175 



A much larger yield is actually obtained by cutting into the cam- 

 bium than by cutting almost into it. 



Another thing to be avoided is cutting too deep. When a cut 

 goes through the cambium into the wood the healing commences 

 at the edges of the cut cambium, and has to spread slowly, making 

 new cambium before it can make new bark or wood. If too much 

 wood is exposed in this case it will often dry up before the cam- 

 bium can heal over and in that case it never heals. I have seen 

 an old machete cut with a half inch of wood exposed, with the 

 bark thoroughly healed all round it. I was told that it had been 

 that way without healing for two years. 



As to the time to tap, there appears to be no reason why the 

 trees should not be tapped at any time during the rainy season. I 

 should imagine that the dryest season in March and April would 

 be a poor time, but I have not been here during that season. Rain 

 generally makes the milk rather watery and makes it flow more 

 freely, but I have never seen it so watery that it would not pay to 

 tap, except in a tree which had been recently tapped. Tapping in 

 heavy rain would not do, as it would wash the latex, which does 

 not flow into the cups and might fill up the cups and spill the 

 latex in them. 



Temperature affects the flow of latex very noticeahly. The 

 yield of rubber is much greater in the early morning than at any 

 other time of the day, and always decreases toward noon and in- 

 creases toward night. This is not so noticeable on cool cloudy 

 days. It would probably not be so noticeable in a shady planta- 

 tion and for this reason some people have claimed that shade 

 grown trees yield more. I believe that the reason temperature af- 

 fects the flow is because a large amount of the water is evaporated 

 and the latex is more solid and does not flow so fi'eely. 



Experiments of others have shown that young trees and younger 

 parts of old trees contain a large percentage of resin in their rub- 

 ber. I have made one observation which suggests a reason for 

 this. In cutting a temporary branch, or leaf stem, it is noticeable 

 that the latex comes very close to the outside bark and that there 

 appears to be a second ring of tubes in the inner bark. Micros- 

 copic examination of these parts shows a large number of collen- 

 dyma cells close to the outside bark. These cells are similar to 

 bast fibres, but the the thick part of the walls is not uniform. Col- 

 lenchyma cells are never formed by older trees except in their 

 young parts. I think it possible that these coUendyma cells carry 

 latex which is richer in resins than ordinary latex and which may 

 possibly be entirely resin. Of course these coUenchyma cells re- 

 main in the plant as it grows older but form a very small propor- 

 tion of its tissue at that time. It is possible that rubber or resin 

 may have some chemical relation to the cellulose of which the 

 thick walls of both coUenchyma and bast fibres are formed. 



