THE TAPPING OF RUBBER PLANTS 59 



biiim are kno^vn as secondary wood and secondary bast, 

 to distinguish them from the primary wood and bast 

 of the young shoots which are developed from the meris- 

 matic tissue situated at the growing point of the stem. 

 The primary structure is only seen in the green portion of 

 the stem within a short distance from the growing point ; 

 in the older portions secondary thickening will have 

 commenced, and the formation of wood and bast by the 

 cambium, with the consequent increase in the girth of 

 the stem, results sooner or later in the disorganisation 

 of the external primary cortex. The " bark " of rubber 

 trees when ready for tapping is consequently composed 

 of secondary cortex, and it is the laticiferous vessels 

 developed in this tissue which are of practical importance 

 to the rubber planter. 



In the secondary cortex the laticiferous vessels run in 

 a longitudinal direction, i.e. parallel to the axis of the 

 stem, and they are most numerous in the inner layers 

 near the cambium, where they are arranged in series 

 of concentric rings encircling the stem. The generally 

 accepted view of the origin of these vessels is that, like 

 the vessels of the wood and the sieve-tubes of the bast, 

 they are formed from rows of special cells produced by 

 the cambium. The cells cut off from the cambium 

 which are destined to develop into laticiferous vessels 

 are arranged in longitudinal rows, and at a very early 

 stage the transverse walls dividing them are absorbed 

 with the formation of a continuous tube which soon 

 becomes filled with latex. According to this view, the 

 laticiferous vessels in the secondary cortex are produced 

 solely by the cambium, which is continually adding 

 fresh layers to the inner side of the cortex to replace 

 the exhausted tissue of the external layers. 



This opinion regarding the development of the latici- 

 ferous tissue is, however, contested by Wright, who asserts 

 that the vessels are formed de novo in the cortex by the 

 absorption of the walls of ordinary cortical cells. 



With reference to the functions of the wood and cortex 

 it may be briefly stated that the vessels of the wood 

 serve to conduct an upward current of water, containing 

 various substances in solution, from the roots to the 

 leaves, whilst the sieve-tubes of the cortex conduct a 

 downward current of liquid, containing food-products 



