The Origin and Occurrence of Rubber. 1 83 



RATE OF RUBBER SECRETION RELATIVE TO GROWTH. 



The material which I have studied in order to determine the relation 

 of growth to the rate of rubber secretion was collected during and follow- 

 ing the growing-season of 1908, which began about June i. Growth is 

 rapid for the first part of the season, during which several centimeters of 

 stem-length are attained and one to three flower-stalks are developed. A 

 period follows in which there is little lengthening, and more or less second- 

 ary thickening occurs, according to the length of the period during which 

 growth of any kind may take place. During the first part there is no evi- 

 dence of secretion of rubber in the new parts ; during the second, which 

 began in 1908 in late July or August, there is a slight evidence of secretive 

 activity as regards rubber, though the secretion of resin is synchronous 

 with growth. The relation may best be expressed by saying that the secre- 

 tion of rubber is a secondary physiological process, its rate of appear- 

 ance being inversely to the rate of growth. The rate relation is brought 

 out best by plants grown under experimental conditions, in which the 

 more rapid growth is accompanied by a less rapid secretion of rubber. No 

 exact quantitative statement can be made, since the conditions under 

 which experimental plants have been grown have not been fully con- 

 trolled. In studying material, I have tabulated numerous observations 

 in field and irrigated seedlings, of various ages and at different periods of 

 the year, and compared the rubber-content of the cells in all the tissues 

 with that in irrigated seedlings. The same has been done for mature field 

 and irrigated plants. For this purpose the material which has frequently 

 been alluded to was at hand, viz, the branches and stocks of irrigated 

 plants at Cedros (plate 4, fig. B) and at Caopas (plate 46, fig. B), both 

 immediately at the close of growth-periods and after a period of drought. 

 The attempt was made to grade the preparations on the rubber-content 

 of the cells, and while this method of procedure has little to recommend 

 it for more than approximate accuracy, it enables us to draw reasonable 

 conclusions as to the rate of progress of secretion. My observations have 

 been digested in the following notes, which will serve to present sufficient 

 concrete evidence to support my conclusion. 



1. At the close of the dry season (May 1908) all the cells of rubber- 

 bearing tissues produced by growth during 1907, both in new shoots and 

 in new tissues in older shoots in field plants, contained rubber in maximum 

 quantities (plate 42, fig. 7). 



2. The same may be said, generally, for the field seedlings. There is, 

 however, evidence that in the cells of the pith near the top of the seedling 

 the maximum content of rubber is not reached. Seedlings (plate 1 7 , fig. 

 A) of rapid growth in 1908 had not reached the maximum content (as 

 shown both microscopically and by the analysis on p. 187) in April 1909. 

 In the cells of the root it was more densely agglomerated than in the stem. 

 Here the rubber had the same appearance as in irrigated plants. It was 

 only partly agglomerated, and only partially filled the cells. It is quite 

 probable that this condition occurs occasionally in mature plants in drier 

 habitats after exceptional rainfall and regularly in moister conditions. 



3. A medium-sized twig, grown in 1908, beginning about June i, 

 measuring 3.3 mm. in diameter at the base and 1.2 mm. at the tip, was 



