ON GUM AND SUGAR TREES 



will not flow in abundance or it is of such quality 

 as to have no value. 



The manner of production of the sap may 

 be more or less accurately inferred from what we 

 have already learned of plant physiology. We 

 know that the leaves of the tree metamorphose 

 water and carbon into sugary substances which in 

 turn are transferred to various parts of the plant 

 to be stored, usually in the form of starch. In the 

 case of the maple, we may assume that the carbo- 

 hydrates, as they are manufactured in the leaf- 

 laboratories, are transferred in the current of sap 

 that flows downward from the leaves through 

 branches and trunk as a countercurrent in the 

 cambium until it finally finds its way to the roots 

 of the tree and is there stored for the winter. 



When spring comes and it is time for the new 

 leaf buds to put forth, the supplies of nourishment 

 are retransformed into soluble sugars, dissolved in 

 the water that is taken in by the rootlets, and trans- 

 ferred from cell to cell and along the little canals 

 in the wood under the cambium layer of the bark, 

 until they reach the twigs where the leaf buds they 

 are to nourish are located. 



It is doubtless the so-called "root pressure" 

 (which we have been led to interpret as due to 

 osmosis) forcing the sap upward that causes it to 

 flow from the wound in the tree made by the 



[245] 



