62 TRANSACTIONS OF THE [Sess. lxxvi. 



repeatedly formed subjects for investigation. So far as 

 the developmental history is concerned, and apart from 

 Payer's x contributions, which in the present instance need 

 only be indirectly considered, there are available only the 

 investigations of Rostowzew, 2 and these I propose in the 

 following to supplement on some points. 



Thus one topic will be the male inflorescence, while 

 another will be the "fruit" of Ambrosia. 



A. The Male Inflorescences. 



As is known, the Ambrosiacese are monoecious to this 

 extent, that the male inflorescences occupy the apex of 

 shoots at the base of which are placed the female ones. 



The individual flower-heads ('-'capitula") of Ambrosia 

 tripartita are arranged in large numbers indefinitely 

 (botryose) on an elongated axis, which in turn is 

 terminated by a flower-head. 



The lateral capitula have this noteworthy characteristic, 

 that the flowers are not placed as usual on the upper 

 (adaxial) side, but on the under side — that is, the side 

 turned away from the primary axis of the whole in- 

 florescence (abaxial). It is natural, at first sight, to ascribe 

 this occurrence to a torsion of the flower-stalk. This is 

 indeed the opinion of Rostowzew, who says that the 

 flower- head undergoes a torsion, in that the peduncle 

 grows more rapidly on its upper side than on its lower. 



This interpretation in itself does not agree with the 

 developmental history. If one examines the early stages, 

 it is seen that the position of the capitula is " reversed " at 

 a very early stage. 



The capitula are differentiated from the embryonal tissue 

 as hemispherical primordia. The cells of the primordia on 

 the adaxial (upper) side pass first into the phase of elonga- 

 tion-orowth, with the exception of the terminal part (a, 

 fio-. 1), which develops as the first leaf. On the other hand, 

 the lower side remains embryonal. This is utilised for the 

 development of the growing-point of the capitulum (b, 



1 Payer, " Traite d'Organogenie comparee cle la fleur" (Paris, 1857, 



p. 638). 



- T. Rostowzew, " Die Entwicklung der Bliiten und des Bhitenstandes 

 bei einigen Arten der Gruppe Ambrosie;e" (" Bibliotheca botanica," 20). 



