40 RUBBER AND 



and air; the second is to provide a channel for conduct- 

 ing the necessary mineral substances from the roots to 

 the leaves in a state of very dilute solution, and also for 

 conducting the elaborated food supply downwards from 

 the leaves to the roots. The liquid which thus circulates 

 through the different organs of the tree is popularly 

 known as the sap. A third function carried out by the 

 trunk or stem of most plants is the storage of reserve food 

 materials, which are accumulated in special cells, often 

 in the form of starch. 



As it is from the trunk of the tree that rubber is 

 derived in the vast majority of cases, it is necessary to 

 enter rather more fully into the structure and functions of 

 this region. The trunk of a tree is well known to consist 

 of two main portions the wood and the bark, including 

 under the latter term the layers described by botanists 

 as the cortex. The cortex may be roughly defined as 

 the softer internal part of the bark which adjoins the 

 wood. If the bark is stripped from the wood, separation 

 takes place at an extremely soft and delicate layer of 

 tissue known as the cambium. The cambium, as we 

 shall see later, is the seat of growth in thickness for both 

 wood and bark. 



Channels for the conduction of sap occur both in the 

 wood and in the bark, and two entirely different streams 

 of sap are associated with these two regions. An up- 

 ward current of sap occurs in the outer part of the wood 

 through a series of minute vessels, in which the mineral 

 substances absorbed by the roots are carried to the 



