64 AMERICAN JOURNAL OF BOTANY [Vol. 10, 



Furthermore, the influences that affect the sex of a single flower often 

 extend to groups of flowers. Thus there is a period of maleness, which is 

 followed by a period of femaleness or of bisexualism, and this in turn is 

 followed by maleness. Flowers in the same condition as to sex are grouped 

 along the raceme. There is a series of cyclic changes all occurring during 

 the period of continuous bloom. 



These qualitative changes in sex in flowers of Cleome spinosa do not 

 involve the transformation of organs of one sex into organs of the other sex 

 after differentiation has begun, as is the case in many of the intersexes 

 reported in animals (Goldschmidt and Poppelbaum, 1914; Goldschmidt, 

 1916; Banta, 1916; Lillie, 1917; Sexton and Huxley, 1921). Here the 

 change is accomplished, as it is in dioecious plants, by the abortion of one 

 or the other kind of sex organs. The relative position of each in the flower 

 as a whole is maintained, but the differentiation giving male and female 

 flowers (along with bisexual flowers) is as complete as is seen in many species 

 of dioecious plants. The differential determination of sex in repeated 

 cyclic alternative changes as they occur in Cleome spinosa shows to what 

 degree the internal correlative differentiations in development may be 

 extended to the organs of sex after the plant as a whole has passed from the 

 exclusively vegetative to the reproductive stage. At the time of the transi- 

 tion to the reproductive stage, the change is not necessarily complete and 

 discontinuous, nor are the flowers produced in succession necessarily of the 

 same grades of sex. Even when the flowers appear to be morphologically 

 the same there may be a decided cyclic change in their physiological char- 

 acter, as is the case with Brassica chinensis and B. pekinensis (Stout, 1922). 

 The contrast between these species of Brassica and Cleome spinosa illustrates 

 well the different types of sterility that may develop in plants and the 

 different expressions of cyclic regulation of them. In these Brassicas there 

 is frequently rather decided abortion of flowers at the time of transition 

 from vegetative to reproductive organs; in Cleome no indication of such 

 abortion is present, the first flowers to appear being often fully developed 

 as hermaphrodites. In the Brassicas there is a somewhat extended period 

 of flower formation with flowers all morphologically bisexual — but in which 

 the physiological relations in fertilization may vary in a very definite and 

 single cycle; in Cleome spinosa there is no variation in the physiological 

 nature of stamens and pistils that are at all functional in so far as these 

 may be tested by the relations of fertilization, but there is the cyclic alter- 

 nation in the morphological development of the organs of sex. This 

 comparison illustrates two rather widely different expressions of sex in its 

 relation to fertility and sterility. 



The conditions in Cleome spinosa favor the view that, as held by Yam- 

 polsky (1920), there is a general tendency away from hermaphroditism 

 toward dioecism among the higher plants. In the persistence of perfect 

 flowers in greater or less numbers along with those which are more or less 



