34 DIMORPHIC BRANCHES IN TROPICAL CROP PLANTS. 



obtained from first tappings, later attempts to secure latex from the 

 vicinity of an old cut may be very disappointing. In the Para rub- 

 ber tree (Hevea) there is a so-called ''wound response" that results 

 in continued and increased yields of latex from the paring back of 

 the edges of the wounds, but in the Central American rubber (Cas- 

 tilla) the tapping of the bark in the vicinity of old cuts may bring 

 out very little latex. The bark pressure that forces the latex out of 

 new cuts is not restored around the old cuts. Only a small propor- 

 tion of the latex is extracted by the present methods of tapping; 

 the rest remains and dries up in the bark. If bark could be pro- 

 duced more rapidly by vegetative propagation, it might become prac- 

 ticable to harvest the bark as well as the latex and extract the rubber 

 by mechanical means. Branches from the more iDroductive trees 

 would be available for extending the plantation. 



THE PRUNING OF RUBBER TREES. 



The fact that the rubber tree prunes itself so extensively leaves 

 little work of this kind for the planter to do, but two precautions are 

 not unworthy of consideration. The self-pruning mechanism does 

 not always work successfully. If growth is very rapid the trunk 

 may enlarge around the bases of the temporary branches and hold 

 them in place, even after they are dead. This is also likely to hap- 

 pen when a branch has been injured or dwarfed, and thus lacks the 

 Aveight necessary to break it away from its socket. Such decaying 

 branches may give fungi or insects an entrance to the wood of the 

 tree and thus induce decay. It would require very little additional 

 labor to keep the plantation entirely clear of them. In most cases a 

 pole with a simple hook or elbow at the end would enable them to be 

 pulled out of their sockets, which would be better than cutting them 

 off. The pruning away of some of the permanent branches may be 

 desirable in the occasional instances where these come out too low 

 down. The earlier these are removed the better, to keep the trunk 

 of the tree smooth and erect for purposes of tapping. 



DIMORPHIC BRANCHES OF COFFEE. 



The upright branches or limbs of the coffee shrub are the equiva- 

 lents of the original main stem; they bear no fruit, but can give rise 

 to other uprights arid to lateral branches. (See PI. III.) The 

 laterals bear flowers and fruit, and can also give rise to other 

 branches of the same form and function, called secondary laterals, or 

 simply secondaries, but no lateral branch ever produces a true up- 

 right branch. Unlike the cotton plant and the rubber tree, each 

 internode of coffee bears two opposite leaves and is capable of pro- 

 ducing two sets of branches, two axillary and two extra-axillary. In 



198 



