Abstract
IN NATURE of April 5 appeared a letter by Mr. Folkard under the heading “The Bleeding of Cut Trees in Spring”. Prof. Priestley's remarks on that letter in the same issue bring out points which are of particular interest to me. Some years ago I carried out work on sugar producing palms, during the course of which I made careful measurements of the yields of sap from the cut stems of a number of wild date palms (Phönyx sylvestris) throughout the cold weather. Daily records were also kept of the maximum and minimum air temperatures and the sugar content of the sap was determined. The air temperature exercises a very marked effect on the yield of sap. A rise in temperature is always followed by a falling off in yield, whereas a fall in temperature is always followed by a rapid increase in yield. Moreover, the coldest months of the year are the best from the point of view of sap yield. During these experiments the maximum day temperatures ranged from 71° to 83° F., and the minimum night temperatures from 46° to 66° F. Cloudy nights have an adverse effect on sap yield and on its quality. Newlands (“Handbook for Sugar Planters”) also records that in the case of the maple tree the best yields of sap are obtained during cold clear nights following bright warm days.
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