igiy] STOBER- WINTER AND SUMMER LEAVES 97 



shape of the leaf, to a certain extent, determines the shape of the 

 epidermal cells. In narrow and elongated, or linear, leaves, such 

 as those of the stems of Arabis brachycarpa, A. lyrata, Linaria 

 villosa, and Artemisia candata, the epidermal cells also are elongated 

 or linear. In such elongated or linear cells the lateral walls are 

 quite regular. The upper epidermal cells, however, are usually 

 more regular than the lower, except in such stem or rosette leaves 

 as are almost equally exposed to light. Such leaves have both 

 surfaces almost equally exposed to desiccating influences, hence the 

 shape and size of the epidermal cells on both sides of the leaf are 

 practically the same. This is very apparent in such xerophytic 

 stem leaves as those of Linaria villosa, Arabis lyrata, A. brachy- 

 carpa, A. laevigata, and Campanula rotundijolia. 



The shape and size of epidermal cells vary greatly, not only in 

 different species and in individuals of the same species, but even 

 in stem and rosette leaves of the same individual. There may 

 even occur considerable variation in different parts of the same leaf. 

 Thus in Leonnrus Cardiaca the sinuosity of the lateral walls increases 

 slightly from the lower to the upper stem leaves. In Geum album 

 the sinuosity seems to increase from the upper to the lower rosette 

 leaves. In Lepidium virginiciim the sinuosity is practically the 

 same from the upper stem leaves to the lowest rosette leaves. 

 However, the sinuosity in the lower epidermis, in the case of 

 Lepidium, is greater than in the upper epidermis. The lateral 

 walls of the lower epidermal cells are, as a rule, more sinuous than 

 those of the upper epidermis, and in the majority of instances (70 

 per cent) the sinuosity is greater in rosette than in stem leaves. 

 Sinuosity of the lateral wall culminates under the most mesophytic 

 conditions. Increased transpiration tends to produce relatively 

 straight lateral walls. Hence we find the epidermal walls of the 

 stem leaves less sinuous than those of rosette leaves, and those of 

 the upper surface of both stem and rosette leaves less sinuous than 

 those of the lower surface, since stem leaves are more xerophytic 

 than rosette leaves, and the upper side of leaves more xerophytic 

 than the lower side. Sinuosity of lateral epidermal walls is not 

 known to be of special significance to plants. It may add a little to 

 the strength of the epidermis and afford a larger diffusion surface 



