102 PLANT SOCIOLOGY 



the hemlock, Tsuga canadensis, according to Lutz (1928), ordinarily 

 germinates and passes its early life in full or partial shade. The 

 minimum light requirement of the hemlock has been shown by Bums 

 (1923) to be approximately the same as that of the beech O-io)- 



Photoperiodism. — Some plants, however, are more influenced by 

 length of day, that is, by the daily relative duration of light and 

 darkness, than by the influence of shade. Many species are more or 

 less responsive to the daily light period, and this response has been 

 termed "photoperiodism" (Garner and Allard, 1920; Kellerman, 1926). 

 Most vernal plants, such as Viola, Hepatica, and Anemone, flower at 

 the season of the year when the light days are only 10 hr. long. Many 

 of these may be made to flower at any season of the year, however, by 

 exposing the plants daily to 10 hr. only of sunlight. Similarly, some 

 autumnal plants like Chrysanthemum spp. and the cultivated Cosmos 

 flower only during the short days of the autumn but may be brought 

 into bloom at any season of the year by being exposed to a short-light 

 day. 



Many summer-blooming plants, on the contrary, flower only with 

 long daily illumination. Allard (1932) has shown that Sedum tele- 

 phium, grown as a garden ornamental, flowers in Vermont with a 

 summer day somewhat more than 15 hr. long. When grown at 

 Washington, D. C, with a 14-hour day it remains strictly vegetative. 

 The range of this species in its native home in Eurasia seems to be 

 north of 45 deg. and hence in regions with summer days at least 15 

 hr. long. Species of Rudbeckia seem to have a similar limit of bloom- 

 ing only with a light day of more than 14 hr. (Fig. 56). 



Other plants are not so sensitive to photoperiods and, like the dan- 

 delion and buckwheat, flower with any daily illumination of 6 to 18 

 hr. It would therefore seem that three classes of plants may be 

 distinguished on the basis of this light response: short-day, long-day, 

 and everblooming plants. 



Photoperiods affect growth as well as flowering of plants. An 

 apple seedling grows most vigorously with a 10-hr. day, while a young 

 Acer negundo plant will not thrive with less than 14 hr. daily illumi- 

 nation. Different parts of the same plant will sometimes respond to 

 different photoperiods. If the common Cosmos is made to develop 

 two nearly equal branches and one branch given 10 and the other 15 

 hr. of light daily, flowers and ripened seed will develop on the 10-hr. 

 branch, while the 15-hr. branch will continue vegetative growth for 

 an indefinitely long period without flowering. 



Garner and Allard (1931), who have made remarkable discoveries 

 in this field, were able to maintain Sedum spectahile, S. woodwardii, 



