodine. Its very prompt appearance, as soon as the apparatus became active, led to the opinion formerly held, that the work of the latter was complete only when the starch was formed. We have seen that the starch is preceded by the formation of sugar, and its appearance is now interpreted as a sign of surplus manufacture. As much sugar as is produced in excess of the immediate requirements of the cell is converted into the insoluble form of starch by the plastids of the chlorophyll apparatus, and is so withdrawn from the sphere of action, thereby enabling the construction of further quantities of sugar to take place. The presence of too much sugar in solution in the sap of the cell inhibits the activity of the chloroplasts; hence the necessity for its removal. Starch, indeed, wherever it appears in the plant seems to be a reserve store of carbohydrate material, deposited where it is found for longer or shorter periods till it is needed for consumption. The readiness with which it is converted into sugar fits it especially to be a reserve or stored material.
Proteid Formation.—We have seen that it has been suggested that the chlorophyll apparatus may perhaps be concerned in the manufacture of proteids as well as of carbohydrates. If not, there must exist in the green plant, side by side with it, another mechanism which is concerned with the manufacture of the complex compounds in which nitrogen is present The independence of the two is suggested by the fact that fungi can live, thrive and grow in nutritive media which contain carbohydrates together with certain salts of ammonia, but which are free from proteids. It is certain that their protoplasm cannot be nourished by inorganic compounds of nitrogen, any more than that of animals. We must therefore surmise their possession of a mechanism which can construct proteids, if supplied with these compounds of nitrogen together with sugar.
The probability is that this mechanism is to be found in green plants in the leaves—at any rate there is a certain body of evidence pointing in this direction. It may be, however, that there is no special mechanism, but that this power is a particular differentiation of a physiological kind, existing in all vegetable protoplasm, or in that of certain cells. The idea of an identity of protoplasm does not involve a denial of special powers developed in it in different situations, and the possession of such a power by the vegetable cell is not more striking than the location of the powers of co-ordination and thought in the protoplasm of cells of the human brain.
But if we accept either view we have still to examine the process of construction in detail, with a view to ascertaining the stages by which proteid is built up. Here unfortunately we find ourselves in the region of speculation and hypothesis rather than in that of fact. The nitrogen is absorbed by the plant in some form of combination from the soil. The green plant prefers as a rule nitrates of various metals, such as calcium, magnesium or potassium. The fungus seems to do better when supplied with compounds of ammonia. The nitrogen of the atmosphere is not called into requisition, except by a few plants and under special conditions, as will be explained later. The fate of these inorganic compounds has not been certainly traced, but they give rise later on to the presence in the plant of various amino acid amides, such as leucin, glycin, asparagin, &c. That these are stages on the way to proteids as been inferred from the fact that when proteids are split up by various means, and especially by the digestive secretions, these nitrogen-containing acids are among the products which result.
While we know little of the processes of proteid-construction, we are almost completely in the dark also as to what are the particular proteids which are first constructed.
Opinions are conflicting also as to the conditions under which proteids are formed. There is a certain amount of evidence that at any rate in some cases light is necessary, and that the violet rays of the spectrum are chiefly concerned. But the subject requires elucidation from both chemical and biological points of view.
The normal green plant is seen thus to be in possession of a complete machinery for the manufacture of its own flood. The way in which such food when manufactured is incorporated into, and enabled to build up, the living substance is again hidden in obscurity. This is, however, also the case with the nutrition of animal protoplasm.
The building up and nutrition of the living substance by the foods manufactured or absorbed is properly spoken of as the assimilation of such food. Up to very recently the original absorption and subsequent treatment of the carbon dioxide and the compounds of nitrogen has been called by the same term. We frequently find the expression used, “the ‘assimilation’ of carbon dioxide, or of
