312 Morphogenetic Factors 



this tissue is much better developed in roots than in stems may be re- 

 lated to the fact that roots normally grow in darkness and stems in light. 

 Leaf structure is often different in different light intensities. It has 

 long been known (for example, Nordhausen, 1903) that in many cases 

 leaves on the south side of the tree ("sun leaves") are thicker and better 

 differentiated than those on the north side or the interior of the crown 

 ("shade leaves"; Fig. 13-3). This has been discussed by Lundegardh 

 (1931) and others. A particularly striking case is described by Cormack 

 ( 1955). The question has been raised (p. 327) as to whether this effect is 

 actually due to light intensity or to differences in water relations, par- 



Fig. 13-3. Transverse sections through the blades of sun leaves (above) and shade 

 leaves ( below ) of a, Acer; b, Quercus; c, Fagus; and d, Tilia. ( After Schramm. ) 



ticularly rate of transpiration, for sun leaves tend to be xeromorphic in 

 character. Talbert and Holch (1957) studied the leaves of 37 species 

 and found that sun leaves usually had smaller laminar area, shorter blade 

 perimeter, deeper lobes, more pronounced veining and marginal serra- 

 tions, more hairy surfaces, and shorter petioles than shade leaves. 



Anderson ( 1955 ) studied the development of sun and shade leaves in 

 Cornus and Viburnum and finds that in the latter the large leaf size is 

 the result of earlier and more rapid growth. The greater thickness of sun 

 leaves is due to greater cell elongation. Differentiation takes place earlier 

 in shade leaves. 



