MORPHOLOGICAL NOTES I95 



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 tri- 

 partita 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 infloresence (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 diff"erentiated 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-growth, with the exception of the terminal part {a, fig. i), 

 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^ fig. i), 

 except the basal part (nearest the primary axis), which later 

 gives rise to the intercalary meristem in the elongating 

 peduncle of the capitulum. 



How shall we interpret this peculiar development? Two 

 difl"erent views are obviously possible. 



It may be that the first leaf {a, fig. i) arising from the 

 growing-point of the capitulum is the first leaf of the in- 

 volucre of the capitulum, the individual leaves of which, 



