191 



which coagulates from the milk, but as the butter which may be 

 separated both from the curd and from the still more watery con- 

 stituents 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 Castilloa tree, and even in different 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.* 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 im- 

 possible and in general reasoning on the subject the inquirer must 

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

 quality and quantity of latex in trees known to produce rubber. 



FUNCTIONS ASCRIBED TO LATEX. 



The nature and functions of latex in plants are difficult prob- 

 lems. Many dissertations have been contributed to swell the ex- 

 perimental and controversial literature of the subject. Many in- 

 teresting 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 concrete problems of rubber pro- 

 duction until a general theory of the function of latex or milky 

 juice in plants can be formed. Very different suggestions re- 

 garding the uses of latex have been defended by different investi- 

 gators 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 reser- 

 voirs forthe storage of elaborated food materials, while others believe 

 that latex is an excretory or waste product, even to the proteids, 

 starch, and sugar with which the milky fluid is commonly 

 charged. Protection against insects and snails has also been urged 

 as the function of latex. One of the most recent writers on the 

 subjectf reviews and dismisses all the previous suggestions appa- 

 rently for the reason that none is of general validity and, after 



*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 hut tlie elasticity 

 Boon ilinappeare when the gum is exposed to the air and repeatedly stretched betwe.n 

 the fiiigers. 



fPercy Groom on the Function of Laticiferous Tubes, Anna! of Botany 3: 157,1898. 



