igi7] STOBBR— WINTER AND SUMMER LEAVES 95 



lyrata, A. brachycarpa, A. laevigata, and Satufeja glabra; and in 

 both stem and rosette leaves of Artemisia caudata and Lechea 

 villosa. All these leaves are linear or oblong. Not all linear or 

 oblong leaves have their stomata longitudinally oriented, but such 

 orientation is characteristic of linear and oblong leaves, especially 

 if the epidermal cells are longitudinally elongated. 



The stomata of the species investigated are not sunken below 

 the surface in either stem or rosette leaves, except in the sand dune 

 xerophytes, Artemisia canadensis and A. caudata, where they are 

 depressed about half the thickness of the epidermis. In a few 

 instances the stomata seemed even to be elevated slightly above 

 the surface. In Mitella diphylla, Lcoiuinis Cardiaca, Aqnilegia 

 canadensis, and Cliclidonium majus, the stomata are confined to 

 the ventral surface of the leaf. 



Rosette stomata are not only larger but also more elongated 

 than stem stomata. Stem stomata are not only smaller but also 

 more nearly round than rosette stomata. Perhaps the number 

 of stomata ought to be correlated with the mass of the chloren- 

 chyma. The smaller number of stomata in the broad, thin 

 (frequently thicker than stem leaves), mesophytic rosette leaves, 

 when compared with the smaller mass of chlorenchyma to be 

 aerated, may be relatively as abundant as the larger number per 

 unit surface in the long, narrow, thick xerophytic stem leaves, 

 where a larger mass of chlorenchyma must be aerated through a 

 given surface area; that is, the number of stomata is correlated 

 with the amount of chlorenchyma to be aerated, and not with 

 the mere surface area of the leaf. The number of stomata also 

 seems to be correlated with the thickness of the cuticle and 

 cutinized outer wall of the epidermis. The greater the thickness, 

 the less is the possibility of gases passing through, and the greater 

 is the need for stomata. It is probably for these two reasons, the 

 greater mass of chlorenchyma per leaf surface and the greater 

 thickness of the cuticle and cutinized outer wall of the epidermal 

 cells, that xerophytic leaves have an increased number of stomata 

 in a given surface area. The relatively thinner and frequently 

 more shaded rosette leaves are broader and have a thinner cuticle, 

 a thinner outer epidermal wall, and a greater development of air 



