BUREAU OF PLANT INDUSTRY. 147 



recorded on the relation of soil temperature and soil moisture to 

 infection of wheat by the bunt organism, and on the relation of crop 

 rotation to the control of this disease. 



KooT DISEASES OF WHEAT AND OATS. — An extensive study of wheat 

 soil " sickness " has been begun w4th a view to determining the cause 

 or causes of poor yields on soil cropped consecutively for many years 

 with a single cereal ; and further to study methods of restoring such 

 soil to its maximum crop capacity. From wheat and oats showing 

 evidence of root diseases three species of imperfect fungi have been 

 isolated that may prove to be causes of the crop injury. 



Plant-infesting nematodes. — It has been found that nematodes 

 can be introduced on importations of sugar-beet seed. Living speci- 

 mens of three diiferent species have been found, none of which, how- 

 ever, are known to be dangerous parasites. Dead specimens of 

 Heterodera schachtii, the sugar-beet nematode which is frequently the 

 cause of serious loss, have been determined, but up to the present time 

 no living specimens have been discovered. The identity of species 

 Tylenchus Mformis^ which has been found attacking sugar cane and 

 bananas in Hawaii, has been determined. 



PLANT PHYSIOLOGICAL INVESTIGATIONS. 



Tobacco " grain," mosaic, etc. — The minute structure of the tobacco 

 leaf, with special reference to the so-called " grain," has been exten- 

 sively studied. The nature of the " grain " has been ascertained and it 

 has been found that the character of the " grain " and the extent of its 

 development have an important influence on the commercial quality 

 of the leaf. An investigation of the causes of poor burning qualities 

 of cigar leaf has shown that the use of fertilizers high in chlorine is 

 an important factor and that poor burn of tobacco from certain areas 

 is correlated with lack of grain development. During the past year 

 further advances have been made in the study of the mosaic disease, 

 especially w^ith reference to the distribution of the virus in the plant 

 parts and its relation to certain constituents of the plant. The results 

 indicate that the disease is parasitic rather than physiological. In a 

 study of the ripening of the tobacco leaf it has been found that the 

 process is analogous to curing in the barn in many respects, and that 

 the more important changes taking place are chiefly dependent on 

 seasonal conditions rather than on the character of the soil. Work 

 on the relation of potash to the development of the tobacco plant 

 and its important qualities has been continued, and it has been found 

 that lack of this element produces very characteristic pathological 

 symptoms which can be easily recognized by farmers. 



Water requirement. — ^Water-requirement measurements have 

 shown that crop plants differ markedly in their efficiency in the use 

 of water. The most efficient crop plants so far investigated are the 

 millets and sorghums, which under dry-land conditions require 

 from 250 to 350 pounds of water for each pound of dry matter 

 produced, while alfalfa has the highest water requirement of the 

 crop plants. Not only do the various crops exhibit a great range in 

 water requirement, but marked differences are also found in the 

 water requirement of different varieties of the same crop. Grimm 

 alfalfa, for example, requires 200 pounds more water than Peruvian 



