SUN AND SHADE LEAVES 155 



efficiency of sun and shade leaves, not always with very 

 satisfactory methods. The results, however, agree well 

 with the interpretation given above. Thus Mliller (1904) 

 found, for the walnut and elder, that, in the shade, sun 

 leaves and shade leaves assimilated at equal rates for equal 

 areas, while in the sun the sun leaf was superior. For 

 equal weights the shade leaf was slightly superior in 

 the sun, markedly in the shade. For the lily-of-the-valley 

 Hesselman (1904) found the shade leaf superior in sun and 

 shade. For the pine and the spruce Stalfelt (1922) found 

 the shade leaves superior for equal weights. For the 

 elder Boysen- Jensen found the sun leaf more efficient, for 

 equal areas, in the sun. We may refer to Blackman and 

 Matthaei's demonstration that equal areas of very different 

 types of leaves assimilate at equal rates when light is 

 Hmiting. This agrees with Miiller's result. 



The difference between the sun and the shade leaf is 

 due to the direct effect of different factors only to a limited 

 extent . Nordhausen ( 1 903 , 1 9 1 2) and Schramm ( 1 9 1 2) have 

 shown that the particular type of leaf is already determined 

 in the bud, so that it may be regarded as showing after 

 effects of the conditions of a former vegetative period. 

 Trees from an exposed position, transplanted to a shady one, 

 may show the sun type of leaf for several years, and vice versa. 

 They have further traced in detail an important parallel 

 between the leaves formed early in the development of the 

 individual and the shade leaves on the one hand, and between 

 the leaves of more mature growth and the sun leaves on the 

 other. 



It is an almost universal phenomenon that the leaves 

 of a plant pass through a definite series of changes in form 

 and structure as the plant grows. We may speak of the 

 earlier and later leaves as youth and adult forms. Such 

 differences also exist, though less markedly, between the 

 basal and apical leaves of a single shoot. Our example of 

 the harebell shows this difference well. The round leaves 

 are youth forms. In shady places they may be retained 

 throughout the plant's life, though such a plant does not 



