34 CENTRAL AMERICAN RUBBER TREE. 



stage is reached. Between the vegetable and animal milk no complete 

 analogy can be maintained, but it serves to illustrate the present point 

 if we think of the rubber not as the curd which coagulates from the 

 milk, but as the butter which may be separated both from the curd 

 and from the still more watery constituents of the milk. As the 

 churned butter is different, both mechanically and otherwise, from the 

 fat globules floating in the milk, so does rubber differ, and probably 

 to an even greater extent, from the semifluid globules of the latex 

 emulsion. Rubber, as such, has no function in the plant, and there is 

 nothing to indicate that the qualities which make it valuable to us are 

 of any significance in the vegetable economy. Furthermore, it 

 appears that at different stages of the Castilla tree, and even in differ- 

 ent parts of the same tree, the substance which becomes rubber may 

 be replaced by another, which hardens with exposure into a worthless, 

 nonelastic resin; indeed, resin and not rubber is a constituent of the 

 latex of the numerous relatives of the rubber-producing trees. a It 

 appears, then, that to trace any direct connection between rubber and 

 the economy of the tree is likely to be very difficult, if not quite 

 impossible, and in general reasoning on the subject the inquirer must 

 be content to learn, if possible, the causes which influence the quality 

 and quantit} T of latex in trees known to produce rubber. 



FUNCTIONS ASCEIBED TO LATEX. 



The nature and functions of latex in plants are difficult problems. 

 Many dissertations have been contributed to swell the experimental 

 and controversial literature of the subject. Many interesting details 

 have been discovered regarding many lactiferous plants, and many 

 suggestions and theories have been contributed to the subject of plant 

 physiology, but thus far no very practical result seems to have been 

 reached in this direction. Indeed, progress may have been impeded 

 by the idea that it is necessary to postpone the investigation of con- 

 crete problems of rubber production until a general theory of the 

 function of latex or milky juice in plants can be formed. Very differ- 

 ent suggestions regarding the uses of latex have been defended by 

 different investigators on the basis of studies of different plants. The 

 first observer compared them to the blood of animals and described 

 the globules of gum as corpuscles, a highly fanciful notion which later 

 writers have so zealously disavowed that they have felt it necessary to 

 deny any circulation at all. Some have held that the milk tubes are 

 reservoirs for the storage of elaborated food materials, while others 



a In the State of Vera Cruz, Mexico, grows a large-leaved species of Ficus, the milk 

 of which coagulates promptly into an elastic substance like true rubber, but the 

 elasticity soon disappears when the gum is exposed to the air and repeatedly stretched 

 between the fingers. 



