The Ecology of the Driva Valley. 369 
Rhodiola. This plant is exceedingly abundant both in the valley 
and on the surrounding mountains, occurring in greatest quantity 
at about 4,900 feet. At the lower altitudes it grows well in its 
normal habitat in the cracks and fissures of rocks, but at the higher 
altitudes it occurs in a somewhat stunted form in the lichen- 
formation, and even among wet mosses at the sides of the small 
streams! Two plants were specially preserved for anatomical 
investigation, one from a constantly wet place, and another which 
was growing close by the first one, but on the dry summit of a rock. 
They showed marked differences in the structure of the leaves. 
That growing in the dry place had relatively thick leaves with large 
mesophyll cells and relatively few and small intercellular spaces. 
The cells of the upper epidermis were also considerably larger than 
those of the lower epidermis. The plant growing in the wet 
situation had thinner leaves and much smaller mesophyll cells. The 
intercellular spaces were numerous and often large, and both upper 
and lower epidermis were of a similar character (vide Fig. 29a and b). 
In and about the trickles of water coming from melting snow 
between the two summits of Knutsho, and in one of the bleakest 
and most exposed situations on the whole mountain, were tufts of 
Phippsia (Catabrosa) algida . This inhospitable spot was about 
5,000 feet and among some extensive snow slopes. The leaf of this 
grass is short and blunt, reaching a length of about an inch. It is 
a leaf characteristic of cold, exposed situations, and in transverse 
Fig. 30. Leaf of Phippsia algida (=Catabrosa algida ). A, transverse section 
to show the folding of the leaf, the seven vascular bundles ( v.b .), the position 
of the motor cells (m) and the very small amount of stereome ( st .) B, part of 
transverse section of leaf to show the nature of the epidermis and the meso¬ 
phyll. w, upper surface ; l, lower surface. 
section is nearest to the leaf of Aira prcecox x . It is a folded leaf, 
with two folds, and motor-cells at each fold, as in Catabrosa aquatica. 
1 L. Lewton-Brain, “ Anat. Leaves Brit. Grasses,” Trans. Linn. 
Soc. Bot., 2nd. ser., Bot. vi., 1904, t. 36, f. 5. 
