rIT HE rapid midsummer growth of plants in high latitudes has been often remarked. In June, I932, the author photographed wheat and oats I8 to 20 inches tall at Fort Vermilion, Alta., 58? 22' N. A week later at Beaverlodge, 220 miles farther south, the same varieties, seeded almost at the same time, were only a foot tall. In tests carried out 'over an eight-year period wheat at Fort Vermilion matured in I6 days less time than at Beaverlodge and in a day and a half less than at Lethbridge, 600 miles farther south. Several other examples of the extraordinary vigor of northern vegetation have been described in the author's preceding paper "Gardens of the Mackenzie."' What is the explanation of such phenomena? Day length is the familiar answer, but just how does the length of day influence vegetation, and what other factors, if any, are involved? The question is difficult to answer. Nature's outdoor laboratory is characterized by a complex of interacting forces.