192 University of California Publications i)i Botany [yoi.. 5 



noticeable fact in this connection is the readiness with which the 

 flowers fall from the pedicels at about the same time (peculiar 

 to each hybrid) after anthesis. This situation seems to be con- 

 nected with the formation of a definite absciss or separating 

 layer within the individual pedicel at a distance approximately 

 1.5 mm. above its point of origin from the peduncle. This par- 

 ticular matter is at present under further investigation, but it 

 appears to be true that this absciss layer is similar to that 

 formed normally in connection with the fall of the leaf in 

 deciduous trees, that it may in certain cases be formed some time 

 before the flower bud opens, and in other cases begins its ac- 

 tivity much later, and finally that it fails at times to form at all. 

 The various F^ hybrids exhibit considerable differences in con- 

 nection with this particular point. A slight shaking of the 

 main axis of one of the plants representing the cross H36 will 

 cause practically all its flowers past anthesis to fall and also 

 numbers of freshly- opened flowers and even buds. The same is 

 true, though in a less pronounced manner, of the plants grown 

 from the seed of the reciprocal cross. In the case of the cross 

 H38. on the other hand, the F^ hybrids hold their flowers much 

 more firmly and, though the great majority of the withering 

 flowers finally fall, still when newly opened fully 50 per cent 

 of the flowers will resist considerable strain before becoming 

 separated from the pedicel. The situation in F^ of the crosses 

 H43 and H45 and their reciprocal is complicated in that very 

 certain evidences of segregation are to be seen and various degrees 

 of resistance to dropping of flowers seem at present to be cor- 

 related therewith. This will be further considered below. The 

 flowers of the F^ hyl)rid of the cross H40 fall almost as readily 

 as do those on Fj plants of the cross H36 mentioned above. The 

 Fi plants of H41 have not as yet come fully into flower (Dee. 

 10), but give evidence of possessing this peculiarity of loss of 

 flowers to about the extent exhibited by the F^ plants of H38. 

 In all these hybrids it can be certainly stated that climatic varia- 

 tions, as they occur out in the field at any rate, are in no sense 

 responsible for the dropping of flowers, nor can it be said that 

 a period of rainy, cloudy weather will cause any very appre- 

 ciable increase in this tendency (Goodspeed, loc. cit., p. 141). 



