EFFECT OF RADIANT ENERGY ON GROWTH 605 



(Impatiens), for example, flowers readily when exposed to a day-length of 14 

 hours, is greatly delayed in flowering by a day-length of 133/2 hours, and at 

 day-lengths of shorter duration is strictly vegetative. While the dividing line 

 between the range of photoperiods which favors reproductive development and 

 that which does not is not as sharp in all species, usually a fairly definite 

 critical day-length can be recognized. 



In addition to the Maryland Mammoth variety of tobacco, other examples 

 of short-day plants are cosmos, salvia, coleus, asters, dahlia, poinsettia, chrysan- 

 themum, nasturtium, violets, and all early spring or late summer blooming 

 wild flowers of the temperate zone. 



Examples of long-day plants include spinach, beets, radish, lettuce, grains, 

 timothy, clover, hibiscus and all late spring and early summer blooming wild 

 flowers of the temperate zone. 



Some of the better known examples of "indeterminate" species are sun- 

 flower, dandelion, chickweed, tomato, cotton, and buckwheat. 



The season of the year at which a plant will bloom is largely controlled, 

 at least in temperate regions, by its type of reaction to day-length conditions. 

 The natural blooming period of long-day plants is in the late spring and early 

 summer. Short day species which can grow at relatively low temperatures 

 bloom in the early spring, the flower buds in many such species being 

 formed during the preceding autumn. The majority of the members of this 

 group do not develop flowers until the advent of the shortening days of late 

 summer or early autumn. This latter t>'pe of behavior is invariably found in 

 the annual species belonging to this group and is characteristic of many per- 

 ennial species as well. Indeterminate species, on the other hand, may flower 

 at almost any season during which other environmental conditions are 

 favorable. 



The ecological distribution of plants is also limited in part by photo- 

 periodism. Tropical and sub-tropical species are mostly short-day plants. 

 Species growing at high latitudes (60° and farther north) are mostly of the 

 long-day type. In temperate zones both long-day and short-day species flour- 

 ish, but flower at different seasons as already described. Indeterminate species 

 can reproduce over such a wide range of day-lengths that their distribution is 

 limited by other factors than the daily duration of the light period. 



Garner and Allard (1923) also succeeded in demonstrating that the reac- 

 tion of plants to differences in day-length may be highly localized. They 

 arranged two similar branches of a cosmos plant in such a way that one was 

 exposed to a short day-length, and the other to a long-day length. The former 

 of these two branches soon produced flowers and fruits, and subsequently died 

 (cosmos is a short-day plant), but the latter continued to grow vegetatively 



