32 BURSA BURSA-PASTORIS AND BURSA HEEGERI : 



of working value, but in the taxonomy of the future they must be reckoned 

 with, because they are the real natural entities with which all students of 

 biology must deal. The old name and the old delimitation of Bursa biirsa- 

 pastoris may remain as the only thing practicable for the amateur collector of 

 plants, but the morphologist, the physiologist, the ecologist, and the evolu- 

 tionist must be more discriminating. It appears to me that systematic 

 botany stands at the parting of the ways. Either it is to be left stranded 

 as a caterer to the amateur or it must adopt the cultural method in lieu of 

 the herbarium method, which has until recently held almost supreme sway 

 among systematists. 



I am not fully convinced, however, that the variable families here de- 

 scribed belong to a single biotype. The fact that both extremes have in 

 certain cases bred true leads to the question whether there is not some way 

 of accounting for the anomalous behavior of the other families on the 

 ground of hybridization. I believe that such an explanation can be foiind 

 on the principle of latent characters which I have recently discussed else- 

 where (Shull, 1908). At certain times the cultures have become very 

 unhealthy on account of the attacks of thrips and other causes, such as 

 overheating during the summer, crowding in the seed-pans, etc., and the 

 specimens weakened in this way may very well have failed to show their 

 distinctive characters, owing to what I have called "latency due to fluc- 

 tuation," thus destroying completely the ratios by which hybridization 

 phenomena would have been rendered evident. 



Whether these plants belong to a single biotype of wide variability or 

 to a hybrid group whose nature has been obscured by latency will have to 

 await further cultures. Lotsy (1906) also found that while some forms 

 bred true with very slight fluctuation, the variability of others was consid- 

 erable. If hybridization with latency accounts for the behavior of my 

 variable families it may also perhaps account for a similar condition in 

 Lotsy 's material. Whatever the situation may be in these highly variable 

 cultures, it is perfectly certain that the true-breeding forms are distinct 

 and elementary, as will be more fully demonstrated in the next section. 



HYBRIDS BETWEEN BIOTYPES OF BURSA BURSA-PASTORIS. 



Although artificially produced hybrids between the several biotypes above 

 described have been studied as yet only in the first generation, except in 

 the case of one F2 family to be described hereafter (see page 42), I have 

 had under observation 31 hybrid families belonging to a second or later 

 generations. My good fortune in being able to report on these second- 

 generation hybrids is due to the facts that two of the original families were 

 the offspring of hybrid individuals growing in nature and several other 

 individuals in the original cultures were obviously produced by pollination 

 from another biotype. The first of these natural hybrids (040.4) was col- 



