The Origin and Occurrence of Rubber. 187 



irrigated shrub, and the character of the growth lends itself to this. This 

 would seem to be necessary in the event that the relative amount of rub- 

 ber in the cortex can not be raised above 3.5 to 4 per cent, not only because 

 of this probable difficulty of agglomerating the more finely divided rubber, 

 but because of the interference with this of the fragments of splintery 

 wood, which will tend materially to obstruct agglomeration in any event. 



In the second place, the individual masses of rubber in the irrigated 

 plant are smaller and further away from each other than in field plants. 

 Hence, as above said, it is more difficult to agglomerate the rubber. This 

 is noted in trying to isolate the rubber from irrigated tissues by mastica- 

 tion, a process more difficult than for field plants. It may be found neces- 

 sary to introduce a machine especially adapted to mastication of the 

 material after passing through the pebble-mill, in which rollers with differ- 

 ential speeds will cause the massing of the minute particles of rubber. But 

 the practical solution of such problems is not to be obtained merely by 

 reasoning about them. The laboratory and factory are mutually of value, 

 but the one does not always solve the difficulties of the other. 



VARIATION IN RELATIVE AMOUNT OF RUBBER IN FIELD 

 PLANTS. 



I have already pointed out that rubber does not appear in newly 

 formed tissues for some time after the cessation of growth ; it may be for a 

 period of some months. It therefore appears that the new growth of field 

 plants taken at some periods of the year has a content and distribution 

 of rubber similar to that in irrigated plants, aside from the relative bulk 

 of the tissues themselves. To illustrate, I take the following analysis of 

 seedlings, from Station 2, Quadrat 4 (plate 17, fig. A), collected April 1909, 

 germinated in 1908 (table 54). The leaves and stems with tap-roots were 

 analyzed separately. 



TABLE 54. 



Of interest in this table are the rubber-content of the leaves taken 

 separately and the low content of the stems and tap-roots. The leaves 

 probably represent the usual condition, as they were old, fully matured 

 leaves which had remained attached to the plants throughout a long 

 drought period. The plants, however, were of rapid growth, indeed 

 remarkably rapid for field plants, and the low rubber-content stands in 

 relation to this. There is no doubt that this rubber-content is much lower 

 than for seedlings of the same size of slow growth. 



In this respect, therefore, there is no hard and fast difference as 

 between field and irrigated plants, nor indeed is this the case for the relative 

 volumes of the tissues themselves, as I have previously shown (p. 117). 

 The response of the guayule under irrigation, therefore, is but an extreme 

 expression of what occurs in nature, correlated with the climatic differ- 

 ences which obtain from year to year, and in different localities. 



