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 cuti- 
cularized outer layer may be seen. 
The palisade-tissue consists of 1—2 layers, and the cells 
are at most 804 in length. On comparing Danish and northern 
Chl 
Fig. 30. Vaccinium Oxycoccos. 
The leaf. Cf. note p. 81. (Finmark: Bosekop.) (H.E.P.) 
specimens I 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 lamelle 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 
