103 
surface of the leaf, that side of the 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, 1 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 
(477 
Prete. | | 
pi / XF & 
# x 
+ TS nA rn FE 
Bin rad À 
Cin SA 2 
Ir Us À 
FED > øk A 
¢ ERE Wr X > 
N'a ts Sy 
AD 2 
Bi 
Fig. 19. Cassiope tetragona. 
The leaf. 1, 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. 
I observed the Kraus winter-condition in a specimen from 
Danmarks Ö (Greenland; N. Hartz). Like Lisssaver I have not 
