substances from one portion of the vegetative tract to another along conduits which lie near the surface. On such compounds, as well as on chlorophyll, the blue-violet rays (see Fig. 2) exercise a disintegrating effect. In quite a large number of plants, the lines of vessels in stalks, midribs, and petioles of leaves are shielded from the direct action of such rays by means of external layers or bands of anthocyan, of some shade of red or purple. It is possible in some instances to trace the line of the conducting vessels by the lines of color appearing near the surface. The direct connection between the food substances and the presence of the coloring matter is strongly indicated by the example detailed by Kerner, in which the pearly-white rhizome of Dentaria when taken from the soil and exposed to the light will become a deep violet in a few hours. Whether the connection is a direct one or not, it is also true that many young and rapidly growing shoots exhibit marked reddish or violet colors at a time when reserve food is being conveyed to them in greatest quantity, and when the thin, tender tissues are otherwise so translucent as to allow the sun's rays to strike through them in a manner calculated to work great damage in the complex compounds in the young leaves. When the leaves mature and are not so pervious to light, the colors may disappear. This is well illustrated by the behavior of the young leaves of rhubarb, cherry, and grape. Many instances of this character are known, as well as the fact that storage organs are often provided with coloring layers or shields, when partially exposed to the light under normal conditions. In plants with deciduous leaves, or the shoots which die down to the root stock each year, it is highly important that the material in the protoplasmic structures of the portion dying away should not be entirely lost, as it represents a large outlay of energy. As a matter of fact, in plants of this character the protoplasm, chlorophyll, and other nitrogenous substances are usually broken down and begin to be gradually withdrawn into the surviving portion of the plant about the time of the formation of the first stages of the absciss layer which finally cuts off the leaf stalk, or about the time the activity of the herbaceous shoot begins to slow down. The disintegration of the chlorophyll would leave the leaf almost colorless and translucent, and the sun's rays would strike directly through it, resulting in the total decomposition of the proteids and a consequent waste to the plant, but during the decomposition of the chlorophyll there occurs, as a result or accompaniment of the process, the formation of much brilliant coloring matter of various shades, to which are due the brilliant autumnal tints of deciduous leaves. These coloring matters sustain the same general relation to sunlight as the other colors described above. They generally absorb the entire violet
Page:Popular Science Monthly Volume 49.djvu/90
POPULAR SCIENCE MONTHLY.