Page:Popular Science Monthly Volume 81.djvu/335

Rh 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 combined, 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 teleologist will doubtless see in rubber a means adapted to conservation of water in the desert plant. Field guayule, however, wilts almost immediately 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 production 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