117 
project slightly. Besides the guard-cells the epidermis of the 
lower surface consists partly of small and partly of large cells; 
the latter, as it appears, function as a kind of aqueous tissue, 
such as often occurs in the neighbourhood of vascular bundles. 
According to Livrorss chlorophyll grains occur both in the upper 
and in the lower epidermis. 
The palisade-cells occur in a single layer only, but are 
long (about 80 yw), nearly as long as the spongy parenchyma, 
and seen in longitudinal section to be placed somewhat obliquely, 
especially in the neighbourhood of the vascular bundles. The 
cells of the spongy parenchyma, which are but slightly branched, 
are usually elongated parallel to the surface of the leaf. Large 
intercellular spaces occur; in connection with the latter it may 
be mentioned that the epidermis of the lower surface easily 
separates from the spongy parenchyma, often forming unusually 
large air-cavities. 
The stem. The epidermis and the cortex of the young 
stem are similar to those of A. Uva-ursi. When the secondary 
cortex arises, and the formation of cork begins at the boun- 
dary between the primary and the secondary cortex, a layer, 
which has already previously been distinctly differentiated, is 
developed as a new epidermis which, as far as I can judge, 
persists for a shorter or longer time (several years) (Fig. 28, 5). 
But when once this new epidermis falls off it is not renewed, 
and then the external investment of the stem is formed by 
the cork-layer itself, which develops early (see Fig. 28,5) and 
exhibits no points of interest. The annual rings in the woody 
part resemble those in A. Uva-ursi; usually they are distinct, 
with a more or less marked difference between the autumn and 
spring wood, but sometimes these are somewhat indistinguishable. 
Normally, vessels are formed in the spring wood and tracheids 
in the autumn wood. 
I have not examined the roots. 
