2o6 



General Botany 



Short day 



Long day 



Long day 



Short day 



Figs. 120 and 121. Like the evening primrose and in contrast to the tobacco shown in the 

 preceding illustration, red clover (Fig. 120) is a long-day plant. The plants growing in the 

 two pails were photographed June 28 and had each received the same treatment, except 

 that those on the left were illuminated for only 10 hours daily, while those on the right re- 

 ceived the light during the whole day. The ones exposed for the shorter period grew but 7 

 inches high and produced no flowers, while those illuminated during the whole day flowered 

 abundantly and the tallest plants grew to a height of 33 inches. 



On the other hand, the dahlia, shown in Figure 121, is, like the tobacco, a short-day plant. 

 The plant on the right, beginning May 12, was exposed to 10 hours of light daily and flowered 

 July 8, when the photograph was made. On the control plant, under the natural length of 

 day, the first blossoms appeared September 27. {Gamer and Aliard, u. S. D.A.) 



light intensity is often reduced locally by fogs and clouds. In 

 the northern Lake states cloudy days may form a considerable 

 part of the growing season, and the total light that reaches plants 

 is much less than on the plains and deserts. Germany, France, 

 Great Britain, and our own Northeastern states are noted for 

 their high yields of potatoes, turnips, carrots, beets, and other 

 vegetative crops. 



The slope of the land, especially in mountain regions, may 

 increase or decrease the intensity and the length of dayhght. 

 Finally, plants may have their light reduced or cut off by trees 

 or other objects. Commercial growers of ginseng cover their 



