Makch 11, 1921] 



SCIENCE 



229 



in the parcels of potatoes of the variety Calvaehe. 

 But although the malady has increased very rap- 

 idly and is abundant in the tomato plots, it has 

 not flourished in those of the potato. 



"Where did this new parasite come from? We 

 have not met -with it up to the present on any of 

 our wild SolanacesB, so as to enable us to infer 

 that it has been transferred from them to the 

 potato and tomato; neither has seed been received 

 from Costa Eica so we could believe that it has 

 come from that locality. The trouble, as it has 

 manifested itself, has appeared on plots grown 

 from North American seed, in a way to make us 

 think that this new plague is to be referred to the 

 United States. 



Mr. Paehano informs me by letter that the 

 disease was not so prominent during 1919 as 

 it was in 1918, but had the same relative pre- 

 dominance on the tomato, especially on the 

 North American varieties. He has also modi- 

 fied his views regarding its origin. We may 

 assume, I think, that the susceptibility of 

 Iforth American varieties has no special sig- 

 nificance in connection with the question of 

 the native host or habitat. The snapdragon 

 rust has been known since 1897, and has 

 spread throughout the United States, but only 

 recently has it been traced to its native Cali- 

 fornian hosts. In fact I think we can safely 

 assume that the appearance of the potato rust 

 in the gardens of central Ecuador indicates 

 that the rust can be found on uncultivated 

 native plants in that same region. The 

 Solanum rusts of tropical and semi-tropical 

 America are numerous, but have been little 

 studied, and those of Ecuador almost not 

 at all. 



There is a rust described from Colombia 

 on Sarache edulis, a close relative of Solanum, 

 which much resembles the potato rust except 

 that it has slightly larger spores. This same 

 rust on another si)eeies of Sarache was found 

 in the vicinity of potato rust on Mt. Irazu in 

 Costa Eica by E. W. D. Holway, who tells 

 me that the plant is common in gardens there, 

 going by the name "yerba mora." There is 

 also a very similar rust known on the wild 

 Solanum triqueirum, a vine ranging south- 

 ward from central Texas, into the adjacent 

 region of Mexico, but this form has slightly 



smaller spores than the potato rust. Only 

 actual trial can show if these forms can be 

 transferred from one host to another, and if 

 the size of the spores is in anywise dependent 

 upon the host. 



A variation in spore-size apparently de- 

 pendent on the host is found to occur in the 

 case of the snapdragon rust, and cases of such 

 size variation are knovra for other species, 

 some of them authenticated by pedigree cul- 

 tures. The spores from the potato and tomato 

 are remarkably uniform in size. Whether the 

 three forms of Solanaceous rusts here referred 

 to are the same or not, it is fairly safe to 

 predict that the potato rust has originated 

 somewhere between Ecuador and Costa Eica 

 on hosts native to the localities. 



J. C. Arthur 



Purdue Univeesitt, 

 Lapatette, Indiana 



SCIENTIFIC EVENTS 



A WORLD ATLAS OF COMMERCIAL GEOLOGY 



With the growth of American industries 

 the known and the possible sources of our 

 supplies of raw materials have become of 

 greater and more pressing interest. Even the 

 United States — ^most favored of nations in 

 abundance and variety of raw materials — can 

 not be self-suificient; it must look beyond its 

 shores for supplies as well as for markets. 

 The study of the distribution df mineral raw 

 materials and their relations to the promotion 

 of trade and the control of industry is a 

 branch of geology and may best be termed 

 commercial geology. Under the complex re- 

 quirements of present-day life no continent, 

 not even North America, can be self-sustain- 

 ing. It is no longer enough for us to make 

 an inventory of the mineral wealth of the 

 United States; we must supplement that in- 

 ventory by a broad understanding of world 

 demand and supply. To set forth graphically 

 and to describe concisely the basic facts con- 

 cerning both the present and the future 

 sources of the useful minerals is the purpose 

 of a World Atlas of Commercial Geology just 



