CHAPTER VII. 



THE ORIGIN AND OCCURRENCE OF RUBBER. 1 



Well-nigh nothing is known about the cytology of rubber-secreting 

 cells. The great initial difficulties in the investigation have arisen from 

 the fact that in most rubber-producing plants this material occurs in 

 latex. In the guayule, as in a few other known plants, the rubber is laid 

 down within certain cells, in a manner analogous to the formation of starch. 

 Although the study of the early cytological activities which lead to the 

 accumulation of rubber still presents great difficulties, since some of the 

 agents used dissolve out the rubber, nevertheless it has been possible to 

 determine the relation of growth and of some of the more important ex- 

 ternal conditions to rubber secretion. These results are important eco- 

 nomically, since we are able to determine the time at which the maximum, 

 or near the maximum, amount of rubber occurs, and during what period 

 rubber is absent from the new tissues, and thus establish rules of pro- 

 cedure in the harvesting of the shrub. 



METHODS. 



The solubility of rubber in xylol and the like prevents the use of 

 paraffin. The preparations must therefore be studied in such a fashion 

 that the rubber is intact. When present in large quantities it is easily 

 recognized, after one has become acquainted with its appearance. 2 When 

 in small quantities, however, it may easily be mistaken for droplets of oil 

 or resin, or for protoplasmic or other granulations, and inasmuch as oils 

 and resins as well as rubber are stained by alkanet, these substances, if 

 present, must be removed by suitable solvents which will leave the rubber 

 unaffected. For this purpose I have treated sections with high-grade and 

 absolute alcohols, acetone, and potassium hydrate, applying alkanet both 

 before and after. There remains the possibility that the substances which 

 remain and which react to alkanet are not always rubber in its final form, 

 but there can be little doubt that the materials which are referred to below 

 are either rubber or are substances in the course of change into rubber. 

 The evidence seems to indicate, however, that it is rubber which we are 

 dealing with. 



In seeking to determine with accuracy the facts of the distribution 

 of rubber in the tissues, the accident of displacement of rubber in the act 

 of sectioning must be properly guarded against. When rubber is present, 



1 The substance of this chapter was presented in a paper entitled "The 

 responses of the guayule, Parthenium argentatum Gray, to irrigation," before the 

 Botanical Society of America, Boston, December 1909. 



2 When in readily appreciable quantities, resin and rubber in the guayule may 

 readily be distinguished by alkanet. Resin takes on a brilliant scarlet, while rubber 

 has a purplish tinge, and is, to the naked eye, blood-red. 



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