104 
been able to find any annual rings in the material at my 
disposal. 
On the whole the leaf of Cassiope tetragona is of very 
peculiar structure. By the erect position of the leaves and by 
their close adhesion to the stem, their surface has no doubt 
been greatly reduced, a fact which must be extremely favourable 
in cases of continuous drought. The consequent danger to the 
stomata, viz. that they are now turned directly towards the 
Fig. 20. Cassiope tetragona. 
The leaf. 2 and 2 shew the difference which may occur in the development of the 
two flanks when the light does not act with the same degree of intensity upon both of 
them. 3, Slanting palisade-cells. 4 Stoma. (Greenland.) (H. E. P.) 
wind, is very happily avoided by the growth of the margins of 
the leaves. The stomata are situated in the interior of a 
relatively large air-cavity, a fact which greatly checks all move- 
ment of air. The thin epidermis which lines this cavity, as 
also the large intercellular spaces of the leaf resulting from 
the form of the leaf and the position of the palisade-cells, 
bear witness to the great protection afforded by this air-cavity. 
It is evident that it must be of vital importance to a leaf that 
persists through the winter that it should prevent too much dry air 
from entering into its interior, which would certainly happen in a 
