BIOTYPES AND HYBRIDS. 55 



pastoris, despite the considerable differences of general aspect both of foliage 

 and of inflorescence. It is possible that these differences of aspect may 

 rest upon the presence of still other units which have not been taken into 

 account in these studies, but if such should prove to be the case, the gen- 

 eral conclusions must be the same. 



I wish to acknowledge my appreciation of the facilities which have been 

 placed at my disposal by the Station for Experimental Evolution of the 

 Carnegie Institution of Washington, and of the faithfulness of my assistants, 

 who have greatly aided me with the technical side of this investigation. 

 My thanks are due to my brother, J. Marion Shull, dendrological artist of. 

 the United States Forest Service, for the drawings reproduced in figs. 1 

 to 23. I particularly desire to acknowledge my indebtedness to Dr. E. N. 

 Transeau, in whose charge the cultures of B. bursa-paslbris X heegeri were 

 left during my absence in California in the spring of 1907. Much of 

 the work of grouping the F 2 into the appropriate classes was done by him. 



SUMMARY. 



(1) Bursa {Capsella) bursa-pastoris is a composite species made up of 

 at least 4 and possibly many distinct elementary species or biotypes ; 

 4 of these are described under the names Bursa bursa-pastoris heteris, 

 B. bp. tennis, B. bp. rhomboidea, and B. bp. simplex. Except for the sup- 

 pression of characteristics due to bad treatment, the fluctuations of these 

 forms are slight. 



(2) These 4 biotypes cross readily, giving in each case a uniform 

 F! and typical Mendelian splitting in F 2 . They are differentiated from 

 each other by 2 unit-characters, namely, elongated primary lobes of the 

 climax-leaves and a rounded secondary lobe in the distal axil of the primary 

 lobes. When B. bp. heteris is crossed with B. bp. simplex, and when B. 

 bp. tennis is crossed with B. bp. rhomboidea, the F t is intermediate between 

 B. bp. heteris and B. bp. rhomboidea because of the imperfect dominance of 

 B. bp. heteris, and the F 2 consists of heteris, tennis, rhomboidea, and simplex 

 in the ratio 9:3:3:1. 



(3) The sterile or nearly sterile plants supposed by Almquist to be 

 hybrids were probably not hybrids, as similar forms were noted in guarded 

 cultures, and in all of my hybrids no decrease in fertility was apparent. 



(4) Bursa heegeri differs from B. bursa-pastoris in the aspect of its 

 rosette and inflorescence, but most notably in the form of the capsule. Its 

 climax-leaves are of the same general type as those of B. bp. heteris. 



(5) Bursa heegeri may be readily crossed with B. bursa-pastoris. When 

 crossed with B. bp. simplex, an F! is obtained which more nearly resem- 

 bles B. heegeri in rosette characters, but has the capsule-form of B. bursa- 

 pastoris. In F 2 there appear the 4 forms of rosette already mentioned, in 



