No. 632] SHORTER ARTICLES AND DISCUSSION 275 



making back crosses on a large scale, it may be practicable to 

 depollinate the flowers of the intended female parent with a 

 water jet instead of actually castrating the buds. 



As regards other desiderata to be considered in selecting ma- 

 terial for genetic study these two species are very promising. 

 They are easily reared in greenhouse or field, the seeds ger- 

 minating quickly in glass germinators, thus permitting easy 

 manipulation and careful checking of viability when desired. 

 The life cycle varies from three to six months except in rare cases 

 of retarded development and little or no rest period is necessary 

 in the seed stage, so that it is possible to grow two or three gen- 

 erations in a year with proper facilities for culture under glass. 

 Partial or complete self-fertility is the rule in both these species, 

 although in some strains of virens the individual plant is nearly 

 self-sterile. No evidence of parthenogenesis or apogamy has been 

 found in these species. In general, therefore, it will be possible 

 to secure numerous sexually propagated pure lines, differing 

 from one another in one or more allelomorphic pairs, which will 

 serve as the basic material for working out the "chromosome 

 content" in these species. It is only the problem of securing 

 seed from a large number of different localities and of growing 

 and carefully studying a sufficient number of plants that must 

 be solved in order to furnish the pure lines desired. The sooner 

 this can be accomplished the sooner can the extensive analysis 

 of the chromosome content of these species be gotten under way. 

 Finally the critical question as to whether these two species can 

 be hybridized has been answered in the affirmative by the pre- 

 liminary experiments of the present year. 4 



Sufficient has been said, I trust, to convince the reader that 

 we have in Crepis a wealth of material which may fairly be ex- 

 pected to furnish date of the greatest value in testing the gen- 

 erality of the chromosome theory of heredity, and that this group 

 is unique in the promise it holds of carrying out that test in much 

 shorter time than would be required if we should depend only 

 on the data which is slowly accumulating from other plants now 

 under investigation. It should be clearly realized, however, that 

 to accomplish the results aimed at, even with Crepis, will require 

 a considerable period of time, the length of the period being 

 largely conditioned by the number of investigators attacking the 



