1911-12.] BOTANICAL SOCIETY OF EDINBURGH 67 



former, a double one, or is it single 1 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-primor- 

 dium. 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 geneticallv 

 lateral in origin becoming ultimately terminal. We may 

 cite the spikelets of many grasses, also the carpels which 

 in many flowers monopolise all the residual part of the 

 growing-point of the flower, and the terminal stamen of 

 X'jios. CoU'itr'iche, etc. Ambrosia is. however, a specially 

 well-marked example of this procedure. The development 

 of the female flower need not be described further, except 

 to point out that there is only a mere trace of the corolla, 

 while the stamens and calyx have disappeared without 

 leaving a trace. 



Thus we see that Ambrosia has carried still further in 

 its female inflorescence that reduction already indicated in 

 Xanthium. Even in Xanthium the male organs of the 

 capitulum are completely suppressed and the number of 

 female flowers is reduced to two. As new structures there 

 are present the hook-like spines on the outside of the con- 

 crescent floral-bracts, which, taking the place of the pappus 

 originally present, facilitate dispersal of the fruit In 

 Ambrosia these spines dwindle to a few rudiments and 

 the number of flowers diminishes to a single one. 



In the male inflorescences, however, the number of capitula 

 is probably increased considerably in comparison with the 

 type-form with hermaphrodite flowers. With this may also 

 be correlated, that these male capitula have departed from 

 the prevailing scheme of development, and as seen in their 

 "reversal" have progressed along new lines. Thus with 

 no great effort, and with a basis of facts admissible in any 

 new speculation, we are able to trace, in what seems to me a 

 satisfactory manner, the historical evolution of the structure 



