564 



THE INDIA RUBBER WORLD 



[August 1, 1913. 



new combinations of environmental factors may prove fatal, 

 whereas any one factor, taken in different combination, might 

 not. So it comes about that the guayule can with^^tand a greater 

 cold much more efTectually under arid than under moist con- 

 ditions. Of a lot of guayule plant grown outdoors at Auburn, 

 Ala., both from seed and from stocks, in 1910, only one in 25 

 was living in 1912. In no case did death overtake them in the 

 warmer periods, nor early in the winter, but rather late in winter 

 during a cold snap following heavy precipitation. The evidence 

 that the winter of Alabama ofTers untoward conditions is clear, 

 however, before the plant is killed — it has a bad color, an<l a 

 generally disheveled look, due to the numerous dead unshed 

 leaves. The wet and cold weather conjoined constitute the limit- 

 ing factor. This may be taken to mean saturated soil and a 

 minimum temperature of 10 degs. Fahr., approximately that of 

 the normal habitat. The part immediately affected appears to 

 be the root, the cortex of which quickly disintegrates. 



The tenure of life in a new habitat depends further upon the 

 ability of the plant to resist new enemies. In Alabama, 'hrips 

 and nematodes have been seen to affect the guayule badly. A 

 strong tendency to form large and irregular Icnticels on the 

 lower portions of the plant in contact with the soil and nearby 

 above its surface may be due to this proximity alone or to insect 

 stimulation. The seed is poorly developed. 



In the semi-arid desert, on the contrary, no case of death has 

 been noted during three years, save as the result of insufficient 

 irrigation. It is clear from this behavior that the resistance of 

 an irrigated plant to drought is much less efTective than that of 



TyPIC.VL CkLI- hkii.M U.\RK ; Rl BBEK IViAMi 



Filling the Cell. 



Appe.\r.\nce, 



one grown under the usual conditions. Though attacked by a 

 fungous parasite, it has never been known to be seriously af- 

 fected. The development of seed is abundant, and it has a vital- 

 ity approximately equal to that of lield plants. 



H.VTE OF GROWTH. 



When grown under irrigation in the semi-arid desert plants 

 from transplanted stocks have attained in two seasons a maxi- 

 mum weight of 4.5 pounds, 3.5 jrounds being a good-sized plant. 

 These were a meter and over in diameter. The average weight 

 for such plants would probably be 2 pounds. The dry weight of 

 such plants, by which the rubber content must be calculated, is 

 about 30 per cent, of the fresh weight. Allowing, therefore, a 

 dry weight per plant of one pound, a not too conservative esti- 

 mate, and assuming 10.000 plants to the acre, a 2 per cent, rubber 

 content would yield 200 pounds of rubber in two growth sea- 

 sons, followed by, say, six months of suppressed irrigation. At 

 60 cents per pound we have a return of $120 per acre for two 

 years, or $60 per acre per year, against which should be charged, 

 in addition to the usual costs, that of obtaining plants from seed 

 for planting the giound. There is no evidence forthcoming that 



the percentage of rubber is greater, under the ahove-described 

 conditions, than about 2 to 4 per cent. Assuming it, however, to 

 be 6 to 7 per cent., guayule would even then scarcely be able to 

 compete with onions in irrigated land. 



The assumption is made that it would not pay to grow the 

 plants longer than two years. In what follows under the head 

 of rate of rubber secretion the reason for this will be seen in 

 I):irt. As to the rate of growth merely it may be said that the 



Stem of Good Field 



Guayule from 



Zacatecas. 



Stem uf Glayule 



FROM IsiDRO. 



Chihuahua. 



Irrigaied Guayule 



FROM 



Zacatecas. 



[Same amount of wood in each stem.] 



rate of increase of weight will be found to drop off rapidly after 

 the second season's growth, to which must be added the fact that 

 the smaller and therefore younger twigs contribute little to the 

 accumulation of rubber. The rapid production of cork character- 

 istic of the irrigated plant, results, it is probable, in the destruction 

 of rubber. I find that the rubber in the cells which are active 

 in giving of? the innermost and therefore younger cork cells be- 

 comes broken up into small globules, and these disappear before 

 the cork cell in which they are found gets very old. The cork as 

 a whole appears, as a result, to be devoid of rubber, although in 

 point of fact minute quantities are present in the cells referred 

 to. It is practically impossible to measure this loss directly, but 

 it may very well offset the further gain by growth quite ma- 

 terially. The death of various twigs and branches due to com- 

 petition inter se must also contribute to loss of rubber. These 

 circumstances, coupled with the reduction of the growth rate after 

 the second season, indicate the economic maximum to have been 

 reached. 



In the moister climate of Auburn. Alabama, one suitable, as 

 everyone knows, to the development of cotton, the rate of 

 growth is less rapid than in the desert under irrigation. In the 

 first season, a height of <S to 10 inches is attained, but in the sec- 

 ond season the additional growth is meagre. This fact, coupled 

 witli the uncertain tenure of life during the winter, would force 

 the issue of considering the rubber secretion of the first season 

 alone. This, as we shall see. is practically nil. It must be con- 

 cluded that the possil)ility of growing guayule in this region is 

 excluded by its abnormal development and slow growth, quite 

 aside from other considerations. I do not raise the question of 

 the character of the soil and its possible causal relation to the 

 manner of growth, beyond saying that it is not related, it would 

 seem, to the lime content, since the plant grown in the limestone 

 soil of Austin, Texas, behaved in a very similar manner. 



R.VTE .VXD CH.\R-\CTER OF RUBBER SECRETION. 



A'onnal field filaiils. In the study of the rate and character of 

 rubber secretion in experimental plants, the field plant has al- 

 ways been taken as a standard. The method, that of microscopic 

 observation, does not enable one to state the results in terms of 

 percentage relative to the total weight of the plant. Only chem- 

 ical methods can be used for this purpose. Nevertheless much 

 useful information can be gained by means of the microscope, 

 particularly in regard to the method of secretion and the amount 

 of rubber within the cells which secrete it. 



