,1911-12.] BOTANICAL SOCIETY OF EDINBURGH 65 



There is little doubt that this anatomical structure is 

 the result of reduction ; that is, the peduncle of the 

 primitive type possessed more than two vascular bundles. 

 Linked with these bundles are those which ramify throughout 

 the involucral leaves and the flower, but I have not followed 

 these in detail. It may be mentioned, however, that the 

 vascular bundles of the involucral leaves are normally 

 orientated in that their xylem lies towards the growing- 

 point of the capitulum. The orientation is that described 

 above, and for the reasons given it does not seem to me 

 possible to arrive at any definite conclusion as to the 

 morphological interpretation of this leaf (a) from the 

 anatomical structure. 



It might be advanced in opposition to the view that the 

 leaf (a) is a bract, that later on it does not always stand 

 opposite the peduncle ; this might, however, result from 

 displacement. In the younger stages I saw it in the median 

 position occupying the whole breadth of the inflorescence- 

 primordium (rig. -4 ). As regards the further development of 

 the male capitula. two facts may be pointed out. Firstly. 

 that the external side (that on which the letter (a ) is placed) 

 appeared to be much further advanced than the inner side : 

 this is seen at once in tig. 4. Secondly, that the arrange- 

 ment of the flowers is not " dichasial '" sympodial (as one of 

 the recent authors wrongly states), but corresponds essen- 

 tially with the other Composite. One sees quite clearly 

 the broad growing-point of the capitulum < V.. fig. 4), from 

 which the individual flowers arise : some of the outer ones 

 have bracts even now. The characteristic arrangement of 

 the flowers is determined by the fact that the capitulum as 

 a whole is dorsiventral in structure, with a more advanced 

 development on the outer side. 



B. The Female Inflorescences. 



As indicated by Rostowzew. these are dichasially arranged 

 (fig. 5). Each consists of a single flower enclosed by an 

 " envelope " (fig. 8). To understand their constitution it 

 will be necessary to first compare the inflorescences of other 

 Ambrosias. Here one finds the following series : — 



1. Mixed inflorescences with female marginal florets. 



2. Inflorescences with sexes distinct, in so far that in 



TRANS. BOT. SOC. EDIN. VOL. XXVI. 5 



