BIOTYPES AND HYBRIDS. 11 



chances appear favorable for the lodgment of pollen from the outer circle 

 of flowers, or from the flowers of other plants, upon the exposed stigma-tops 

 of the second circle. 



Adaptation to self-fertilization is found in the facts: (a) that the stigma 

 is also receptive on its under (proximal) surface as well as on the upper 

 (distal) surface, and () that the anthers dehisce while they are still held 

 in contact with this under surface of the stigma by the erect segments 

 of the perianth (fig. 1, B). Soon after the anthers open the petals begin 

 to spread, thus exposing the pollen to be carried away to other flowers by 

 visiting insects (fig. 1, c). 



At the beginning of these experiments paraffin-paper bags of suitable 

 size were not available, and as insects were not abundant in the room used 

 for the cultures, the only precaution taken to guard against cross-pollination 

 was to set the flowering specimens intended as seed-plants somewhat apart 

 from each other and from other flowering specimens. As will appear 

 later, a few individuals, of unexpected character, are doubtless to be attrib- 

 uted to this unguarded condition of the earlier cultures, but it will also be 

 seen that the percentage of such chance crosses is extremel} r small. As 

 soon as possible, suitable paper bags were secured, and since then the cul- 

 tures, with few exceptions, have been carefully guarded. 



Although crossing among the unguarded cultures in the greenhouse has 

 been of rare occurrence, it was evidently more frequent in the material 

 secured from nature. Of the 21 original cultures 2 proved to be of hybrid 

 origin, while a fraction of 1 per cent of the rest indicated by their atypic 

 condition that they were probably the result of cross-pollination. Only 

 when the pollen comes from some form which is dominant to the pistil- 

 parent is the fact that a cross has taken place obvious in the first genera- 

 tion. It seems fair to assume that on the average as many crosses take 

 place with a recessive pollen-parent as with a dominant, and this assump- 

 tion would require that hybridizations occur with twice the frequency with 

 which they become obvious in the F T offspring. On this basis the fre- 

 quency of cross-pollination between different biotypes of Bursa in nature, 

 as indicated by these cultures, is about 1 to 65 as compared with the fre- 

 quency of self-pollination and crosses between flowers of the same plant 

 or between plants of the same biotype. Of course this ratio is based upon 

 a very limited number of specimens and can be expected to vary greatly 

 in different lots of material of the same magnitude, but at least the great 

 preponderance of self-fertilization may be safely inferred in this species 

 when in a state of nature. 



These characteristics of Bursa which make the production of pure self- 

 fertilized lines easy are opposed to the ease with which cross-fertilization 

 may be controlled. The fact that self-fertilization takes place before the 

 petals spread makes it necessary to carefully remove the stamens about a 



