CLIMATIC FACTORS 



103 



and Cassia marylandica in flowerless condition for eight years under 

 conditions of short-day illumination. Controls flowered each year 

 under the influence of the full length of the summer day at Washington, 

 D. C. The sedums, in which flowering had been suppressed for eight 

 years of short-day illumination, flowered in an apparently normal 

 manner when they were exposed to long-day illumination. 

 These investigators conclude that: 



A wild plant in its natural invasions from an original center of occurrence 

 must find itself subjected to the factor of length of day during the growing 

 season as well as to other climatic and soil conditions. If its distribution 



Fig. 56.— Rudbeckia nitida showing responses to different lengths of day. The 

 plant ceases flowering with a light day somewhere between 14.5 and 14 hr. long. The 

 control with a light day nearly 15 hr. long flowered perfectly; days of 14, 13.5, and 13 hr. 

 allowed the development of leafy rosettes only. {Photo hij AUard, courtesy of V . S. 

 Department of Agriculture.) 



depends upon successful seed production, its limits of distribution must 

 depend largely upon those conditions of summer-day length favoraljle to 

 successful flowering and the production of fertile seed. Whether it can main- 

 tain itself in a region unfavorable to flowering would depend upon whether 

 natural methods of vegetative propagation are at its disposal and whether 

 sufficient nutrition reserves can be maintained to support healthy growth in 

 the plants from year to year. 



Measurement of Light Intensity. — Methods of measuring light 

 are discussed in detail by Wiesner (1907), Riibel (1922, 1928), Braid 



