MORPHOLOGICAL NOTES I99 



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^ 

 Sd, fig. 6, III.) in whose axils the female flowers are placed. 

 With reference to the development, the reader is referred to 

 my recent account.^ 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.- How dispersal is eff'ected 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 former, 

 a double one, or is it single ? That there is some suggestion 

 of a double envelope is evident by the fact that the apex of 

 the envelope (originally laid down as a closed ring ; see fig. 7, 

 right-hand figure) is distinctly two-lobed in the later stages 

 of growth. This indicates that two leaf-primordia take 

 part in the formation of the envelope. Only one of these, 

 however, has an axillary flower-primordium. Obviously a 

 characteristic retrogression has taken place here : the flower 

 monopolises the whole area within the envelope ; hence it 

 arises not as a lateral outgrowth on the growing-point of a 

 capitulum, but is terminal. There are, of course, many 

 examples of an organ genetically lateral in origin becoming 

 ultimately terminal. We may cite the spikelets of many 



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



- They might, of course, become lodged between the claws or in the hoof. 



