86 GROWTH AND WORK OF PLANTS 



grow in the shade the leaves do not point north and south and the 

 blade is horizontal. 



137. Movement of leaves in response to light. When 

 leaves are young and their position has not become fixed, they 

 often show marked movements in response to light stimulus. 

 While many plants manifest this peculiarity, it is more marked in 

 some than in others. In the case of many seedlings when placed 

 where they receive a one-sided illumination, as near a window, 

 or in a box open only on one side, the leaves, as well as the stems, 

 turn so as to face the light. If the position of the seedlings is 

 changed, the leaves will turn again. In the sunflower plant 

 throughout its growth the younger leaves " follow the sun" all 

 day on bright days. The leaves near the upper end of the stem 

 are drawn somewhat together, so that they form a rosette, and 

 turn so that their upper surfaces face toward the rising sun, the 

 stem also turning to assist in bringing them into this position. 

 This rosette of leaves then " follows the sun" all day and at 

 sunset it is facing the west. After sunset the stem straightens up, 

 and the leaves assume a horizontal position because the strongest 

 rays of light are now from the zenith. On cloudy days the leaves 

 remain in this horizontal position. Many other plants show this 

 same peculiarity, the cotton plant, ragweed, sweet clover, and 

 especially those plants belonging to the family known as sensitive 

 plants, and to the legumes or Leguminosae. 



138. Night and day movements of leaves. While leaves are 

 very young, as in the bud, or plumule, growth of the cells is usually 

 more rapid on the under side than on the upper side of the leaf. 

 This causes the leaves to bend upward or inward toward the axis 

 of growth of the stem. During later growth, however, growth is 

 more rapid on the upper side, and this causes the leaves to "open" 

 from the bud and to extend outward or even to bend downward. 

 This upward growth tendency* of leaves in the bud is an advan- 



* The tendency of leaves to turn upward during early growth is called 

 hyponasty, or hyponastic growth. The tendency later to turn downward 

 by greater elongation of the cells on the upper surface is called epinasty, or 

 epinastic growth. 



