i 9 22] STOUT— STERILITY 117 



aborted flowers, but usually only a few of the first flowers abort. 

 Flower abortion also appears in many plants of B. chinensis which 

 have very loose rosettes of leaves. 



In these species flower abortion occurs as a transitional stage 

 between a period of vigorous vegetative vigor and a period of flower 

 formation and seed production. The plants which exhibit abortion 

 are not able to pass at once into complete reproductive activity 

 in producing potent flowers. The amount of abortion is greatest 

 in the varieties of B. pekinensis in which vegetative vigor is most 

 marked and in which excessive vegetative growth can readily be 

 induced by good cultural conditions and which have been selected 

 and bred for this feature. Flower abortion occurs in numerous 

 plants of these sorts that are grown in pots, as it does in many 

 plants of the loose-leaved kinds, but it apparently tends to be less 

 marked in these. 



Flower abortion is here undoubtedly correlated with the degree 

 of vegetative vigor. It is not merely due to a stifling of flowers 

 from simple direct injury because of inclosure within a head, 

 however, but to a constitutional feature of which the formation 

 of a leafy head or rosette is an extreme expression. In this sense 

 the abortion of flowers is self -induced and to some degree hereditary. 

 Usually the transition from aborted flowers to apparently normal 

 flowers is sudden and complete (fig. 1). Sometimes, however, the 

 first flowers to appear after the aborted ones, or the first flowers 

 when there are no aborted ones, are poorly developed, are plainly 

 immature and undersized, and especially in B. chinensis there may 

 be premature opening. 



(2) Arrested development of last flowers. — At least some 

 of the last flowers which begin to form remain immature and func- 

 tionless. In the first of such flowers the corollas wither quickly 

 and may become dry and papery without falling. Then the flowers 

 become smaller in size and more incomplete in development until 

 at last they are mere stubs of tissue. Usually from six to ten 

 flowers in these various stages of arrested development may be 

 counted at the tip of each branch. On short, lower, lateral branches 

 and on secondary or later laterals all of the flowers may fail to de- 

 velop fully. This condition is shown in the illustrations. The dis- 

 tribution of flowers that fail to develop is indicated in figs. 2 and 3. 



