32 CENTRAL AMEBIC AN RUBBER TREE. 



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, however, a very slow process, owing to 

 the time required to bring the generations to seed-bearing maturity. 

 With Castilla 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 plantations as possible with cuttings from their most pro- 

 ductive trees, and to watch for the best ;i 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 

 selective improvement of rubber plants may be pushed forward with- 

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

 mines its formation in the plant, since all that the planter needs to 

 know is that rubber is present in more than average quantity in cer- 

 tain of his trees, and he may expect that by propagating from these 

 under the same conditions a higher average of production 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: 



You can no more grow a rubber tree without the rubber milk in it than you can 

 grow a sugar-maple tree without the sugar 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 gather- 

 ing 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 experience in 

 agriculture should have prevented the inference that because a plant 

 thrives when young it is certain to reach a productive maturity. 

 Many of the early experimenters in rubber culture have found to 

 their cost that the Central American rubber tree, at least, can grow 

 with the most promising vigor and yet fail to deliver any approxima- 

 tion of the estimated quantities of gum. Indeed, this fact might have 

 been learned with vastly less expense of time and money by consult- 

 ing the native rubber gatherers, who are thoroughly aware that many 

 u ule" trees give no return for tapping. The realization of this sim- 

 ple and fundamentally important fact has been delayed through exist- 

 ence in some of the Central American rubber districts of a second 

 species of Castilla.** called by the natives ;i ule macho,** or "male 

 rubber.'* because it o-ives little or no milk. 



