154 THE BIOLOGY OF FLOWERING PLANTS 



not to area. The average area of the leaf is small in the 

 sun, and again small in extreme shade, with a maximum at 

 an intermediate light intensity. Stomata are much more 

 numerous in the sun leaf : the sun leaf of the beech has 

 413 stomata per sq. mm,, the shade leaf only 113 (cp. 

 Nordhausen, 1903, Schramm, 19 12). These sun and 

 shade leaves of a single plant are contrasted in the same 

 way as the sun and shade plants we have just considered. 

 They offer a better basis for comparison. Their environ- 

 ments differ in temperature, humidity and illumination. 

 We know something of their assimilation, but much exact 

 work, especially on transpiration, will be necessary before 

 their relations to their respective environments can be 

 properly understood. 



In such a case as that of Campanula we can trace a rela- 

 tion between the plant's requirements and the leaf types ; 

 the broad, rounded leaf of the shade station offers a large 

 surface for the absorption of the low light intensities 

 available ; the narrow upright leaf of the exposed station 

 might serve as an example of a panphotometric leaf. We 

 may also relate the structure of sun and shade leaves of 

 trees like the beech, maple, or elder to the type of illumi- 

 nation they receive in nature. The thin palisade of the 

 shade leaf may be supposed to absorb efficiently light of 

 low intensity, and the available leaf substance is spread 

 over a greater area giving a greater absorbing surface. The 

 sun leaf, on the other hand, with deep palisade, may be 

 regarded as absorbing more efficiently light of higher 

 intensity ; the increased energy supply thus available is 

 paralleled by an increased supply of carbon dioxide through 

 the greater number of stomata. As in the thin leaves of 

 shade plants, the respiration of the shade leaf of a tree 

 must be small per unit area, and a low light intensity will 

 permit assimilation to balance respiration: Harder (1923) 

 found this critical illumination for the shade leaves of the 

 ivy to be one-half that for the sun leaves, and Boysen- 

 Jensen obtained a similar result for the elder. A good 

 deal of experimental work has been done on the relative 



