44 RUBBER AND 



soon break down, and a more or less free passage 

 is established throughout. The interstices of the net- 

 work are occupied by the medullary rays t which are 

 groups of thin-walled cells running by way of the 

 cambium radially from the bark into the wood. The 

 medullary rays serve in part as conducting channels for 

 food materials between the wood and the bark, and 

 in part for the storage of reserve food supplies. The 

 whole laticiferous system consists of a series of such 

 networks one within the other, extending round the 

 stem. The space between two adjoining networks is 

 occupied by the cells and tubes of the phloem. From 

 the arrangement of the laticiferous vessels it will be 

 readily understood that their contents can flow more 

 easily in the longitudinal direction than transversely 

 round the stem. 



Hevea. Gross structure of the bark. 



If we begin an examination of the bark of Hevea 

 from the outside of the tree, we find first a brown or 

 grey layer of cork, which is generally thin in young and 

 untapped trees. The function of the cork is purely 

 protective. Secondly, beneath the cork there occurs in 

 healthy trees a thin dark green layer of living cells. 

 Discolouration of this layer may be taken as indicating 

 that the health of the tree requires attention. Thirdly 

 comes a granular yellowish or pinkish layer of tissue, 

 which makes up the greater part of the thickness of the 



