LIGHT AND VEGETATION. 193 



of that memorable ride through the carion, 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 vil- 

 lages, a delay of a few hours was made to permit some mining en- 

 gineers 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 " Klon- 

 dike," 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 for- 

 tunes lay in the other direction, and that night we were aboard the 

 Islander, bound for Victoria and the south. 



+t» 



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 oxy- 

 gen approximately equal to that of the carbon dioxide taken up is ex- 



VOL. LIV. — 14 



