64 



Pink Disease of Plantation Rubber 



medullary rays, the mycelium travelling in the cells of the rays spreads 

 out and enters the vessels (Fig. 3). All the elements of the wood become 

 permeated with the hyphae which pass readily through the large pits 

 without constriction. The deep medullary rays favour a quick passage 

 transversely while the large pits allow a ready passage for the hyphae 

 among the elements of the wood. 



The most characteristic feature in the wood of Hevea brasiliensis 

 attacked by Pink Disease is the presence of tyloses in the vessels 

 (Figs. 4 and 5). Every specimen examined showed these bladder- 

 like ingrowths from the living cells bordering the vessels, plugging up 



Rg. 2. Section showing hyphae in medullary ray cells, x 



liin. 



the water courses. Specimens of healthy wood, of wood taken below 

 the tapping area, and of wood attacked by Diplodia cacaoicola were 

 examined, but in no case was there any indication of tyloses. Thus 

 the formation of tyloses in Hevea brasiliensis appears to be a response 

 to the attacks of Corticium salmonicohr. 



The tyloses are of two types, (a) those in which the cells retain 

 their thin cellulose walls (Fig. 1), (fc) those in which the walls become 

 lignified (Fig. 5). Both types are found in the same branch in 

 adjoining vessels though the second type is rarely produced. The 

 tyloses which become lignified lose their protoplasm and appear like 



