temperature favors nectar secretion, since the membranes of the nectary are rendered more permeable, the solvent power of water is increased, and chemical changes in the plant take place more readily. Records of honey production for 29 years at Clarinda, Iowa, showed that the largest yield from white clover was on days with a temperature from 80 to 90 degrees F. According to the reports of 19 large honey producers in Iowa, Missouri, and Illinois, the largest yields of nectar are obtained when the temperature ranges from 80 to 100 degrees. Alfalfa yields nectar most freely when there is ample moisture in the soil of irrigated land, during a succession of hot days. On the other hand, very high temperature, exceeding 95 to 100 degrees, may be injurious.
Several very important honey plants yield nectar abundantly at moderate, or in some instances relatively low, temperatures. Willow-herb will yield nectar freely during weather that would stop all storing from basswood and clover. “I have seen,” says Hutchinson, “bees working on raspberry when the weather was so cool that white clover would not yield a drop of nectar.” Sainfoin in England will yield between 60 and 65 degrees, even with very little sunshine. Sladen records that on a September day, in England, bees brought in from heather most honey between 8 a. m., when the temperature was only 48 degrees, and 10 a. m., when it was 52 degrees. I have also a report from British Columbia of bees working on flowers when the temperature did not rise above 52 degrees all day.
Cold or cool nights followed by warm days are better for nectar secretion than a uniform temperature. Sugar is formed most rapidly at a high temperature, but its accumulation in the plant is favored by a lower temperature. On a warm, clear day sugar is made in the cells of leaves more rapidly than it can be carried away, and in the form of starch it is stored in the granules (chloroplasts) containing the leaf-green pigment. During the night, with its lower temperature, it is moved out of the leaves in the form of glucose or grape sugar (dextrose), and is carried to different parts of the plant where it is used or stored. The best, or optimum, temperature for the manufacture of sugar by leaves, in many plants, is between 68 degrees F. and 72 degrees F.; but it may be made slowly in some cases at much higher and lower temperatures. Plants can not make sugar in the night time, because sunlight, the force which drives the machinery of the leaf, is turned off when the sun drops below the horizon.
But growth may take place in darkness as well as in the light. The best temperature for growth with most plants is between 77 degrees F. and 90 degrees F., for there is an optimum temperature for growth as well as for the making of sugar. But it will be noted that a higher temperature is required for growth than for the manufacture of sugar. In the Temperate Zone the temperature at night is usually much lower than in the daytime, and in spring often falls nearly to or even below the freezing-point. During a cold night growth is checked, or stopped, and the sugar formed during the day remains unused, until with the return of light the temperature again rises. There will thus be more sugar available after a cold than after a warm night, during which much of it has been used for growth. Woody shrubs and trees contain the largest amount of starch in the fall, which during the winter is changed into sugar found in the bark and winter leaves. A rise of temperature in the spring causes the sugar to change back into starch. The conversion of insoluble starch into soluble sugar and the reverse is effected by the agency of ferments known as enzymes, the action of which is influenced by temperature.
LIGHT AND THE MAKING OF SUGAR
The secretion of nectar is closely related to the amount of light the plant receives. The force, or energy, required for the manufacture of food material is