1919] Hall-Goodspccd: Chnjsil 211 



The microscopical method, in brief, consists in the cutting of cross 

 sections of a mature portion of the sample and the making therefrom, 

 after the action of suitable solvents, stains and mounting media, of a 

 preparation in which the rubber, if present, may be seen under the 

 microscope. As will be shown in the detailed description given below, 

 the process of making such a preparation is simple and may be suf- 

 ficiently well controlled to yield results that will be fairly uniform and 

 reliable. 



As in Guayule, the rubber in Clirysothamnus and Haplopappus 

 is present within the individual cells of the plant, and is not a latex 

 rubber. Like Guayule again it is found principally in the parenchy- 

 matous elements of the cortex ; indeed it may occur in any undifferen- 

 tiated elements lying without the cambium or, in other words, in what 

 is often spoken of as the "soft" or "inner bark." It may also be 

 noted here that rubber does not appear to be laid down during the 

 first year of growth of a tissue and indeed, unless present in large 

 amount, is not readily detected by the histological method in por- 

 tions of the plant less than three or four years old. A section of a 

 plant taken slightly above the soil line will exhibit tissues containing 

 practically maximum quantities of rubber. In investigating new and 

 untried species by the microscopical method it is wise to make sections 

 from well down the root as well as at the soil line. This is indicated 

 by our results on Haplopappus mentioned in the third part of this 

 report. In certain species of that genus practically no rubber was 

 found in sections of stem tissue, while quite appreciable amounts were 

 shown in sections taken well down the main root. We have found 

 in Chrijsothaninus that maximum quantities are borne by the stem, 

 although for a limited distance below the soil line the root may bear 

 an almost equivalent amount. 



In selecting and removing the material to be sectioned care must 

 be exercised to retain the bark. This is often diificiilt, as the dry 

 corky tissues readily split off, together with the living cortex, along 

 the delicate cambium region, from the hard, woody cylinder. When 

 the tissues are stripped off in this way it is useless to attempt the cut- 

 ting of sections, since practically the entire rubber-bearing cortical 

 area will be missing. It has been our practice to cut out the desired 

 piece of mature woody tissue^- and allow it to soak in water for twenty- 

 four hours before sectioning. This treatment softens all the tissues and 



1^ We liave found that a c-opino; saw can be very successfully employed to cut 

 out the piece of stem to be sectioned. 



