Relation to Humidity.

Humidity in general is more or less directly connected with precipitation and fog formation. These aspects of it have already been discussed sufficiently for the present. Humidity due to evaporation is of some slight interest. Vegetation is always much more luxuriant along river banks and other masses of water particularly in this Eastern Area. Even in the cold temperate belt, where precipitation is high, along the smallest water-courses the bryophyte flora is especially rich. The pendulous mosses and liverworts such as Weymouthia and Lapicolea abound.

Of greater interest, however, is the lowering of humidity brought about by wind. This agent, by removing the comparatively saturated atmosphere from the immediate neighbourhood of plants, causes steepening of the saturation gradient in the vicinity of the transpiring surfaces, thus greatly increasing the rate of transpiration. The result of this is an appreciable reduction in leaf-surface, and, as a consequence, decreased rate of assimilation, and, hence, growth. In the most adverse conditions such as exist on Mount Omega, 1100 m., which is subject to severe dry north-westerly winds descending from the main divide, the leaves of Nothofagus Menziesii are reduced to half the area of those growing in similar but sheltered places, such as certain places on the timber line, 1200 m., on Mount Alpha. The average length of leaves in the two places is about 7 mm. and 10 mm. respectively. The average rate of radial increment per annum of the uppermost branches is about .5 mm. and 1 mm. in the two respective places. Leaves of Olearia Colensoi, Nothopanax Colensoi, and Pimelea longifolia taken from the scrub formation from windward and leeward sides of several ridges, where the mechanical force of the wind is not evident, showed even greater variation. Although the respective places were only a few metres apart and separated