Plants ripen more rapidly in 

 some regions than in others. 

 These two chrysanthemums 

 were grown under identical 

 conditions except that the 

 one on the right was shaded 

 with black cloth from 4.30 P.M. 

 every afternoon, beginning 

 the first of August. The 

 shaded plant was in full 

 bloom by September 5, be- 

 having like plants .growing 

 where the days are relatively 

 short in late summer. The 

 other was then just begin- 

 ning to form buds 



p. W. Zimmerman and A. E. Hitchcock, from Boyce Thompson Institute 



DO PLANTS KNOW THE CALENDAR? 



more daylight hours than the same calendar days in a southern region. In 

 the northern latitude, accordingly, plants need fewer days to receive enough 

 light to complete their growth than they would in a southern region. We 

 may say, then, that if conditions are otherwise suitable, ripening will take 

 place after a certain amount of exposure to sunlight. 



Another seasonal adjustment that is related to light is seen in the forma- 

 tion of tubers by the artichoke or in the late blooming of the aster. The 

 plant does not ''know" what is going to happen later in the season. But as 

 the nights become longer in late summer, more carbohydrate material moves 

 into underground parts and accumulates as starch (see illustration opposite). 

 By shading an aster plant part of each day we can hasten the blooming. 



Illumination and Leaf Growth The leaves near the top of a tall tree 

 (which are constantly exposed to light) are generally smaller and greener 

 than those in the lower and shaded portions. This is a definite response to 

 differences in light. If we examine cross sections of the leaves with a micro- 

 scope, we find that there is much more chlorophyl in the smaller leaves. 

 Although the leaf can make more food in the light than in the shade, it 

 apparently grows more rapidly in the shade. In accordance with this fact, 

 the tree seems to fill out its leaf surface to the best advantage (see illustra- 

 tion, p. 254). 



Changing Illumination All who have had a chance to observe either 

 house plants or garden plants have been impressed by the fact that the 

 leaves face the light, and that stems bend toward the light. If we turn the 

 plants in the window halfway around, we shall find on the following day 

 that the leaves have actually turned to face the light again. If we keep a 



252 



