The streams which the road here crosses descend from different parts of the ridge of Nagkanda. They occupy the bottom of deep ravines, and are in their whole course densely wooded. These ravines are, in their upper part especially, extremely steep and rocky, often with precipitous walls, and scarcely practicable even on foot. The denseness of the forest is principally due to their northern exposure, and to the consequent more equable temperature and greater humidity. They contain many trees not previously observed on the journey from Simla, though all of them, I believe even the horse-chesnut, occur in the very similar steep rocky ravines below Fagu. The alder is a common tree at 4-5000 feet in the north-western Himalaya, always in valleys and on the banks of streams.
In this shady forest I collected a considerable number of plants which do not occur at Simla. A scandent _Hydrangea_, the loosely-adhering bark of which separates in long rolls like that of the birch, and is used as a substitute for paper, was seen twining round the trunks of trees. I observed also a fine _Calanthe_, and abundance of _Adenocaulon_, a remarkable genus of _Compositæ_, which, till Mr. Edgeworth discovered a species in the Himalaya, was only known as a native of South America. In the thickest part of the forest in this ravine, I was also fortunate enough to meet with a few specimens of _Balanophora_, which here probably attains its western limit. All these plants are abundant forms in the most humid parts of Nepal and Sikkim, and their presence may, I think, be regarded as indicative of a more equable temperature throughout the year than prevails in the more open parts of the Sutlej Himalaya. The range of mountains on which Nagkanda stands certainly intercepts a great deal of moisture during the rainy season, and therefore makes the valleys on its northern aspect less humid at that period of the year. This would appear to be more than counterbalanced by the effect of the dense forest in keeping up moisture and preventing radiation during winter, for the cold and dryness of that season seem to have a much greater effect in determining the cessation of the forms characteristic of the eastern Himalaya, than the diminished rain-fall during the three months of the rainy season.