142. Photosynthesis cannot take place in etiolated plants.—Moreover photosynthesis is usually confined to the green plants, and if by any means one of the ordinary green plants loses its green color this process cannot take place in that plant, even when brought into the sunlight, until the green color has appeared under the influence of light.
This may be very easily demonstrated by growing seedlings of the bean, squash, corn, pea, etc. (pine seedlings are green even when grown in the dark), in a dark room, or in a dark receiver of some kind which will shut out the rays of light. The room or receiver must be quite dark. As the seedlings are “coming up,” and as long as they remain in the dark chamber, they will present some other color than green; usually they are somewhat yellowed. Such plants are said to be etiolated. If they are brought into the sunlight now for a few hours and then tested for the presence of starch the result will be negative. But if the plant is left in the light, in a few days the leaves begin to take on a green color, and then we find that carbon dioxide assimilation begins.
143. Chlorophyll and chloroplasts.—The green substance in plants is then one of the important factors in this complicated process of forming starch. This green substance is chlorophyll, and it usually occurs in definite bodies, the chlorophyll bodies, or chloroplasts.
The material for new growth of plants grown in the dark is derived from the seed. Plants grown in the dark consist largely of water and protoplasm, the walls being very thin.
144. Form of the chlorophyll bodies.—Chlorophyll bodies vary in form in some different plants, especially in some of the lower plants. This we have already seen in the case of spirogyra, where the chlorophyll body is in the form of a very irregular band, which courses around the inner side of the cell wall in a spiral manner. In zygnema, which is related to spirogyra, the chlorophyll bodies are star-shaped. In the desmids the form varies greatly. In œdogonium, another of the thread-like algæ, illustrated in [fig. 144], the chlorophyll bodies are more or less flattened oval disks. In vaucheria, too, a branched thread-like alga shown in [fig. 138], the chlorophyll bodies are oval in outline. These two plants, œdogonium and vaucheria, should be examined here if possible, in order to become familiar with their form, since they will be studied later under morphology (see chapters on [œdogonium] and [vaucheria], for the occurrence and form of these plants). The form of the chlorophyll body found in œdogonium and vaucheria is that which is common to many of the green algæ, and also occurs in the mosses, liverworts, ferns, and the higher plants. It is a more or less rounded, oval, flattened body.
Fig. 69a.
Section of ivy leaf, palisade cells above, loose parenchyma,
with large intercellular spaces in center. Epidermal cells
on either edge, with no chlorophyll bodies.
145. Chlorophyll is a pigment which resides in the chloroplast.—That the chlorophyll is a coloring substance which resides in the chloroplastid, and does not form the body itself, can be demonstrated by dissolving out the chlorophyll when the framework of the chloroplastid is apparent. The green parts of plants which have been placed for some time in alcohol lose their green color. The alcohol at the same time becomes tinged with green. In sectioning such plant tissue we find that the chlorophyll bodies, or chloroplastids as they are more properly called, are still intact, though the green color is absent. From this we know that chlorophyll is a substance distinct from that of the chloroplastid.
146. Chlorophyll absorbs energy from sunlight for photosynthesis.—It has been found by analysis with the spectroscope that chlorophyll absorbs certain of the rays of the sunlight. The energy which is thus obtained from the sun, called kinetic energy, acts on the molecules of CH₂O₃, separating them into molecules of C, H, and O. (When the CO₂ from the air enters the plant cell it immediately unites with some of the water, forming carbonic acid = CH₂O₃.) After a series of complicated chemical changes starch is formed by the union of carbon, oxygen, and hydrogen. In this process of the reduction of the CH₂O₃ and the formation of starch there is a surplus of oxygen, which accounts for the giving off of oxygen during the process.
147. Rays of light concerned in photosynthesis.—If a solution of chlorophyll be made, and light be passed through it, and this light be examined with the spectroscope, there appear what are called absorption bands. These are dark bands which lie across certain portions of the spectrum. These bands lie in the red, orange, yellow, green, blue, and violet, but the bands are stronger in the red, which shows that chlorophyll absorbs more of the red rays of light than of the other rays. These are the rays of low refrangibility. The kinetic energy derived by the absorption of these rays of light is transformed into potential energy. That is, the molecule of CH₂O₃ is broken up, and then by a different combination of certain elements starch is formed.[8]