328 THE POPULAR SCIENCE MONTHLY 



desired substances in the same amount as in the wild. When the 

 problem of cultivating the guayule presented itself, the first question 

 that arose, and most naturally, was the relation of secretion of rubber 

 to the supply of soil water. Though there is no very good a priori 

 ground for expecting one result or the other, it was known that the 

 amount of rubber obtained from certain latex trees is influenced by the 

 amount of soil water during the period of tapping. It was also found 

 that in the guayule the occurrence of rubber in the new growths of the 

 particular season intervenes only after a distinct interval of time, for 

 which reason the collecting of the shrub at the close of the rainy season 

 involves a great waste, since the increased weight is in no degree to be 

 referred to a greater rubber content. 



The careful study of the guayule plants which were under cultiva- 

 tion and had been freely irrigated developed, in addition to several 

 other points of interest, the knowledge that the greater the amount of 

 growth (itself dependent on the amount of water available), the slower 

 the secretion of rubber and the smaller its total amount relative to the 

 volume of the tissues. On the other hand, the rate of growth under 

 irrigation is so much more rapid that the total volume of rubber-bear- 

 ing tissue is very greatly in excess of that in field plants. Irrigated 

 plants present other features of difference which must be taken into 

 account by the manufacturer and which are of very great interest to 

 students of the physiology of desert plants. I need mention only the 

 more salient here, namely, the greater relative volume of the woody 

 portion of the stem, its harder and more rigid character, the smaller 

 volume of the cortex (bark) (Fig. 16) and the quantity of rubber in 

 individual secreting cells. 



The increased hardness of the " wood " in the irrigated plant is due, 

 in part at least, to the reduced volume of the medullary rays. And as 



Fig. 16. Transverse Sections of Guayule Stems, of the same transverse 

 area of wood, grown under irrigation (a) and wild (i), c). c was from a plant of 

 unusually rapid growth. 



