several rains. In Australia flatweed (Hypochaeris radicata), a plant resembling the dandelion, sends up flowers at all times of the year after showers following a period of dry weather.
But too much rain may induce a rank vegetative growth which is detrimental to the development of flowers. Much rain will prevent the blooming of mesquite a second time. Several years ago mesquite in Texas twice started “to tag,” and heavy rains prevented the buds from maturing. In 1921, in Arizona, according to Vorhies, after an unusually dry winter, there was a fair flow from mesquite. But in July and August the rainfall was exceptionally large, the precipitation at Tucson being 6.2 inches in July alone. A second blooming was expected, but the superabundant moisture induced a heavy vegetative growth and the trees scarcely blossomed at all a second time.
As has been already pointed out, nectaries secrete nectar only when their cells are distended, or turgid. The nectar must exert a strong pressure on the elastic cell-walls. If the cells are flaccid no nectar will be secreted. The advent of dry weather has often cut short the honey flow. If want of water causes the leaves and flowers to droop no nectar will be produced. Kenoyer found that plants which had been allowed to dry to the withering-point several times in the course of their growth yielded a little less than one-third as much nectar as normal plants. The nectar of open wheel-shaped flowers, like those of buckwheat, may be largely washed away by rain, as has been proven by the use of an artificial spray.
Humidity is the quantity of water vapor in the air. On a rainy day the air is saturated with moisture and the humidity is 100 per cent.; on a dry day it is about 50 per cent. In the semi-arid region of Nevada, where the average rainfall is about 10.6 inches, the average annual humidity is 51 per cent. In Iowa, where the average annual rainfall is 30.9 inches, the average annual humidity is 76 per cent. Honey stored in a humid atmosphere contains more water than that gathered in a dry air. Analyses of honeys from Wisconsin showed an average moisture content 3.79 per cent, greater than was found in honeys from Nevada. Less water evaporates in humid air than in dry air. Nectaries secrete a larger quantity of water in humid air, because the evaporation of water from the leaves is checked and accumulates in the plant cells under greater pressure than when there is little moisture in the air. In a saturated air, water may escape also in drops from the ends and teeth of the leaves. But while the amount of water passing through the nectaries increases, the quantity of sugar secreted remains about the same.
Scholl states that cotton in Texas yields nectar most abundantly in early morning, but that the quantity decreases as the air becomes dryer at mid-day. Toward evening it again increases unless the air is very hot and dry. During cloudy days the yield is good throughout the day. Moist air, says Sladen, with slow evaporation, seems to be necessary for a good gathering from heather in England. Buckwheat flowers secrete much more liquid when placed under a bell jar than in the open air.
TEMPERATURE AND NECTAR SECRETION
Temperature exerts a greater influence on the functions of flowers, including nectar secretion, than does light, humidity, or rainfall. According to recent investigations in France, the movements of the parts of flowers, their opening and closing, are caused by the daily rise and fall of temperature. During a partial eclipse of the sun there was a fall of ten degrees, and flowers ceased to open their corollas. The temperature at which nectar secretion begins, and at which the largest amount of nectar is secreted, varies greatly with different honey plants. In general, high