166 



only much later did it become obvious that many of the supposed 

 simplex plants were producing a greater elongation of some of 

 the lobes than is to be found in pure B. bp. simplex. The ratio 

 2.08 : 1 here reported for this family, was derived by waiting 

 until the flower-stems were about 5 — 10 centimeters high, and 

 then calling everything tenuis which produced at least one lobe 

 more elongated than those of pure-bred B. bp. simplex. Plate V 

 shows the most highly developed leaf-characters attained in each 

 of twenty-six individuals taken quite at random from plants in 

 this family, which had been finally classified as B. bp. tenuis. 

 In normal, well- developed specimens of B. bp. tenuis there is a 

 long series of leaves in the middle („climax") region of the 

 rosette, in which there is marked elongation of the lobes (see 

 plate III), but in family 09275 many of the plants which were 

 finally regarded as B. bp. tenuis, had but one or two leaves in 

 which recognized tenuis characters appeared. Thus the wide gap 

 which ordinarily separates the dominant and recessive types in 

 these hybrid families of Bursa, was in this particular family not 

 only reduced to zero, but it appears certain that the heterozy- 

 gous and recessive categories overlapped to such an extent that 

 many individuals which belonged in the former were necessarily 

 classified in the latter : hence the defective ratio is to be explained 

 by the failure of dominance of the tenuis characters in the heterozy- 

 gotes. 



The cause of this failure of dominance is not apparent. The 

 environment has a very considerable influence in determining the 

 various features of Bursa plants, and especially in limiting the 

 development of such distinctive characters as ordinarily appear 

 only in the climax leaves. Crowding in the seed-pans, poor 

 illumination, and other unfavorable conditions, have caused many 

 plants in certain of my cultures, to develop fiowers and ripen 

 seeds without having developed their leaves beyond the early 

 juvenile stages. While the rosettes in 09275 were not in any 

 sense juvenile, the simplex characters do represent a less highly 

 specialized type than tenuis, and therefore any influence which 

 tends to abbreviate the cycle of development, might conceivably 

 reduce tenuis plants to a form indistinguishable from simplex. This 

 large family of 656 individuals was germinated in a single Square 

 seed-pan, 30 X 30 cm, and the young plants grew in this seed-pan 

 for seven weeks before they were potted. Perhaps this long 



