i89 



selected seed or cuttings, and there is every probability that the 

 same will be true of rubber. In fact, the great variation in the 

 amount of rubber both in wild and in cultivated trees is itself an 

 indication that a ready response to selection may be expected 

 The selective improvement of trees propagated from seed is, how- 

 ever, a very slow process, owing to the time required to bring the 

 generations to seed-bearing maturity. With Castilloa much more 

 prompt results could be obtained by the use of cuttings made, of 

 course, from true or permanent branches. It would be excellent 

 policy on the part of planters to set as large a part of their plan- 

 tations as possible with cuttings from their most productive trees, 

 and to watch for the best " milkers" in each generation, just as 

 the sugar growers test the sugar content of individual canes and of 

 individual beets which are to be used for propagation. The se- 

 lective improvement of rubber plants may be pushed forward with- 

 out waiting to find out what the function of rubber is or what de- 

 termines its formation in the plant, since all that the planter needs 

 to know is that rubber is present in more than average quantity 

 in certain of his trees, and he may expect that by propagating 

 from these under the same conditions a higher average of produc- 

 tion may be secured. 



PROBLEMS PRESENTED BY THE LATEX, OR "MILK." 

 Of what use is the rubber milk to the trees, or why do the trees 

 make rubber ? These are the questions which seem to underlie 

 the scientific investigation of the cultural production of rubber. 

 At first it was taken for granted that the elaboration of rubber is 

 the special function of the rubber tree, an idea apparently indorsed 

 by some of the tree-planting companies in such statements as the 

 following : — 



Yon can uf) more grow a rubber tree without the rubbber milk in it than you 

 can grow a sngar-niaple tree without the sng.ir sap in it. The growing of rubber 

 trees in their own soil and climate is just as practical, just as safe, and just as 

 sure as gathering elm seed and growing elm trees therefrom. 



Rubber is not, however, the fruit of the rubber tree, except in 

 the financial and commercial sense, and even the slightest expe- 

 rience in Agriculture should have prevented the inference that be- 

 cause a plant thrives when young it is certain to reach a produc- 

 tive maturity. Many of the early experimenters in rubber culture 

 have found to their cost that the Central Americrn rubber tree, at 

 least, can grow with the most promising vigour and yet fail to de- 

 liver any approximation of the estimated quantities of gum. In- 

 deed, this fact might have been learned with vastly less expense 

 of time and money by consulting the native rubber gatherers, who 

 are thoroughly aware that many " ule" trees give no return for 

 tapping. The realization of this simple and fundamentally im- 

 portant fact has been delayed through existence in some of the 

 Central American rubber districts of a second species of Castilloa," 

 called by the natives " ule macho," or " male rubber," because it 

 gives little or no milk. 



