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Moisture and Fruit-growing.
degrees of latitude (reaching northwards to about 27°). Beyond all these bounds there are special localities in which fruits of the adjacent zone may thrive for a series of years, and the fruits of contiguous zones overpass. The strawberry is probably the most tractable of all our fruits as respects climates, because its stature and habit allow it to be protected from extreme cold and its short period of growth allows it to thrive in the cool season of the warmest sub-tropical regions.
The annual temperature of a region is chiefly determined by three factors, - the latitude, the altitude, and the proximity or remoteness of large bodies of water.
The moisture determinant.- The second chief factor of climate in determining the fruit-zones is relative humidity. Whilst the isotherms-or lines of equal temperatures-run easterly and westerly, the isohyetals or lines of equal rainfall-have no intrinsic direction. They are determined by physiographical characters. In the United States, there are three general fruit-zones which are marked by peculiarities of rainfall. These are the Atlantic zone, a moist area which is bounded westward approximately by the Mississippi River; the plain zone, extending westward to the mountains; and the Pacific slope zone. The two latter are relatively dry.* The interior or plains region is particularly
*We should, perhaps, make a fourth division, to comprise the arid or Sonoran zone of New Mexico and Arizona, but this area is yet too little known in a pomological way to demand specific treatment here.
trying to fruits because of the strong and dry winter winds, which evaporate the moisture from the trees whilst the ground is often so deeply frozen that the roots cannot supply moisture. There is probably always evaporation from tree tops in winter when the air is drier than the wood.
The fact that moisture may be lost from winter twigs is a most important consideration in the study of the winter injury of trees, and it throws light upon the severe damage which often follows the "dry freezing" of nursery trees in transit and of fall-planted trees. A few figures will show the extent to which evaporation may take place through the bark of dormant twigs.*
The extent to which loss of moisture may take place through the bark of dormant twigs may be determined by cutting off the twigs and quickly sealing over the ends with wax, weighing them, and then detecting the loss in weight from time to time. The following figures of such measurements will serve to emphasize the fact that moisture is lost from winter twigs, although they are not designed to show the actual rate of this loss when the twigs occupy their natural position on the tree.
April 7, a cion of apple weighing 4.425 grams was placed on a balance, and the loss by evaporation measured at intervals during three days. The cut end of the cion was sealed with wax to confine evaporation to to that which may take place through the bark. The balance or scales was placed
*Bailey, Cornell Exp. Sta., Bull. 117, pp. 385-388. Work done in Michigan.
Loss of Moisture from Winter Twigs. 11
a living-room, where the the readings could be taken at frequent intervals. It will be noticed that the rate of evaporation was nearly constant, averaging about one-half a centigram per hour:
It has been said that the rate of the loss of moisture from trees in winter determines the relative hardiness of different varieties of apples, and of some other fruits; and similar inferences have been made respecting the ability of foliage to endure summer droughts. The following table shows studies of twigs of varieties of different degrees of hardiness, but it will be seen that the per cent of loss of moisture bears no relation to the supposed hardiness of the varieties.
Early in April, twigs from the previous year's growth were taken from several varieties of apples, which vary much in their ability to endure our climate (those marked by asterisks being supposed to be the hardiest varieties). The twigs were carefully weighed, and the cut ends were then sealed with wax to prevent evaporation save through the bark. At the expiration of two days the wax was removed and the twigs were again weighed. The twigs were kept in an open shed:
The following table shows that there is great variation in the rate of water loss between twigs of the same variety of apple:
Evaporation from Trees in Winter.
This subject of moisture loss from dormant trees seems to be a most important one, and it is strange that the matter seems to have escaped the attention of pomologists. In order to spread a knowledge of the subject, further studies in the evaporation from winter twigs have been made for me by my student, A. L. Knisely, M.S.:
"In January, 1897, I cut twigs of various kinds about one foot in length, and took them to the laboratory. When ready to weigh the twigs, they were cut down to about four or five inches in length, the object of the second cutting being to leave as little time as possible between the cutting of the twigs and the weighing. As soon as the short twigs were cut, they were weighed, and the freshly cut ends were then dipped in melted paraffine, thus sealing the cut surfaces and preventing evaporation except through the bark and buds of the twigs. After dipping in the paraffine, they were weighed again, and then put in places as much exposed as were the trees from which they came; in fact, in some cases, the twigs were tied on the trees and left there for 72 hours, and then weighed. They were afterward exposed for another 72 hours and weighed again, making a total length of time of 144 hours, or 6 days, that the twigs were exposed. During all this period, the thermometer registered below the freezing point. That there is loss of moisture by evaporation is shown by the following table, which gives the data obtained from the twigs of a number of our most common fruit and shade trees: