19"] CURRENT LITERATURE 261 



promote elongation of the heads, while the third (C) acts as an inhibitor which 

 checks the longitudinal development of the heads. When all of these factors are 

 absent, a moderately dense head results, as exemplified by the " squarehead " 

 varieties. When C is present and both L% and L 2 are absent, the extremely 

 dense "compactum" form is produced. Although considerable transgressive 

 fluctuation renders the analysis doubtful in individual cases, the total result 

 is sufficiently decisive to leave little doubt of the essential correctness of the 

 interpretations. The discovery that several genes may affect quantitatively 

 the same external characteristic has given an explanation of some hybrid 

 progenies which have seemed to breed true to characters intermediate between 

 the parents, and it also explains the intensification of parental characters in 

 F 2 individuals which have often been observed. As an example of the latter 

 phenomenon, a cross between two wheats of intermediate density, having 

 the formulae CL t L 2 and clj 2 , produces some F 2 plants with very dense heads 

 {Chl 2 ) , and some with very lax ones (cLiJU) . In respect to rust-resistance, the 

 difficulties of analysis are still greater and the author makes no attempt to 

 identify particular genes, but the results of a large number of tests in second 

 and third generations show very clearly two important facts, namely, that there 

 is a segregation of different grades of resistance in the F 2 , and that the matter is 

 not generally as simple as Biffen found it to be in his crosses dealing with this 

 problem. In none of Nilsson-Ehle's crosses was there an indication of a 

 simple monohybrid ratio (3 : 1) for rust-resistance, as was found by Biffen. 



Ka janus 1 * reports an instance in which the spelta-chamcter (zigzag rachis 

 and adherent glumes) is recessive to the zw/gare-character (straight rachis and 

 free glumes), a situation exactly the reverse of that found by von Tschermak. 

 This indicates that there are two genotypes of one or the other of these two 

 phenotypes, thus paralleling the now frequently demonstrated existence of 

 dominant and recessive whites. Kajanus found presence of awns recessive 

 to their absence, and hairiness of the glumes dominant to its absence, 

 as in all other reported crosses in which these characters have been involved. 



Geo. H. Shull. 



Cytology and sexuality of Olpidiopsis. — Overcoming considerable 

 difficulties in the matter of obtaining and managing material, Barrett 16 has 

 greatly increased our knowledge of the cytology and especially of the sexuality 

 of the submerse chytrids. Three species of Olpidiopsis, parasitic on Sapro- 

 legnia and Aphanomyces, were studied, two of which (0. vexans and O. luxurians) 

 are described as new. The first Dart of the DaDer consists of biological 



observa 



will 



taking similar work. The zoospores are shown 



1$ Kajanus, B,, Zur Genetik des Weizens. Botaniska Notiser 191 1 : 293-296. 



16 Barrett, J. T., Development and sexuality of some species of Olpidiopsis 

 (Cornu) Fischer. Ann. Botany 26: 209-238. pis. 23-26. 1912. 



