The Ecology of the Driva Valley. 365 
Phaca frigida extends right through the shrubland and is much 
affected by a Puccinia. 
Thalictrum alpinum also occurs in small quantity through this 
zone, and was frequently attacked by an JEcidium. 
Nothing could be more conspicuous than the leaves of Vactinium 
uliginosum and V. Vitis-idcea attacked by the parasite Exobasidium 
VaccinE, the bright crimson leaves being quite a feature of the 
upper reaches of this zone. 
The upper limit of the shrubland is not very well defined, but 
is approximately 4,500 feet, although Betula nana in straggling 
dwarfed individuals goes to 4,800 feet, and stunted specimens of 
Salix lanata were observed up to 4,900 feet. As the larger shrubs 
begin to thin out, Salix reticulata 
becomes very abundant, attaining 
its maximum development about 
4,400 feet and extending up to 
4,800 feet. Salix herbacca becomes 
dominant where the 5. reticidata 
ceases, and reaches its maximum 
abundance about 4,700—4,800 
feet, after which it rapidly dimi¬ 
nishes in amount up to 5,000 feet. 
Where Salix herbacca is thus 
abundant it carpets the ground, forming a dense mat to the 
exclusion of most other plants. 
Betula nana near its highest limit is much stunted and its 
leaves much reduced in size. Fig. 27 a — e shows the striking con¬ 
trast in the size of the leaves of plants growing at 3,200 feet and 
those growing at or near the upper limit of 4,800 feet. There are 
also differences in anatomical structure which are due to this 
difference in altitude. The large leaves from the lower altitudes 
are somewhat thicker, with a palisade tissue of two and a portion 
of a third or often of three complete layers, whereas the palisade of 
the small leaves from high altitudes consists of one complete layer 
and a second smaller and more or less incomplete layer. The 
spongy mesophyll cells are also larger and more inflated ( vide Fig. 
28 a and b). 
One character, possessed by all the leaves of Betula imna 
examined, irrespective of altitude, is the presence of an incomplete 
layer of hypodermal cells beneath the upper epidermis. This layer 
is derived from the upper epidermis, the cells of which are of variable 
Fig. 27. A and B, leaves of Betula 
nana from an altitude of 3,200 feet ; 
C -E, leaves from an altitude of 
4,800 feet. Natural size. 
