BIOTYPES AND HYBRIDS. 43 



guarded, so that hybrid! ty was excluded with such care as is possible. 

 Intermediacy in the size of flowers is not at all strange in the case described 

 by Almquist, for the culture in which the supposed hybrids occurred was 

 that of a very large- flowered form. Intermediacy in such a case simply 

 means a reduction in the size of the flowers. In all of my hybrids there 

 has been no apparent decrease in the number or viability of seeds produced, 

 and it will be recalled that there is only one clear case of intermediacy, 

 namely, in the hybrids between B. dp. heteris and B. bp. rhomboidea, in 

 which it is due to the incomplete dominance of the former. Perhaps in 

 other cases also dominance is not quite complete, but it is so nearly so that 

 it is impossible to distinguish certainly between the heterozygote and one 

 of its parents. 



HISTORY OF BURSA HEEGERI. 



Eleven years ago Professor Heeger found some specimens of a crucifer- 

 ous plant growing in the market-place at Landau, Germany, which he could 

 not identify. In general habit these plants resembled the almost cosmo- 

 politan species Bursa bursa-pastoris . They differed from the latter species, 

 however, in having the seed-capsules elliptical in longitudinal section and 

 circular in cross-section instead of flat and triangular or obcordate, as is 

 characteristic of Bursa bursa-pastoris. 



The specimens were submitted to Solms-Laubach, who was inclined at 

 first to refer them to the genus Came 'Una , which is characterized by nearly 

 spherical capsules, and Professor Ascherson, to whom he showed them, was 

 of the same opinion. Cultures made by Solms-Laubach from seeds secured 

 from Professor Heeger soon indicated, however, the near relationship of 

 the new form to Bursa bursa-pastoris, when in 1898 several apparent rever- 

 sions to the capsule-form of B. bursa-pastoris were noted. Solms-Laubach 

 (1900) published an account of the new form, assigning to it the name 

 Capsella heegeri, which becomes, according to the rule of priority, Bursa 

 heegeri ( Solms - Laubach ) . 



This very distinct species of Bursa has attracted considerable attention, 

 for the reason that its occurrence as a component of the flora of a region 

 so well known systematically has left little doubt of its very recent origin 

 from B. bursa-pastoris by mutation, and it is mentioned by De Vries (1901, 

 pp. 477-478; 1905, pp. 582-584) as an instance of mutation in nature. 

 Shortly after the publication of the original account, Bursa heegeri disap- 

 peared from the type locality at Landau, owing to the destruction of its 

 habitat by covering the market-place with gravel, and it has been reported 

 from nature only once since that time, though it has been widely grown in 

 botanical gardens. 



The second report of the discovery of Bursa heegeri in nature was made 

 by Laubert (1905), who found it along the Dahlem turnpike in 1905, but 



