1917] STOBER— WINTER AND SUMMER LEAVES 07 



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 caudata, 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 rotundifolia. 



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 Leonurus 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 virginicum 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 UDDer eDidermis. The lateral 



epidermal cells are, as a rule, more sinuous t 

 epidermis, and in the majority of instances 



per cent) 



stem 



culminates under the most 



conditions. Increased transpiration tends to produce relatively 

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



stem 



than the lower side. 



th stem and rosette leaves less sinuous 

 ace, since stem leaves are more xerop 

 d the upper side of leaves more xerop 

 Sinuosity of lateral epidermal walls i 

 significance to plants. It may add a lit 



•t> 



epiderm 



