400 JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



Inbreeding and Crossbreeding, The Effect of, upon Development. By 



D. F. Jones {U.S.A. Exp. Stn., Conn., Bull. 207, Sept. 1918 ; 3 figs., 30 tables, 

 12 plates). — The main conclusions are given as follows : 



Effects of Inbreeding. — 1. Continued inbreeding results in the segregation 

 of a variable complex into a number of diverse types which are uniform within 

 themselves. 



2. The segregates which differ in visible qualitative characters also differ in 

 quantitative characters ; types with abnormalities appear which cannot reproduce 

 themselves ; others appear which are perpetuated with difficulty ; others are 

 obtained which are perfectly normal in structure and function. These latter are 

 usually less well developed, but may be as well or better developed than the 

 original stock from which they are derived. 



3. The change in size, structure, or function, and reduction in variability, is 

 most noticeable in the earlier generations of inbreeding, rapidly becomes less, 

 and the surviving inbred strains are uniform and constant. 



4. The rate of approach to uniformity and constancy differs in different lines. 



5. These uniform and constant inbred strains are quite comparable to 

 naturally self-fertilized species. 



6. No single effect can be attributed to inbreeding other than the reduction 

 in variability. 



7. All these results are in conformity with Mendel's law and Johannsen's 

 genotype conception. 



Effects of Crossbreeding. — 1. Heterosis (i.e., " hybrid vigour " or " stimulus 

 accompanying heterozygosis ") accompanies heterogeneity in germinal constitu- 

 tion, whether or not the organisms crossed are from the same or diverse stocks. 



2. Heterosis is widespread in its occurrence throughout the plant and animal 

 kingdoms. 



3. Heterosis is shown as an increase in the size of parts, rather than an increase 

 in the number of parts. 



4. Cross-fertilization is without effect until the zygote is formed ; from that 

 time on, heterosis may be apparent throughout the life of the individual. 



5. Heterozygosis has an undiminished effect on plants propagated vegeta- 

 tively. 



6. Heterozygosis may have a stimulating effect on some characters, and a 

 depressing effect on others, in the same organism. 



The author finally concludes that, whatever may be the value or significance 

 of heterosis, to account for this phenomenon it is for the most part unnecessary 

 to assume that there is an indefinite stimulating effect of hybridization along 

 with the expression of definable hereditary factors. Hence the distinction is 

 no longer needed between the effects of self-fertilization and cross-fertiliza- 

 tion and of heredity in development. The heretofore indefinite physiological 

 stimulation resulting from heterozygosis, and the related effects accompanying 

 the loss of this stimulation following inbreeding, can therefore be given a strictly 

 Mendelian interpretation. 



This being so, there is no longer a question as to whether or not inbreeding 

 per se is injurious. Whether good or bad results from inbreeding depends solely 

 on the constitution of the organisms before inbreeding is commenced. In- 

 breeding is concerned only with the manifestation of conditions pre-existing. 

 As a means of analysing and of purifying a cross-bred stock by the elimination 

 of undesirable qualities, inbreeding is therefore a method of first importance in 

 plant and animal improvement. — F. G. A. 



Inheritance of Sex in Strawberries. By R. D. Anthony (U.S.A. Exp. Stn. 

 New York, Tech. Bull. 63, Sept. 191 7). — An incomplete study. When imperfect 

 varieties (those with pistils only) have been pollinated by perfect sorts (those 

 with pistils and stamens), the resulting seedlfngs of the two types have been in 

 about equal numbers. When two perfect sorts have been crossed, almost the 

 whole of the progeny has been perfect ; but when perfect varieties have been 

 self-pollinated, about one-quarter have been imperfect or only semi-perfect 

 (filaments partially developed, but anthers abortive if present). More than one 

 year's observation is sometimes necessary to say whether a seedling will eventually 

 develop into a perfect sort or not. — A. P. 



Insects, Sucking, and Mites of the Apple. By W. H. Brittain (College of Agr., 

 Truro, N.S., Cir. 17, 191 7, pp. 3-1.5 ; 1 fig.). — This pamphlet deals with the 

 life histories and control measures of 8 sucking insects and 2 mites, and points 

 out the equal importance of spraying for sucking insects as for biting insects. 



(1) The Green Apple Bug, Lygus communis var. novascotiensis, Knight ; 

 (2) Black Apple Leaf, Idiocerus fitchii, Rose Leaf, Empoa rosae, and Apple-leaf 



