360 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



He tried both long-day and short-day plants and found the responses 

 different. For example, in a long-day plant like the stock, the great- 

 est response occurred in the red for the wave-length region of 6500 

 to 7200 A, as illustrated in plate 2, figure 2. With short-day plants 

 such as cosmos and salvia, supplementary red light hindered flower- 

 ing. Likewise, a Mazda light of 1 foot-candle prevented these plants 

 from flowering. This is clearly shown for salvia in plate 2, figure 3. 



Dr. Withrow divides the plants he has so far studied into three 

 general groups: Those showing (1) no response of commercial value, 

 such as the rose and carnation, (2) earlier or increased flowering or 

 both, as stock, aster, shasta daisy, pansy, and (3) delayed flowering, 

 as the chrysanthemum. 



These few illustrations show some of the interesting plant re- 

 sponses brought about by the proper lengthening or shortening of 

 the light period. In addition to a general and scientific interest, 

 these responses have a real commercial value. 



The so-called "sleep movements" of plants such as shown by the 

 clover, sorrel, mimosa (sensitive plant), and Desmodium gyrans are 

 undoubtedly related to the normal daily light and dark periods. In 

 the morning these plants open or unfold their leaves and at night 

 close them. This daily rhythm of opening and closing becomes so 

 fixed in the protoplasm of the plants that when they are placed in 

 continuous darkness the movement may continue for several days; 

 each day, however, it becomes weaker until it finally ceases. 



INTENSITY EFFECTS 



There is scarcely a place on the earth's surface either too light or 

 too dark for plants to grow. On the deserts we find plants adapted 

 to intense sunlight. In caverns receiving little or no sunlight other 

 types of vegetation are found. One of these "dark-loving" plants is 

 a tiny moss {Schistostega osniundacea) equipped with a plate of cells 

 forming a set of lenses capable of focusing the scattered light on its 

 chloroplasts, those small bodies bearing chlorophyll which is essential 

 for photosynthesis. 



Many plants exposed to daylight of varying intensities have de- 

 veloped certain characteristic responses which in many cases have 

 proven beneficial. The English ivy {Hedera helix), for example, ar- 

 ranges its leaves in a mosaic pattern that exposes a maximum area 

 to the light. Other plants, like the compass plant {Silphium lacini- 

 atum) and the wild lettuce {Lactuca scariola) turn the edges of their 

 leaves in a general north-south direction. Thus when the light is 

 weakest in morning and evening, the flat surfaces of the leaves are in 

 a position to receive a maximum amount of light, whereas at noon, 

 when the light is most intense, these surfaces are more or less par- 



