THE GUAYULE 329 



these make up a goodly proportion of the volume of the smaller twigs 

 in the field plants, and since the medullary ray cells contain rubber, the 

 greater hardness of the irrigated wood is attained at the cost of rubber- 

 bearing tissue. The lessened volume of the cortex and of the pith 

 contribute to the same results, so that even if the "amount of rubber 

 secreted by the individual cells remained unaffected by irrigation, the 

 amount relative to the volume of the plant will be materially less. But 

 so far as the writer's own observation goes the rubber cells in the plants 

 which have been abundantly watered so as to extend the growing season 

 over six or seven months, do not, even after subsequent sustained 

 drought periods, contain the normal but rather a considerably less 

 amount of rubber. This, from the point of view of the manufacturer, 

 means that the globules of rubber which must be agglomerated into 

 worm rubber in the pebble mill are smaller and farther apart and 

 consequently more difficultly brought together. It means, too, that in 

 obtaining a given amount of rubber much more bagasse must be 

 handled. These difficulties may doubtless be overcome by suitable 

 modifications in the details of the process already outlined. 



The reader will have gathered the impression that the guayule may 

 be grown readily under cultivation. This is true, at least in its areas 

 of distribution and in adjacent or other areas of similar meteorological 

 conditions. It does not, however, thrive in eastern Texas, judging 

 from a specimen planted by Dr. H. H. York at Austin, nor in Alabama. 

 Here it will grow rapidly the first season, but little the second. Further, 

 a large percentage of the plants are killed by frost and dampness com- 

 bined, whereas they will resist severe frost in dry regions. And again 

 the amount of rubber in them is very small indeed. Why this peculiar 

 relation of rubber secretion to soil water can not be said. The teleolo- 

 gist will doubtless see in rubber a means adapted to conservation of 

 water in the desert plant. Field guayule, however, wilts almost imme- 

 diately upon being pulled up, and its congener mariola, which is more 

 drought-resistant, has even less rubber than irrigated guayule. 



On the other hand, the larger development of the wood appears to 

 be correlated with a more rapid growth, and consequently a larger pro- 

 duction of leaves, made possible by a greater water supply. This logic 

 is clearer when it is realized that the wood contains the water-carrying 

 vessels and at the same time the mechanical tissue for the support of 

 the foliage. From this point of view the more extensive rubber-bearing 

 tissues and apparently greater succulence in field plants result from 

 more meager development of the wood and do not speak for a larger 

 " water balance," one criterion, at least, of drought resistance. 



One additional feature of the behavior of guayule under irrigation, 

 especially when grown from transplanted stocks, is worth mentioning 

 in closing. It had been noticed that its congener, the mariola (Par- 



VOL. LXXXI.— 23. 



