BIOTYPES AND HYBRIDS. 45 



HYBRIDS BETWEEN BURSA BURSA-PASTORIS AND 



BURSA HEEGERI. 



The prominent part which mutation may have taken in the production 

 of new species makes it of great interest to know, in each case, just what 

 will be the result when the supposed or the demonstrated mvitant is self- 

 fertilized and when it is crossed with the parental form; for its behavior in 

 these two cases is the first important factor in determining the power of 

 the mutant to maintain itself at the time of its origin and its capacity to 

 give rise to a successful series of genetically related individuals belonging 

 to an independent type. 



The result of self -fertilization was investigated by Sol ms-Laubach before 

 the publication of his original account, and he showed that the characters 

 that differentiate B. hcegeri from B. biirsa-pastoris are fully heritable in a 

 self -fertilized line. As we have seen, the Bursas normally self -fertilize to 

 a predominant extent, and this habit, coupled with a vigorous constitution, 

 would seem to constitute all the factors necessary to successful mainte- 

 nance. I have now determined what will be the result of intercrossing B. 

 heegcri and B. biirsa-pastoris. 



The aspects of Bursa heegeri and B . bursa-pastoris as they appeared in 

 my cultures were so different that at the first I was skeptical concerning 

 the near relationship which has been assumed to exist between them. 

 Bursa heegeri was much more vigorous than B. bursa-pastoris, and the dif- 

 ferences observed in the inflorescence by Laubert (1905) were strikingly 

 apparent, the pedicels of the capsules being shorter, more crowded on the 

 rachis, and diverging from the latter at a wider angle (fig. 21). While 

 the leaves of the rosette of Bursa heegeri are of the heteris type, having 

 the primary and the rounded secondary lobes readily distinguishable, the 

 sinus which sets off the latter is comparatively shallow, and in consequence 

 the secondary lobe appears low and less well-marked. The primary lobe 

 is usually broader and less strongly attenuate than in B. bursa-pastoris 

 heteris. Despite these considerable differences, however, it was found that 

 B. heegeri may be crossed with B. bursa-pastoris with perfect ease in either 

 direction and without any apparent decrease in fertility, though I crossed 

 it with B. bp. simplex, the biotype of the latter species which is most unlike 

 B. heegeri. 



The families of pure-bred B. heegeri and its hybrids with B. bp. simplex 

 may be briefly considered under the pedigree-numbers used during their 

 culture : 



040.9 : Seeds of Bursa heegeri received through Dr. D. T. ]\IacDougal 

 from Professor Solms-Laubach were sown July 31, 1905, and produced 

 26 plants, all of which agreed with the above description, the fluctuating 

 variations being extremely slight. The unguarded seeds of one of these 



