1911-12.] BOTANICAL SOCIETY OF EDINBURGH 61 



Hardly any greater contrast can be presented than 

 between the resplendent flower-head of a Helianthus or a 

 Dahlia, with its remarkable arrangements for dehiscence 

 and for transference of the pollen, and the insignificant 

 male and female floral attire of a Xanthium or an 

 Ambrosia. There is a clear indication here that, quite 

 apart from the lack of a corolla conspicuous in its colora- 

 tion, the structure of the floral organs is in strict correla- 

 tion with their functions. In the flowers no nectar is 

 formed, there is no " concrescence " (in reality it is only 

 agglutination or sticking together) of the anthers, and the 

 pollen is formed in great quantity, as is the case with other 

 anemophilous flowers. The condition of the exine indicates, 

 however, that originally pollination took place by insects 

 as in other Compositae. 



It is well known that the pollen in entomophilous plants 

 is provided with a spiny or sticky exine, whereas in anemo- 

 philous plants the exine of the pollen-grains is smooth and 

 not sticky. The Ambrosiaceaj, so far as I have been able 

 to examine them, still show in the exine the remains of 

 spinose thickenings such as are found in numerous other 

 Composite, but these are so reduced that the pollen is no 

 longer adapted for adhering. 



The male flowers still show on the rudiment of the 

 stigma a hairiness which recalls the " stigmatic brush " of 

 other Composite. In the female flowers, in which the 

 stamens have dwindled without leaving a trace, there 

 is no longer any indication of this stigmatic brush. 

 Thus the Ambrosiacese, in contrast to other Composite, 

 are retrogressive in structure. On the other hand, it 

 is evident that they have developed new characters 

 not attained by other Composites. This is seen more 

 especially in the character of the noteworthy envelopes 

 with which the fruit is enclosed. The fruiting heads of 

 Xanthium are well enough known to the wool-grower as 

 the detested " burrs " (Kletten), which by means of their 

 bent hooks attach themselves to the woolly coats of 

 animals, and are so dispersed. We shall see that as regards 

 these fruiting heads the other Ainbrosiacese have also 

 undergone retrogression in contrast with Xanthium. 



It is not surprising that such noteworthy plants have 



