i 9 i 7 ] STOBER— WINTER AND SUMMER LEAVES 105 



and in addition Leonurus Cardiaca and Verbascum Thapsus, is 



as follows. 



In general the lowest stem and rosette leaves, as well as the 

 basal part of all leaves, are most protected and most shaded, and 

 therefore have the most mesophytic structure. The leaves are 

 thinnest; the outer epidermal wall and cuticle are thinnest; the 

 palisade parenchyma is developed most poorly; and spongy 

 parenchyma, containing a maximum of air spaces and a minimum 

 of chloroplasts, is developed most highly. 



The upper stem leaves are relatively xerophytic in structure. 

 This is especially true of the apical region of these leaves. We 

 frequently find the maximum thickness of leaf, maximum thick- 

 ness of epidermal wall and cuticle, and a maximum development of 

 palisade tissue, which in many instances develops almost equally 

 on both sides of the leaf. The middle and lower stem leaves are 

 almost invariably thinner than the corresponding rosette leaves. 

 The spongy parenchyma is better developed in rosette than in 

 stem leaves. This was true of 75 per cent of all sections studied. 



The palisade parenchyma in stem leaves is better organized, 

 more compact, and the cells relatively longer and narrower as 

 compared with the thickness of the leaf. In rosette leaves the 

 layers of palisade tissue are frequently less perfectly organized, 

 less compact, and the cells larger. Palisade cells of rosette leaves 

 are decidedly broader and usually longer than those of stem leaves; 

 but the amount of palisade tissue and the length of the cells, 

 when compared with the average thickness of the leaves, are less 

 in rosette than in stem leaves. The absolute length of palisade 

 cells in the first layer is greater in rosette leaves than in correspond- 

 ing stem leaves in 70 per cent, in the second layer in 55 per cent, 

 and in the third layer in 28 per cent of all sections studied. In 30 

 per cent of all stem sections studied the second palisade layer was 

 not developed. The same was found to be the case in 29 per cent 

 of rosette sections studied. Likewise, the third palisade layer was 

 not developed in 81 per cent of all stem sections studied, or in 66 

 per cent of all rosette sections studied. The number of sections 

 considered in each case was the same (75 stem and 75 rosette sec- 

 tions). With the exception of the upper stem leaves, where the 



