366 
W. West and G. S. West. 
size. Their outer walls are more or less plane, whereas their inner 
walls are very convex and quite commonly hemispherical. They 
are also of very variable depth. The majority of these cells are 
divided by a wall parallel to the surface of the leaf and nearer the 
outer than the inner wall. 
Thus, a relatively large 
hypodermal cell has been cut 
off from a narrower epider¬ 
mal cell. The dividing-wall 
and the other walls of the 
hypodermal cells are ex¬ 
ceedingly thin, with a still 
thinner lining protoplasmic 
layer, the interior of the cell 
being occupied by a colour¬ 
less cell-sap vacuole. The 
outer walls of the epidermal 
cells are fairly strong and 
well cutinized. Such a hypo¬ 
dermal layer does not appear 
to have been observed in 
species of Bctula, although 
a number of species of Aimes 
possess a hypoderma. 
The hypodermal cells of 
Betula vana are both con- 
Fig. 28. A, section of leaf of Betula nana spicuous and irregular, the 
from 4,800 feet: B, section of leaf from , .. 
3,200 feet. e, epidermal cell; h, hypoder- cells extending up- 
mal cell ; p, palisade cells. wards and filling up the 
irregularities. The walls are so delicate and the vacuoles so 
completely fill these cells, that we can only regard them as con¬ 
stituting a slight development of aqueous tissue. It should be 
remarked that Betula vana grows for the most part in very exposed 
situations, where it is subject to almost continuous winds. 
On the flatter ground near the upper limit of shrubs were a 
number of boggy patches, those on the Nystuhoerne side of the 
valley being much the most extensive (vide b, b, on chart). There 
was almost an entire absence of Sphagnum from these bogs, and the 
part usually played by species of this genus in alpine bogs was here 
taken up by Arnblystegium sarmentosum, which occurred in immense 
quantity. Other less abundant mosses were Paludella squarrosa, 
