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. 1 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. 8, 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 



