CLIMATIC FACTORS : RADIANT ENERGY 137 



ceives a portion of the light available. Genetic differences deter- 

 mine whether the leaves are exposed in the form of a rosette or 

 in a mosaic pattern, or whether they are supported by a spirelike 

 central axis or several spreading branches, each of about equal 

 size. The variations within such a general plan probably result 

 from effects of auxins on growth of petioles and secondary 

 branches. 



Leaves normally become arranged with their broadest surface 

 exposed outward and upward on the side of the plant where they 

 grow. This results in a maximum exposure to the available light 

 at that point. However, plants growing under conditions of ex- 

 cessive light, especially where there is reflection from light-colored 

 soil, not uncommonly have their leaves in a profile position, which, 

 of course, reduces the light to which they are exposed. Turkey 

 oak (Quercus catesbaei), which grows on sand dunes in the south- 

 eastern United States, regularly develops a twist in the petiole 

 that turns every blade vertically. The leaves of wild lettuce (Lac- 

 tuca scariola) are vertical when grown in full sunlight but do not 

 change from a horizontal position in the shade. Several so-called 

 compass plants have leaves that are not only vertical but that also 

 face east and west, exposing only their edges to the sun's rays at 

 midday. 



Plants growing in close stands characteristically lose leaves and 

 usually branches from below when the light penetrates insuffi- 

 ciently to maintain necessary photosynthesis. Most monocots with 

 grasslike leaves and underground stems are unaffected because 

 their upright linear leaves permit light to penetrate to their bases. 

 In forest stands, this self-pruning may be economically impor- 

 tant. Conifers that self-prune grow tall and straight with few 

 knots and smooth grain. In contrast, those with dead branches 

 down to their bases are difficult to handle and produce much less 

 valuable wood when finally cut. 



Leaves grown in full sunlight tend to be smaller, thicker, and 

 tougher than leaves grown in the shade. This is particularly no- 

 ticeable in plants of the same species and may also be observed on 

 the same plant. A forest-grown tree may have sun leaves at the top 

 and shade leaves near the base, or in the interior of its crown. 



Certain structural differences are associated with the two types 



