Page:Popular Science Monthly Volume 49.djvu/88

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76
THE POPULAR SCIENCE MONTHLY.

phyll against the more intense action of the rays. It is also in this purpose of protection of the chlorophyll that many of the colors grouped under anthocyan find their chief function in the plant. In such instances the color is generally in solution in the sap of the layers of cells exterior to the chlorophyll, and light must pass through the coloring matter in order to reach the interior of the leaf. This may be seen by reference to Fig. 4, in which is shown a cross-section of a portion of a leaf of coleus.

That such layers of coloring matter do materially alter the light which passes through them may be demonstrated if the spectrum of light which has passed through a solution of them is examined in the manner described above. Water, instead of alcohol,

PSM V49 D088 Cross section of leaf of coleus.jpg
Fig. 4.—Cross-section of Leaf of Coleus. A, A, epidermal cells tilled with reddish cell sap; B, cells containing chlorophyll bodies.

is used as a solvent, however. If the color of the leaf of the amaranth is used, it will be found that nearly all the light has been absorbed except a portion between B and D (Fig. 1, II).

It may be seen that a large proportion of the light is absorbed by the anthocyan and converted into heat, and furthermore it is inclusive of the portion of the spectrum which exercises the most violent disintegrating effect on chlorophyll, as may be seen by reference to Fig. 2. The portion which promotes synthesis of food materials, on the other hand, is transmitted almost unchanged to the chlorophyll beneath. That the anthocyan does partially retard the disintegration of chlorophyll by light may be seen if two vessels containing solutions of chlorophyll are so arranged that the light which strikes on one of them shall first pass through a parallel-walled vessel containing water, and that which strikes the other through a similar vessel containing a solution of anthocyan. The chlorophyll in the first will soon become much more discolored than in the second, which has received light transmitted through anthocyan. The number of plants in which coloring substance is present in the cell sap or walls of the outer layers of leaves is extremely large, and em|embraces}}