NATURE OF PLANTS 



29 



petioles are variously elongated and curved in order to bring the 

 leaves into proper relation to the light (Fig. 15). Notice the 



FIG. 15. Horizontal branch of Forsythia with leaves in two rows owing to 

 the alternate twisting of each internode, assisted by the curving of the petioles. 

 Compare Fig. 13. 



change in leaf arrangement in horizontal and erect stems of maple. 

 In an example like the maple if the leaves could not change their 

 position they would all be standing edgewise to the light on hori- 

 zontal branches and therefore receive little of it. They not only 

 place their blades at right angles to the light but owing to the 

 greater elongation of each succeeding petiole from the apex 

 toward the base of the stem, all the leaves are arranged one be- 

 yond another so as to overlap very little. This same device is 

 noticed in many plants that produce their leaves close to the 

 ground in rosettes, as mullein, wood betony, plantain, etc. (Fig. 



FIG. 1 6. Leaf rosette of evening primrose. Each leaf blade has a longer 

 petiole than the one above it, so that the leaves are not shaded by one another. 



1 6). Compare the erect and horizontal branches of a variety of 

 plants noting by what devices the leaves are brought into the 



