LEAVES, THEIR FORM AND MOVEMENT 79 



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. 



134. 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 advantage 

 to the younger leaves and to the growing point of the stem since 

 they are protected from drying out when in the delicate stage. 

 When the downward growth comes into play the leaves are usually 

 held in a horizontal position, and do not turn downward during 

 the day, because the stimulus of light (see preceding paragraph) 

 holds them in the most favorable light relation. But at sunset the 

 young leaves of many plants, and all leaves of certain sensitive 

 plants, turn downward, because the stimulus of light is removed 



* 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. 



