SOLANACEiE 583 



lieved to be the result of "factors which hinder the normal 

 development of the plants and tubers or invite disease." 

 Stewart recently describes forms of degeneration known 

 as leaf-roll, curly-dwarf, mosaic and spindling-sprout. He 

 found that the progeny of plants with normal foliage and 

 high yield may very suddenly degenerate into dwarfish plants 

 affected with the above-mentioned diseases. The leaf-roll, 

 curly-dwarf and mosaic troubles are passed from generation to 

 generation by means of the tubers. The nature of spindling- 

 sprout is not this well known. The observations are 

 significant in that they show that seed selection may not 

 always insure against "running out." It is claimed by the 

 Svalof investigators that more vigorous seed tubers are pro- 

 duced in cool, moist conditions than in hot, dry regions. This 

 view is also held by Fitch who worked in Colorado. The 

 cause of this increased vigor is another question. It may be 

 due to a well-developed vascular system in the tuber or an 

 abundance of diastase at the sprouting season. Investi- 

 gators express the view that "where suitable sorts are used, 

 and where suitable tubers of these sorts are utilized for seed- 

 ing purposes each year, the standard of a variety may be 

 maintained indefinitely under all favorable conditions of soil 

 and climate." Hence it seems that the inherent tendency 

 to degenerate is perhaps no stronger in potatoes than in other 

 crops, but that they are more widely and strongly influenced 

 by environmental conditions than are most crops. It is re- 

 ported that at Svalof the variety Dala, introduced about 1 50 

 years ago in the province of Delarne, is still one of the best 

 sorts grown there. 



Environmental Relations.^ — The potato is a native of the 

 high, cool regions of Mexico and South America. In the 

 United States it thrives best in a cool, moist climate as is 

 evidenced by the fact that the five leading potato States 



