103 



surface of the leaf, that side of tlie leaf which is pressed close 

 to the stem. Upon the two sides of the leaf, the parts of the 

 palisade, which form the limit of the air-cavities, are often 

 very unequally developed (Fig. 20), so that the layers of 

 palisade-cells differ rather considerably in number. This difference 

 is most obvious in such parts of the leaf as have not been 

 covered by other leaves, consequently, especially towards the 

 apex of the leaf (Fig. 20, l and 2). This may be explained by 

 the fact that the palisade- cells are formed in the bud, the 

 leaves of which overlap each other considerably, leaving the 



Od 



Fig. 19. Cussiope tetragona. 



The leaf. 2, The outer epidermis with cuticle. 2, Inner epidermis with stomata. 

 3—5, Stomata. (E. Warming.) 



apices free. It may be proved that the part of the four longitudinal 

 layers of the palisade-tissue which is most well-developed, 

 corresponds with the direction in which the light falls most 

 intensely. Symmetrically-developed flanks occur not only in 

 places where the leaves have overlapped each other, but also 

 on such leaves as have been placed symmetrically towards 

 the light. 



1 observed the Kraus winter-condition in a specimen from 

 Danmarks Ö (Greenland; N. Hartz). Like Linsbacer I have not 



