40 



Guayule. 



There can be little doubt that the component elements in such a veg- 

 etation are in a state of ebb and flow, and, in view of the density of the 

 vegetation, in contrast with the condition usually met with in deserts, con- 

 stitute an important question economically. Here the individuals come 

 into actual contact above ground, where the competition is often severe, 

 as well, presumably, as below ground. Referring especially to guayule, 

 it may be accepted that, when a plant is once well started, it is seldom 

 killed outright by contact with its neighbors, but the occupancy of the 

 ground by other species which have superior methods for spreading grad- 

 ually reduces the available surface and water-supply for the guayule. This 

 plant takes advantage of surface-water by means of its superficial roots 

 and plants with which it is associated and which behave similarly {e.g., 

 Jatropha spatulata) must come into severe competition with it in this 

 regard. But, assuming that, for purposes of forestry, it is desirable to 

 thin out other vegetation in order to favor the guayule, the question 

 arises as to the effect upon the germination of seed of this plant, which is 

 undoubtedly favored by partial shade. It may be argued that the superior 

 numbers of seed available and the shade of the guayule plants themselves 

 will suffice, and this seems probable. On the germination of seed in the 

 open more will be said, based upon experimental evidence (Chapter IV). 

 Denuded areas are under observation, and the future may be expected to 

 bring exact observation to bear upon the practical question of the value 

 of clearing land, as well as upon the theoretical aspect of the questions 

 above stated. (See also Chapter IX.) 



PARASITISM. 



Of vegetable parasites affecting the guayule only two are at present 



known. Of lesser importance, so far as we may judge, is a rust hitherto 



known as Uredo parthenii Speg. (fig. 5). Prof. J. C. Arthur, to whom 



material was sent for identification in April, 1908, reports that the fungus 



properly belongs in the genus Puccinia, and may be 

 called Puccinia parthenii (Speg.) Arthur, ivied. , for the 

 purpose of record. 



It has been noticed that the fungus appears 

 chiefly on plants which are on the north slopes of ar- 

 royos, especially near the bottom, where the relative 

 humidity is most favorable, since it is here that the 

 highest vapor-tension exists. It has been found also 

 on plants growing on ridges, and especially on those 

 which are subject to a condition which we have called 

 "witches' broom," in which the leaves are small and 

 very much crowded. It appeared in the spring of 

 1908 also on plants which had been grown under 

 irrigation at Cedros, apparently on the older leaves, 

 which still remained attached from the previous 

 year. The parasite is not at all plentiful, and appears 

 to be absent almost entirely from guayule growing in open situations. 1 



1 A small seedling which germinated in the early summer of 1908 was found 

 in April 1909 with a single infection spot, quite in the open foot-slope (Station 3), 

 in which situation the fungus is seldom seen. 



Fig. 5. Teleuto and ure- 

 dospores of Puccinia 

 parthenii (Speg.) Ar- 

 thur. 



