26 LEAF ARRANGEMENT 



is such as to occupy practically all the space about the stem that 

 receives light. This becomes very noticeable if we look directly 

 down upon a shoot or better if we stand under a tree and look 

 up into the branches. It will be seen that very little direct sun- 

 light finds its way through the branch, so nicely are the leaves 

 adjusted to each other. Some plants like the hickories, catalpa. 

 etc., occupy all the available space with a few large leaves at the 

 tips of the branches. In other cases, as in the willows, some 

 lilies, etc., the same result is accomplished by many small leaves 

 that can be arranged along the stem for considerable distances 

 without shading. The nicety of leaf arrangements is especially 

 noticeable in many horizontal and creeping stems. In such cases 

 the leaves can only be exposed on the sides of the stem and con- 

 sequently the stem may become twisted or the petioles variously 

 elongated and curved in order to bring the leaves into proper 

 relation to the light (Fig. 15). Notice the change in leaf ar- 



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. 



rangement in horizontal and erect stems of maple. In an ex- 

 ample like the maple if the leaves could not change their position 

 they would all be standing edgewise to the light on horizontal 

 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 

 bottom of the stem, all the leaves are arranged one beyond 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. 16). Compare 

 the erect and horizontal branches of a variety of plants noting 



