iQig] CURRENT LITERATURE ■ 273 



the Weber-Fechner formula, if the wave-frequencies be made a function of the 

 constant; (7) the relation of the spectral energy to the presentation time may 

 also be approximately expressed in the Trondle formula, the wave-frequency 

 being made a function of the constant. — Wm. Crocker. 



Breeding for disease resistance. — It has been a popular impression that 

 newly produced disease resistant varieties will gradually lose their immunity 

 in later generations. The idea was that such new varieties might sometimes 

 become slightly infected; this short sojourn of the disease organism in the 

 normally immune host would enable the former to adapt itself to the new con- 

 ditions and gradually acquire virulence, until finally a new biologic form was 

 developed to which the host in question was quite susceptible. Evans* carried 

 the same idea further when he found that a cross between resistant and sus- 

 ceptible races of wheat produced a hybrid even more susceptible to rust than 

 the susceptible parent. Furthermore, rust from the hybrid could now infect 

 the immune parent. Such facts were very discouraging, since they indicated 

 that the artificial breeding of resistant crop plants is rapidly overtaken by the 

 natural breeding of new biologic forms of the disease organism. 



Particularly acceptable, therefore, is the work of Stakman, Parker, and 

 PiEMEiSEL,9 who find that wheats resistant to rust remain resistant regardless 

 of the previous history' of the rust; the gap between immune and susceptible 

 varieties is not bridged by transitional varieties or by artificial hybrids. "Re- 

 sistance is rather an hereditary character, which cannot be produced by the 

 accumulation of fluctuating variations within a susceptible line, nor broken 

 down by changes in the host or parasite." Acceptable as such a conclusion 

 may be, both to commercial breeders and to academic geneticists, it is very 

 questionable how widely it may be applied. It will be difficult, although 

 not hopeless, to explain away much of the contrary evidence. — Merle C. 

 Coulter. 



Nature of monocotyledonous leaves. — Mrs. Arber'" has presented the 

 results of an anatomical investigation of the phyllode theory of the mono- 

 cotyledonous leaf. According to DeCandolle, it is equivalent to the leaf- 

 base and petiole of a dicotyledonous leaf, but Mrs. Arber believes that certain 

 monocotyledonous leaves are still further reduced in that they are equivalent 

 to leaf-bases only. In case the monocotyledonous leaf shows a distinction of 

 petiole and blade, Henslow suggested that the blade is merely an expansion 



* Evans, I. B. P., South African cereal rusts, with observations on the problem 

 of breeding rust resistant wheats. Jour. Agric. Sci. 4:95-104. 191 1. 



9 Stakman, E. C, Parker, John H., and Piemeisel, F. J., Can biologic forms 

 of stem rust on wheat change rapidly enough to interfere with breeding for rust resist- 

 ance? Jour. Agric. Research 14:111-123. pls.i^ij. 1918. 



'" Arber, Agnes, The phyllode theory of the monocotyledonous leaf, with special 

 reference to anatomical evidence. Ann. Botany 32:465-501. figs. 32. 1918. 



