66 TRANSACTIONS OF THE [Sess. lxxvi. 



the female only the marginal florets flower, and 

 only the minority of these, while in the male 

 inflorescence the formation of female flowers is 

 entirely suppressed. 

 3. There is also a diversity in the arrangement of the 

 female and male inflorescences. Whereas the 

 arrangement was originally indefinite (botryose), 

 this is retained only in the case of the male inflore- 

 scences, the female ones showing dichasial arrange- 

 ment. 1 

 In Xanthium the male inflorescences are orientated norm- 

 ally ; the structure of a young male inflorescence in longi- 

 tudinal section is shown in fig. 6, I. The female capitulum 

 consists of two flowers enclosed in the many-spined envelope. 

 This envelope originates from the fusion of two bracts (Sa, 

 8b, fig. 6, II.) in whose axils the female flowers are placed. 

 With reference to the development, the reader is referred to 

 my recent account. 2 This is noteworthy in this respect, that 

 the two floral bracts entirely monopolise the growing-point 

 of the capitulum (except what is required for the flowers 

 arising in their axils), as is shown in the view from above 

 (fig. 6, IV.), and that the flowers are deeply sunk in the 

 axis of the capitulum. 



Comparing this inflorescence with that of Ambrosia, it 

 will be found to have undergone still further reduction. 

 Firstly, we see that the spines or barbed hooks outside the 

 envelope are here reduced to a few small processes (figs. 7 

 and 8, st) ; secondly, each envelope contains only one flower. 

 The processes referred to can no longer be of use in the 

 dispersal of the fruits by animals. 3 How dispersal is 

 effected can only be ascertained in the native land of the 

 plant. The fruits are relatively light, and float in water 

 for a short time at least, and they may even be rolled to 

 a distance by wind. 



It is evident that the envelope of the fruit of Ambrosia 

 corresponds to that of Xanthium. But is it, like the 



1 With regard to diversity of male and female, compare Goebel, 

 " Ueber sexuellen Dimorphismus bei Pflanzen " (" Biolog. Centralblatt," 

 p. 657, 1910) 



2 Goebel, I.e. ("Biolog. Centralblatt," xxx., 1910, p. 722). 



3 They might, of course, become lodged between the claws or in the 

 hoof. 



