Page:Popular Science Monthly Volume 54.djvu/207

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193
LIGHT AND VEGETATION.

of that memorable ride through the cañon, the wheel reversed and throwing water over the pilot house, the boat rocking and swaying to and fro! Before we were fairly aware of the fact we were out into that great, deep, silent basin again and off on the home stretch. Apart from taking on wood and stopping at one or two Indian villages, a delay of a few hours was made to permit some mining engineers to examine a mine. They had just come up from the coast and brought with them news of the gold excitement in the Yukon Valley, and now for the first time we heard that magic word "Klondike," which was soon to "electrify the world and put the gold fields of California, South Africa, and Australia to shame."

At nine o'clock we were in Essington once more. "Klondike, Klondike!" on every side. The whole country seemed to have gone daft. One steamer after another went racing by the mouth of the Skeena on the way to Dyea and the Skagway Trail. But our fortunes lay in the other direction, and that night we were aboard the Islander, bound for Victoria and the south.

 
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LIGHT AND VEGETATION.
By D. T. MACDOUGAL, Ph.D.,
PROFESSOR IN CHARGE OF PLANT PHYSIOLOGY, UNIVERSITY OF MINNESOTA.

LIGHT is the most important of all the external agencies which influence the vegetal organism, and the sun's rays have been the most potent force in shaping the development of existent plant forms.

The sunbeam stands in a manifold relation to the plant. First and foremost, light is the universal source of energy, by the aid of which the chlorophyll apparatus in green leaves builds up complex food substances from simple compounds obtained from the soil and air, a process necessary for the nutrition of the entire living world. Some obscure organisms, such as the "nitrosomonas," soil bacteria, are able to accomplish the construction of complex substances, by means of energy derived from other chemical compounds, which were, however, formed originally by green plants. These food-building processes are designated as photosynthesis, chemosynthesis, electrosyn thesis, thermosynthesis, etc., according to the source of energy used.

By photosynthesis, carbon dioxide from the air and water from the cell are combined in the green cells of leaves, forming sugar and possibly other substances. During this process an amount of oxygen approximately equal to that of the carbon dioxide taken up is ex-