Examples of Constant Races. 81 



occur. Similarly Senecio Jaeobaea lias a forina raJiala 

 and a F. discoidea^ (I^'igs. 8 and 9) both of which grow 

 in this country and are absolutely constant. The Discoi- 

 deus grows in thousands of specimens in the dunes in 

 the province of North Holland; but the Forma radiata 

 grows equally abundantly in South Holland ; both are 

 amongst the commonest and most widely distributed spe- 

 cies of our flora. For twenty and more years I have had 

 them under observation, and never saw any trace of ad- 

 mixture or reversion ; the two varieties were always al> 

 solutely pure in the respective localities. Of late, how- 

 ever, there have been some cases of intermingling near 

 the limits of their areas, probably as the result of seed 

 transportation. The two sorts can therefore be regarded 

 as absolutely constant.^ Matricaria Chamoniilla discoi- 

 dca^ has proved equally constant in my experimental 

 garden, but Murr mentions the occasional occurrence of 

 heads with rays.'^ In 1897 I raised from the seeds of a 

 single plant of M. discoidea 575 plants, all of which were 

 without ligulate florets. On these I only harvested the 

 seeds of the weakest branches of the higher orders and 

 raised 460 plants in 1898, all of which again were with- 



^ See Vol. I, p. 196. 



^A valuable summary dealing with this point is given by. J. 

 Murr, Stvahllose B Hit hen bei heimischen Kompositen, Deutsche Bot. 

 Monatsschr., Vol. 14, 1896, pp. 161-164. See also Bofan. Jalircshcr., 

 T. 24, 2, p. II, where rare instances of forms with rays belonging 

 to normally discoid species and rayless flowers on normally radiata 

 forms, are given. I cite Senecio Jaeobaea as an instance of the latter, 

 in opposition to the observations given in the text. An attempt to 

 discriminate half races amongst these forms (See §3, p. 18) would 

 probably lead to valuable results. 



^ For an account of the rapid spread of this form in Norway sec 

 Jens Holmboe, Nogle Ugracsplanters Invandring i Norge, 1000. 

 A^3'( Maga::. f. Naturv., Vol. XXXVTII, p. 187 (with map), the 

 variety is there also fully constant. 



*J. Murr, loc. cif., pp. 161-164. 



