166 . BOTANICAL GAZETTE [AUGUST 
generalization ; namely that the occurrence of one cross-over in a chromosome 
lessens the like that a second cross-over will take place in the same 
chromosome. This phenomenon the author describes as “interference.” 
Following Morcan, the author explains the phenomenon of crossing over as due 
to the twisting together of homologous chromosomes, and the failure to can. 
_ pletely untwist when the chromosomes are separated—the “‘chiasmatype”’ of 
JANSSENS. Two cross-overs, or even three, may take place in the same 
chromosome when a series of loci sufficiently removed from one another are 
involved, but the frequency of such plural cross-overs is correspondingly low. 
While the percentage of cross-overs between any two “‘linked”’ characters was 
fairly constant in most of the material which has been studied by MorGAN 
and his students, the author points out that in certain strains there was a great 
deal of variation in the intensity of linkage. A part of this variability seems 
to be hereditary, but it is also suggested that some of the variation is probably 
due to conditions of food, etc. Most of the data regarding this variability are 
withheld for presentation and discussion in subsequent publications. 
The author discusses the relation of chromosomes to Mendelian inheritance, 
and gives a list of 17 species of plants and animals in which clear cases of linkage 
have been described, and also a list of chromosome numbers which have been 
found in 25 species of plants and animals used in genetic experiments. He 
points out, as has been done by a number of geneticists, that each unit char- 
acter is directly or indirectly due to the action of numerous Mendelian genes, 
and that each gene may and usually does affect a number of characters. 
terminology used by the author, following that of MorGan, is in one respect 
essentially the reverse of the one now most widely used, in that the symbols 
chosen to represent any Mendelian pair are based on the recessive instead of the. 
dominant character; thus, instead of Cc for the factor for color, Ww is used, 
intending to suggest that in the absence of W a white individual results. This 
is just as usable a method of formulation as that now in general use, and has 
only the disadvantage that would be due to any such reversal of terminology. 
It has the strong pedagogical advantage that any dominant character is less 
likely to be misconstrued by the non-specialized reader, as the sole result of the 
single gene represented by the symbol.—G. H. SHULL 
A case of obligate symbiosis.—RAvNER’ has discovered a very interesting 
case of obligate symbiosis between Calluna vulgaris and one of the mycorhiza. 
He has carried his investigations into careful experimental work, so that the 
details of the symbiosis in connection with the life history of Calluna have 
been discovered. It seems that infection by the fungus takes place shortly 
after germination, the source of the infection being the testa. This infection 
does not cease with the development of the mycorhiza in the roots, but affects 
6 Rayner, M. seemed symbiosis in Calluna vulgaris. Ann. Botany 29:97- 
153. pl. 6. figs. 4. 
