Varietäten, Descendenz, Hybriden. 571 



tions and mutations, pointing out that the attempt to draw a sharp 

 line of distinction between them is, in many cases at least, unsuc- 

 cessful. De Vries' distinction between elementary species and varieties 

 is held to amount to "nothing more than the fact that crosses between 

 certain groups follow Mendel's law, while crosses between others 

 de not." The experiments of de Vries in producing a double 

 variety of Chrysanthemum segetum, which required four years' bree- 

 ding "to reach its füll expression, are cited to show that mutations 

 theraselves may come about gradually. It is also held that Mendelian 

 behavior in hybrids does not prove the existence of units of any 

 sort in the germ plasm. The writer concludes that "the various 

 categories of variations recognized \>y de Vries and others are 

 not sharply separable either on morphological grounds or by their 

 behavior when subjected to crossing experiments." R. R. Gates. 



Price, H. L. and A. W. Drinkard Jr. Inheritance in Tomato 

 hybrids. (Plant World. XII. p. 10—18. fig. 2. 1909.) 



Twenty-one distinct variety crosses were made, involving thir- 

 teen differential pairs of unit characters. Among these characters 

 may be mentioned (dominant character placed first) round )< pear- 

 shaped fruit, 2-celled X many-celled fruit, red X yellow fruit, yellow 

 X transparent skin, normal X potato leaf. With one exception, 

 pubescent X smooth foliage, the behavior was Mendelian. In the 

 exceptional case both parental types appeared in the F 1 . Skin color 

 and flesh color in the fruit behave independently , so that a combi- 

 nation including pink flesh and yellow skin produces a red fruit, 

 the ratio of pink, red and yellow in the F 2 being that of a dihybrid. 

 Inter- and intra-specific crosses gave similar behavior. R. R. Gates. 



Shull, G. H., A simple chemical device to illustrate Mende- 

 • lian inheritance. (Plant World. XII. p. 145—152. 1909.) 



This is a chemical experiment to illustrate Mendelian behavior, 

 and particularly the dominance of "absence" over "presence" of a 

 character. Litmus or some other indicator is used. The germ cells 

 are represented by test tubes containing litmus to which have been 

 added one, two or no portions of acid. Pouring these together in 

 the various combinations gives a result (F x ) which is red in all cases 

 (dominant). Similarly the F 2 generation and the dominance of absence 

 over presence may be demonstrated. R. R. Gates. 



Shull, G. Ü., The "presence and absence" hypothesis. (Amer. 

 Nat. XLIII. p. 410—419 1909.) 



This is a Statement of the view that, in Mendelian inheritance, 

 the presence of a character should be considered to be dominant 

 over its own absence, rather than dominant over another character. 

 The positive homozygote (dominant character) will then be represented 

 by AABB, the negative homozygote by BB, and the heterozygote 

 by ABB, the three types differing in the number of A genes present. 

 In some cases, as horns in cattle, absence of a character is domi- 

 nant over its presence. Such cases are explained in a simple way 

 on a chemical basis. R. R. Gates. 



