nights at alpine heights. Dry air is not sensibly wanned by the sunlight passing through it, but if it contains much water vapor its temperature rapidly rises, while the soil receives less heat. In the daytime the earth at high altitudes receives a large amount of sunlight, which the soil absorbs in a much greater degree than does the air. Thus on a clear September day, in the Pyrenees, at a height of 9400 feet when the air was at 50 degrees F. the soil was at 91 degrees F. The warm soil raises the temperature of the layer of air in contact with it so that the dwarfed plants of alpine heights find both soil and air very much warmer than at night time.
Nectaries are commonly associated with flowers, but they may occur on many extra-floral plant organs, and probably first appeared on leaves. They may be found on involucral leaves, as in cotton and poinsettia; on the flower-stalk, as in cowpeas; on the blade of the leaf, as in the turban squash; on the midrib of the leaf, as in cotton; on the leafstalk, as in the cherry, almond, and peach; on the stipules, as in jewelweed; but in the majority of cases they are useless and wholly or nearly functionless. But from the extra-floral nectaries of cotton, partridge-pea, cowpea, vetch, and a number of other plants, a large amount of honey is obtained. The leaf-nectaries of partridge-pea secrete nectar freely, but the flowers are nectarless — a paradoxical phenomenon. Active leaf-nectaries would appear to be a distinct disadvantage to the partridge-pea, since they tend to draw away insects from the flowers; but they are less injurious than would at first appear; for, no matter how abundant the supply of nectar, bees are compelled to gather pollen for brood-rearing. Thus, despite the absence of nectar, the flowers of partridge-pea are well pollinated by bumblebees and very productive of seed.