119 



partly with the small number of leaves and partly with the 

 often great length of the stem which requires an abundant 

 supply of air. Further, it must also be a fact of importance in 

 this connection that the leaves occur only slightly above the 

 level of the soil and in an atmosphere fairly rich in carbonic 

 acid; this increases the necessity for oxygen. Upon the inner 

 side of the guard-cells and upon the adjacent cells a culi- 

 cularized outer layer may be seen. 



The palisade-tissue consists of 1 — 2 layers, and the cells 

 are at most 80 /^ in length. On comparing Danish and northern 



Fig. 30. Vaccinium Oxycoccos. 



The leaf. Cf. note p. 81. (Finmark: Bosekop.) (H.E. P.) 



specimens ! have not found any special differences, except 

 perhaps that the lower layer of palisade-cells is better deve- 

 loped in the northern specimens. 



The cells of the spongy tissue are not much branched 

 and form lamellæ which lie somewhat transversely across the 

 leaf a circumstance which harmonises well with the often vertical 

 position of the leaf. The cells of the mesophyll are fairly thick- 

 walled. No aqueous tissue occurs, but a considerable amount 

 of stereom. The Kraus winter-condition was observed on 

 Febr. 24, 1907 (Farum bog), but not on March 22, 1907 (Lyngby 



