240 THE STRUCTURE OF FLOWERS. 



before tlie development of the styles and stigmas, bore 

 diminislied whitish anthers, not opening at all, and contai-aing 

 also some shrivelled pollen-grains. Lastly, in a barren sandy 

 locality, many of the stems produced female flowers, v^ith 

 stamens aborted in the same degree as in D. superhus, and 

 not infrequently such female flowers and protaudrous her- 

 maphrodite ones are found on the same stem." Wiegman also 

 found the Bianthus had contabescent stamens v^hen growing 

 on a dry and sterile bank. The conditions here mentioned 

 are very like those more than once described as associated 

 with double flowers, in which the stamens have also de- 

 generated but taken the petaloid form. Hence I think we 

 may directly trace the degeneracy of the anthers and pollen 

 to atrophy ; since chemical analyses of pollen prove that the 

 most important constituents required are potash, nitrogen, 

 and phosphorus pentoxide,* probably wanting in the localities 

 mentioned. 



" Centaurea Jacea " Miiller describes f " as having its 

 flower-heads of the same stem always of the same form, but 

 different stems of the same locality often present astonishing 

 differences in their flower-heads. 



" In the most common and apparently original form, the 

 flower-heads consist of florets which are all of the same 

 tubular shape, and all contain both fully developed anthers 

 and stigma, the divergence of the outer florets giving to the 

 whole head a diameter of 20-30 mm. From this original 

 form variation has gone on in two opposite directions, the 

 final effects of this variation being, on the one side, very 

 conspicuous male flower-heads of 50-55 mm. diameter ; and 

 on the other side less conspicuous female flower-heads of 



* From an analysis of Ash blossomSj by Professor Church, Journal 

 of Botany, 1877, p. 364. 



f Nature, vol. xxv., p. 241. 



