Chicagfo Natural History Museum 



BU^^rTIN 



Formerly Mi/el^M\m News 



Vol. 16 



JANUARY-FEBRUARY, 1945 



No8. 1-2 



GUAYULE— A SOURCE OF RUBBER, AND A NEW CROP FOR AMERICAN FARMERS 



By J. FRANCIS MACBRIDE 



ASSOCIATE CURATOR OF THE HERBARIUM 



A hundred miles or so south of San 

 Francisco near the town of Salinas, on the 

 flat fertile floor of the irrigated valley of the 

 same name, there are now growing thousands 

 of gray-green guayule 

 shrubs. North Ameri- 

 ca's native rubber 

 plant. 



In a previous article 

 (Field Museum News, 

 vol. 14, no. 1, p. 4, 

 Jan. 1943) the history 

 and beginnings of this 

 project were outlined. 

 Here I shall try to 

 give some idea, more 

 particularly, of the 

 knowledge of the 

 plant itself as obtained 

 by the hundred or 

 more plant scientists 

 — botanists if you 

 please, in a broad and 

 for that matter in a 

 real sense — who have 

 devoted time to study- 

 ing it during the past 

 two and a half years. 

 The progress they 

 have made in its de- 

 velopment or domesti- 

 cation, so to speak, 

 and its potential im- 

 portance as a farm crop, improved by 

 science for man's needs, will also be discussed. 



In the preparation of this informal report, 

 I have had the assistance of Dr. A. C. 

 Hildreth, plant physiologist, and Dr. Reed 

 C. Rollins, geneticist of the Guayule Re- 

 search Project, Bureau of Plant Industry, 

 Soils and Agricultural Engineering, who 

 with acute understanding described and 

 interpreted on the Salinas tracts and in the 

 extensive laboratories, their research and 

 that of their associates. I also had the 

 privilege of being accompanied by Willis 

 Linn Jepson, professor of botany emeritus, 

 University of California. As in the case 

 of the previous article I am indebted to 

 Mr. E. L. Perry, assistant to the director, 

 Emergency Rubber Project, for kindly 

 editing and approving officially. 



Botanically, guayule is a kind of Par- 



thenium (P. argentatum) ; it is of the same 

 family of plants as chrysanthemum, the 

 artichoke and dandelion, the last incidentally 

 closely related to one of the plants Russia 

 has grown as a source of rubber. There are 

 numerous species of Parthenium, and one 



eign to common 



GUAYULE PLANTS IN SECOND GROWING SEASON 



Photograph of a field on the government trgct in the Salinas Valley, California. (Furnished by the Bureau of Plant 



Industry, Soils and Agricultural Engineering, United States Department of Agriculture). 



of them is so similar and so closely related 

 to guayule that it sometimes hybridizes with 

 it. This plant, known as mariola, (P. 

 incanum), is greatly inferior, however, as a 

 source of rubber since its pith and medullary 

 rays contain none, but merely lignify, while 

 in guayule nearly all parenchyma tissue is 

 gorged with rubber; on the other hand, 

 mariola is more hardy and more vigorous. 

 Hybrids between the two species may 

 show this desirable feature in their more 

 luxuriant growth. At present, discounting 

 some interesting experiments with subtropi- 

 cal species that are tree-like in habit, part 

 of the permanent improvement of guayule 

 may result, it seems, from a proper blending 

 of the selected desirable characters of these 

 two species, the characteristics not useful 

 to man or necessary for the plant in cultiva- 

 tion being modified or eliminated. 



Now how is modern scientific knowledge 

 and technique accomplishing this miracle? 

 Probably only an idea can be given the lay- 

 man because the processes are involved and 

 the terms used to describe them are for- 

 experience. But one 

 may say that in this 

 case the scientist is 

 aided by two fortui- 

 tous circumstances: (1) 

 reproduction in many 

 of these plants is asex- 

 ual, i.e., viable seeds 

 develop without pol- 

 lination, so that every 

 type remains true; and 

 (2) the inherent ge- 

 netic variability is 

 wide, the chromosome 

 count, with a usual of 

 72, varying from 36 to 

 180 or more. 



However, certain 

 types of guayule are 

 completely sexual and 

 dependent on pollina- 

 tion so that hybridi- 

 zation may be easily 

 accomplished. These 

 facts simply mean 

 that the student can 

 take his materials, that 

 is, the almost infinite 

 products of interbreed- 

 ing, and then by 

 proper controls add or subtract character- 

 istics almost at will and create a new plant 

 (given of course the needed time), as it were, 

 which will approach closely the ideal plant 

 envisioned and will reproduce its kind. 



This may take a few years, but that is 

 nothing compared to the tens of thousands 

 of years that nature and (or) man by chance 

 or happy selection have needed in the devel- 

 opment of fruits and vegetables such as 

 dates, corn or cabbage. For in all probabil- 

 ity as suggested in these pages recently 

 (vol. 15, nos. 7-8, p. 2, July-Aug. 1944) in 

 an article on the wild cabbage, the genetic 

 makeup was such that, given outlet or play, 

 so to speak, through interbreeding, selection, 

 climatic factors etc., something equivalent 

 to modern man's planned works has come 

 now and then to pass. 



In such transformation nowadays the 



