The Environment. 41 



Of more importance, economically, is the "seda" (silk) or dodder 

 (Cuscuta sp.) , which often grows very plentifully. The habit of this par- 

 asite is well known, so that no account of the plant is here necessary. It 

 is very readily recognized as a yellow or orange vine-like leafless organ- 

 ism which winds about the upper twigs and leaves of the host. It is not 

 confined to the guayule, being found also on hojasen (Flourensia cernua), 

 on mariola (Parthenium incanum) , on tatalencho (Gymnosperma corym- 

 bosum), and other perennial plants, and probably on some summer an- 

 nuals. It reproduces itself by means of seeds which germinate after the 

 advent of the summer rains, but is to be found vegetating vigorously long 

 before this time. This is explained by the fact that it passes the win- 

 ter in rather tight, compact clusters of thread-like stems, tightly wound 

 about the uppermost twigs and leaves of the host. (Lloyd, 19080!.) Thus 

 it is independent of seed and is a true perennial. 1 



The effect of the dodder upon the guayule is due to two causes. 

 It diverts water and foods [from the host into its own tissues and thus 

 reduces the rate of growth, and it strangulates the twig and leaves upon 

 which it fastens itself. There is thus produced a dwarfing and distortion 

 which is reflected in the whole habit of the plant. 



As soon as growth commences in the host, the dodder, which is 

 ready at the top of the previous year's growth to take hold of the new 

 tender tissues, begins to twine about the newly forming stem and leaves 

 and soon overtakes and strangulates them. The effect is to produce very 

 slowly growing plants, and it is seen that the presence of much dodder 

 would materially reduce the annual accretion of growth and therefore of 

 rubber. In periods of severe drought the effect of the dodder is even more 

 marked, since it diverts the already meager water-supply and thus causes 

 the death of the portion of the twig at and above the zone at which the 

 dodder is found. Plants with twigs killed in this way, and in which 

 the dodder itself had succumbed, were found at the close of a sustained 

 drought, in April 1909. The dodder should therefore be stamped out 

 wherever it may be found. The best and only practical means is to har- 

 vest with the initial crop all the guayule affected with the parasite. In 

 this way the parasite will be checked, and additional checks will be re- 

 ceived at each harvesting by following the same rule. 



Indications of another vegetable parasite were thought to be seen in 

 the "witches' broom" above mentioned, but material examined by Prof. 

 W. G. Farlow gave no clue to the cause. The densely packed leaves in- 

 deed favor the growth of the rust already described, but this is quite a 

 secondary condition. It is possible that the distortion is due to the crop- 

 ping of the guayule by animals, but not all plants so treated show it, else 

 nearly all would be affected. Plants closely in the field trimmed back 

 (Station 2, quadrats i, 2) show a tendency to produce "witches' broom," 

 indicating that constant or close browsing by animals may after all be 

 the cause of this condition. 



1 Cuscuta is sometimes a perennial as far north as the State of New York. 

 Stewart et al., Bull. 305, Agri. Exp. Sta. N. Y., Nov., 1908. 



