1917] STOBER— WINTER AND SUMMER LEAVES go 



are broad and thin. These are plants that have almost a typical 

 xerophytic shoot and a mesophytic rosette. 



The outer epidermal wall is decidedly thicker in stem than in 

 rosette leaves. In each of the 3 types considered, Lepidium vir- 

 ginicum, Capsclla Bursa-pastoris, and Chrysanthemum Leucantke- 

 mum, the stem leaves are borne considerably above the ground and 

 rather widely separated from each other, thus exposing them freely 

 to air, sunlight, and desiccating winds. The rosette leaves, on the 

 other hand, are close to the ground and considerably shaded; 

 hence we should naturally expect this difference in thickness of 

 epidermal cell walls. There is a slight tendency for the wall to 

 diminish in thickness from the apex to the base of the leaf. The 

 maximum thickness is usually reached in middle stem and apical 

 rosette leaves, while the maximum thinness is probably to be found 

 in the lowest rosette leaves. The outer epidermal wall on the 

 upper surface of the stem leaves is but slightly thicker than that 

 of the lower, especially in those upper stem leaves that grow 

 obliquely upward so as to expose both surfaces almost equally. In 

 the lower stem and rosette leaves this difference is much greater, 

 the epidermal wall on the lower side being considerably thinner. 

 The thickness of the cuticle varies with the thickness of the 

 outer epidermal wall, the thickest wall having the thickest cuticle. 

 The cuticle of the stem leaves of the types treated is decidedly 

 thicker than that in the rosette leaves. It is thicker on the upper 

 than on the lower surface of the leaf, except in the upper stem 

 leaves, where both surfaces are about equally exposed. Here 

 the lower cuticle is almost as well developed as the upper. The 

 greatest decrease in thickness of cuticle is observable in the basal 

 rosette leaves. 



In interpreting the facts set forth it must be borne in mind that 

 only middle stem leaves are compared with middle rosette leaves, 

 and that whatever conclusions may be deduced must rest upon 

 this comparison. Most plants have their rosettes better protected 

 than their shoots. In 83 per cent of 30 plants observed, the cuticle 

 is thicker in rosette than in stem leaves. In at least 75 per cent 

 of the number the epidermal wall is also thicker in rosette than 

 in stem leaves. Thus it seems that when the effective means of 



