BUREAU OF PLANT INDUSTRY. 139 



object of these investigations is to determine as accurately as pos- 

 sible the minimum, optimum, and maximum heat and moisture re- 

 quirements of tlie date palm, in the hope of discovering the causes 

 of offshoot production. The date industry is now an assured com- 

 mercial success, but the rapidity of its future extension as an industry 

 in the Southwest depends largely on the discovery of means for the 

 rapid propagation of offshoots. The results already obtained have 

 shown beyond a doubt that a considerable degree of humidity in the 

 air about the plants is necessary, and some success has been obtained 

 in forcing the growth of offshoots on date palms by inclosing the 

 trees in canvas sheds, thus increasing the humidity of the air in 

 immediate contact with them. 



INJURY TO PLANTS BY LOW^ TEMPERATURES. 



A laboratory study of the process known to gardeners as " harden- 

 ing off" plants by exposing them to low but noninjurious tempera- 

 tures has shown that as the plant becomes increasingly resistant to 

 cold the cell sap becomes more concentrated. When temperatures 

 are carried low enough to produce injury, a change of reaction in 

 the plant juices takes place and proteins are probably precipitated. 

 If the change in reaction does not go too far, the cell recovers, but 

 not otherwise. The biochemical processes involved in these changes 

 are being further investigated. Eesistance to injury from frost is 

 found to vary in cabbages with different individuals, and the selection 

 of cold-resistant strains seems likely to be practicable. 



PLANT-BREEDING INVESTIGATIONS. 



OIL CONTENT OF SEEDS. 



Studies on the relative oil content of the seed of standard cotton 

 varieties have shown that strains may be developed which, in addi- 

 tion to being resistant to certain diseases and possessing other desir- 

 able characteristics, are capable of yielding larger quantities of oil 

 from the seed. 



canio:r-resistant citrus fruits. 



In connection with the campaign for the eradication of citrus 

 canker in the Gulf States and Florida, it has been found to be of 

 the utmost importance to secure canker-resistant citrus fruits suitable 

 for culture in this region — especially in the region from Alabama 

 westward to Texas. The Satsuma orange, now largely grown in that 

 section, is fairly resistant to canker, but, unfortunately, almost all 

 other kinds of citrus fruits grown there are highly susceptible to 

 canker. It is very necessary, therefore^ to substitute canker-resistant 

 varieties of oranges, gi^apefruit, lemons, and limes for the varieties 

 now* grown, and of even more importance to find canker-resistant 

 stocks to substitute for the trifoliate orange for use in nurseries 

 throughout this region. The trifoliate orange is very susceptible to 

 citrus canker and constitutes a menace not only in the nurseries, 

 where thousands of seedlings are grown in close contact and are 

 therefore liable to epidemic outbreaks of the disease, but also in com- 



