140 THE STUDY OF PLANT COMMUNITIES ' Chapter VI 



There are added advantages ( in the thinness and greater area of the 

 shade leaf since both maximum exposure under conditions of re- 

 duced light and penetration of light to a high proportion of in- 

 ternal cells are thus assured. 



Since reduced light favors elongation, vegetative growth, and 

 delicacy of structure, it can readily be understood why several 



A b c 



Fig. 71. Structure of leaves of broad sclerophyll forest trees (A) Castan- 

 opsis chrysophylla, (B) Quercus agrifolia, (C) Quercus durata. Note com- 

 pact structure, multiple layers of palisade, and tendency for all mesophyll to 

 be palisade-like. Note also struts of mechanical tissue from epidermis to epi- 

 dermis.— From Cooper™ 



garden crops used either for leaves or roots are best grown in 

 spring and fall or in regions with many cloudy days. A number of 

 leaf crops are grown under artificial shade. The point is well illus- 

 trated by the production under artificial shading of the large thin 

 leaves of tobacco needed for cigar wrappers. 



Since intense light inhibits vegetative growth and favors, or is 

 actually necessary for, flowering and fruiting, it is not surprising 

 that centers of grain and fruit production characteristically have 

 much clear, cloudless weather during the growing season. Here, 

 too, is a partial explanation of the reduced size of alpine and arctic 

 plants, which produce large and numerous flowers. Likewise it 

 helps explain why trees in the open often fruit more prolifically 

 than those in a closed stand, where overtopped individuals rarely 

 produce a seed crop. 



Photoperiod.— A number of seasonal biological phenomena long 

 have been accepted as such, without much concern as to causes. 

 Violets, miliums, bellworts and many other wildflowers blossom 



