LEAVES 



547 



tensity are not at all easy of explanation. Probably of similar import 

 are the changes in orientation from the base to the apex in the leaves 

 of many plants (as Verbascum or Nicotiana, fig. 786) ; the intermediate 

 leaves show gradations between the large horizontal lower leaves and 

 the small vertical upper leaves. The resultant plant contour is a cone, 

 a shape well fitted for light reception Probably such changes in leaf 

 orientation are due partly to a change in the character of the photo- 

 tropic reaction as the light becomes more intense; but other explana- 

 tions are possible (p. 

 603), and cautious 

 statement is necessary 

 until more experi- 

 mental data are avail- 

 able. In some plants 

 the leaf position is 

 determined by me- 

 chanical factors; for 

 example, banana and 

 palm leaves, though 

 diaphototropic, often 

 hang vertically be- 

 cause of their weight. 

 Growing Yucca leaves 



tend toward verticality, but crowding and other factors cause them to 

 assume various positions. 



The advantages of leaf reactions to light. Diaphototropic leaves. 

 In the open, plants commonly receive much more light than they can use 

 in carbohydrate synthesis, carbon dioxid and temperature being more 

 important as limiting factors. But in the shade the amount of light 

 may be insufficient, hence it follows that there the leaf arrangement is 

 of high importance. Indeed, in dense cultures the lower leaves of 

 many plants soon die, presumably from lack of light ; the absence of 

 leaves on the lower branches of forest trees doubtless has a like explana- 

 tion. Experiments show that synthesis in a given leaf is reduced 

 greatly when the incident light penetrates another leaf, and that it practi- 

 cally ceases if it penetrates two leaves. Hence the avoidance of shading 

 through petiolar growth and otherwise is of great significance. Dia- 

 phototropism favors maximum light exposure, and its advantage is 

 apparent. Less obvious is the advantage resulting from the facing of 



FIG. 785. A portion of a sun plant of the prickly 

 lettuce (Lactuca scariola), showing the characteristic twisted 

 leaf bases and the resultant fixed vertical or profile posi- 

 tion; occasionally, as here, the leaves are in one plane, 

 facing east or west, as in the compass plant. 



