OBSERVATIONS UPON THE PHENOMENA OF PLANT-LIFE. 339 



CAUSE OF SUCTION. 



This theory explains the fluctuation of the o:anges, and accounts for the 

 einscular fact that the upper one shows the most pressure and the greatest 

 variations, inasmuch as the branches and twigs would, of course, be most 

 quickly and powerfully affected by the heat of the sun and the temperature of 

 the atmosphere. The pressure of the expanded gases in a tree in a normal 

 condition would facilitate the re-absorption by the wood of the liberated sap. 

 Their contraction by cold would also cause the cessation of the flow from a 

 tree which was running, and produce the remarkable phenomenon of suction 

 exhibited by the gauges at night or during frosty weather. 



An important and elegant demonstration of this theory was obtained by 

 cutting large branches, fifteen to twenty feet in length, when the thermometer 

 was below zero, from trees of the sugar maple, white birch, elm, hickory, but- 

 tonwood, chestnut, and willow, and suspending them in the warm air of the 

 Durfee plant-house. The maple soon began to bleed at the rate of twenty- 

 four drops per minute, while the buttonwood bled eleven drops, and the 

 hickory exuded a little very sweet sap, precisely as in spring. The birch, elm, 

 chestnut and willow did not flow at all, and were not even moist on the freshly- 

 cut surface. 



A mercurial gauge, attached to the end of a frozen branch of sugar maple, 

 indicated pressure and suction when the temperature was raised and lowered, 

 precisely as it would have done upon a maple tree during the ordinary alter- 

 nations of day and night in the spring of the year when the sap is flowing. 



In the warm regions of Asia, Africa, and America, are found about one 

 thousand species of palm trees, from many of which a sweet sap is obtained in 

 large quantities. This is simply allowed to ferment, and drank as palm-wine 

 or toddy, or distilled for the production of a sort of brandy, or it is evaporated 

 for the extraction of its sugar in the form of syrup, or of a more or less crys- 

 talline solid called jaggery. In the province of Bengal, in India, more than one 

 hundred million pounds of palm-sugar are manufactured annually, while the 

 total product of palm-wine in the world greatly exceeds that of wine from the 

 grape. 



PALM WINE. 



There are three principal methods adopted in different countries for obtain- 

 ing the sweet sap of palms. In Chili, trees fifty feet high are felled in such a 

 way that the top will lie higher than the butt of the trunk, and the single ter- 

 minal bud with the crown of leaves is cut off. The sap flows abundantly from 

 the higher end of this log, and if a fresh slice of wood be removed every day 

 the bleeding continues for several months. The yield is greatest during the 

 warmest days, and amounts in all to an average of ninety gallons, or about 

 seven hundred and twenty-five pounds from each tree. This sap is mostly 

 evaporated and utilized as a very agreeable syrup called palm-honey. 



IN" INDIA, 



it is customary to make incision into the wood of trees near the top, from 

 which, during the cool months, the sap flows freely. From the common wild 

 date-palm the annual yield of sap is about two hundred pounds, containing 

 some eight pounds of sugar, or four times the average product of the sugar 

 maple. Much the larger proportion of palm sap is obtained, however, from 

 the lal^ge branching flower-stalks of the inflorescence. These are produced in 



