246 Univcrsih/ of Call for iiia Piihlicathns i)i Botmnj [Vfn,. 7 



Tlit'iv is no sIriUiiig tlilTcrciico between the pci-ccntii^cs found in the 

 plants from the hot desert valleys of California and tlie cold niounlain 

 valleys of ('olora(ht. 'i'he apparent i'unnin<i down of rubber content 

 as one ai)proaehes the limits of distribution for tlie s])ecios is j)erhai)s 

 to be accounted for on otliei- <i'rounds. [f temporatnro were the con- 

 trollinfi: factoi" it would be dit^eult to explain the unifoi-ndy low 

 rubber content of plants from the warm Coast ranfres of California 

 and also of the cool plateaus at the eastern base of the Rockies. 



The effect of wounds and of pruning- may be a matter of much 

 importance in case the ))lants are ever brouglit under cultivation. 

 The wild shi-ubs are often riddled l)y the attacks of larvae and l)eetles, 

 but their influence, if any, is not known. Experiments now in prog- 

 i-ess have ali'cady determined that the plants may be pruned back 

 without injury and that this greatly increases the number and W'cight 

 of the rubbei' bearing stems. The possible effect upon the formation 

 of rubbei" will be determined later. (See plate 20.) 



('. SEASONAL VAKIATION 



In the case of Guayule a variety of evidence points to the fact 

 that there exists a sti'iking seasonal variation in rubber content. Thus, 

 it has been found that during the active season of growth onl\' ncgli- 

 gibl( (piantities of rubber are deposited while during the following 

 resting period rubb(>r nuikes its appearance in the tissues. These two 

 periods are correlated with the duration of the rainy season in the 

 desert region where Guaj'ule grows wild. During the rains growth 

 is initiated and continues for a time. Shortly after the start of the 

 dry season active vegetative growth ceases and rubber then begins 

 to be deposited in the recently formed tissues. 



With these facts in mind some experiments were made to determine 

 whether a similar situation holds for rubber deposition in Chryso- 

 tharnnus. Table 9 gives the results of these experiments. On Sep- 

 tember 16, 1918, i)()rtions of mature tissues were cut from thirteen 

 plants representing three different varieties of naiiscosiis growing in 

 a small tract near Benton, California. Notes and photographs nuide 

 it possible on December 22, 1918, to remove from the same i)lants 

 portions ai)j)arently etpiivalent to those which had been secured in 

 September. Thei-e was no evidence that the removal of the first 

 portion had in any way imjiaired the i-enutinder of the i)lant or im- 

 peded its normal develo]>ment. The Se|iteml)er date is taken to 

 represent the close ttf the growing period at which time the flowers 



