240 TEXTBOOK OF BOTANY 



piece of stone fits closely between the neighboring pieces. 

 This is why such an arrangement of leaves is spoken of as 

 a leaf mosaic. If the light comes chiefly from one side rather 

 than from above, as in the case of a grape vine growing 

 against a wall, the leaf mosaic is turned toward the side 

 from which the light comes, and not upward. 



254. Responses of Leaves to Light Stimuli. The mosaic 

 arrangement just described results partly from the fact that 

 the leaves are attached to different sides of the stem. But 

 no method of attachment of the leaves to the stem would of 

 itself prevent overlapping ; in particular, it could not provide 

 for the fullest possible illumination of the leaves when the 

 light comes from one side. The leaves of many plants, 

 especially of plants growing in shady places, can adjust their 

 own position so that the upper surface of their blades will 

 receive the greatest amount of direct light. This adjustment 

 is' brought about partly by the greater or less growth in 

 length of the leaf-stalk ; sometimes the leaves on the same 

 plant have stalks that differ greatly in length. The adjust- 

 ment of the leaf to light is also helped by a twisting of the leaf- 

 stalk, and sometimes too by a curving of the leaf-blade. 

 Most of these changes are responses to the stimulus of light, 

 although the stimulus of gravity plays a part also in deter- 

 mining the direction of growth and the position of the leaf. 

 These responses are more complex and more difficult to study 

 than the changes which occur in the direction of growth of 

 stems and branches in response to stimuli. 



There are some plants whose leaves do not so place them- 

 selves as to receive the greatest amount of light. These are 

 especially plants that live in exposed places where the light 

 is so intense, particularly in the middle of the day, that it 

 might injure the leaves. The prickly lettuce is a fairly good 

 illustration. Its leaves are nearly vertical instead of hori- 

 zontal ; some of. them point toward the north, others toward 

 the south. In this position the surfaces of the leaves receive 



