580 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



is shown by the beautiful poinsettia, so highly prized as an orna- 

 mental plant during the Christmas season. Specimens exposed to 

 10 hours of sunlight daily from July 9 flowered and began to 

 color in normal fashion. By the latter part of August the plants 

 showed as fine coloration of the leaves subtending the inflorescence 

 as can be obtained in winter. 



Having shown that various plants which normally flower in 

 autumn, fall, or winter are readily forced into bloom in midsummer 

 simply by shortening the length of the daily exposure to light, it 

 will be of interest next to consider plants which normally flower in 

 midsummer when the days are longest. Some of our best known 

 vegetables fall into this class. The behavior of radish (Eaphanus 

 satbvus L.) is interesting and instructive. As is well known to all 

 gardeners, the ordinary varieties such as Scarlet Globe when planted 

 in spring first produce an edible root of moderate size, after which a 

 flowering stem is sent up, seed are formed, and the plant perishes. 

 The Scarlet Globe when planted in May and allowed to receive only 

 seven hours of light daily continued to increase slowly in size 

 throughout the summer and the succeeding winter without develop- 

 ing any flower stem (see pi. 12, fig. 2). The root finally reached a di- 

 ameter of nearly 5 inches and the leaves attained a length of about 18 

 inches. (See pi. 13, fig. 2.) As the plant was allowed to receive the 

 benefit of the lengthening days of the following spring the usual flow- 

 ering stem appeared. It is well known that while spinach {Spinacea 

 olera-cea L.) may be easily grown as a garden vegetable in spring 

 and fall, it is a " failure " when planted in the summer for the rea- 

 son that the plants quickly " go to seed " instead of merely develop- 

 ing the desired rosette of edible leaves. This behavior of spinach 

 in midsummer has been almost universally regarded as being due 

 solely to high temperature. As a matter of fact, when exposed 

 to a short day-length summer-grown spinach was found to behave 

 essentially the same as when planted in spring or fall, except 

 that the higher temperature undoubtedly hastens the general de- 

 velopment of the plant. The Climbing Hempweed (Mikania scan- 

 dcns L.), which normally begins to flower in July, was given a 

 daily light exposure of seven hours and under this treatment re- 

 mained sterile throughout the summer. (See pi. 14.) It has been 

 found, moreover, that this plant is unable to blossom when grown 

 in the greenhouse through the winter months. Similarly, Hibis- 

 cus moscheutos L., a wild perennial which flowers in late summer, 

 was unable to flower or to make any appreciable growth when ex- 

 posed to seven hours of light daily, as is shown in plate 10. With- 

 out multiplying examples further, it is clear that there is a group 

 of plants which are unable to flower except under the influence of 

 comparatively long days, as contrasted with the group first dis- 



