PART III. | THE MECHANISMS OF FLOWERS. 327 
inner surfaces upwards, and bend their hairy tips so far backwards 
_ and inwards that some of the pollen which remains adhering to 
them is removed from the chance of contact with insects (Fig. 
112, 4). While the pollen is being forced up the anther-cylinder 
projects slightly beyond the corolla (Fig. 112, 3); when the 
stigmas are exposed, the anther-cylinder lies more deeply in the 
_ throat of the corolla, having been drawn down by the contracting 
1 filaments. The stigmas thus come to lie immediately above the 
corolla, in the same spot where the pollen was before. The result 
' is that as the under-sides of insect-visitors sweep over the cap- 
_ itulum they come in contact with many florets and effect many 
 cross-fertilisations at the same time. In this species, twenty or 
more florets (which, however, do not develop simultaneously, but 
_ centripetally) are united in a disk about 3 mm. in diameter ; but 
since five florets at the margin have each an external lobe over 
3mm. long and rather more than 3 mm. broad, the diameter of 
the disk is increased to 9 or 10 mm. The marginal florets have 
~ attained this enormous development of the corolla at the expense 
_ of the stamens, which are quite absent; they possess a style how- 
ever, which divides into two branches, covered on their upper 
_ surfaces with stigmatic papillz but destitute of hairs. In spite of 
the enlargement of the corolla in the marginal florets, the capitula, 
__ when they stand alone, are not very conspicuous ; but very many 
| capitula, often more than one hundred, are united in a corymbose 
inflorescence, which is not only very conspicuous but permits still 
' more extensive cross-fertilisation to be effected at once by an 
 insect-visitor. 
_ 226. AcHILLEA Prarmica, L., is distinguished from A. Mille- 
folium by the much greater size of its separate capitula, which are, 
however, associated in much less numbers in an inflorescence. About 
eighty to over one hundred florets, each scarcely 2} mm. long, are 
- united in a disk 6 to 7mm. in diameter, at whose edge eight to 
_ twelve marginal florets stand. Each of these latter possess an 
' outer lobe 4 to 6mm. long and not quite so broad, whereby the 
_ diameter of the attractive surface of each capitulum is increased to 
_ 15 oreven 18 mm. In other points the flowers agree with those of 
A, Millefolium. Both plants grow in equal abundance in the same 
localities, they flower at the same time, and are visited to the same 
extent by the same insects. In both, but especially in A. Mille- 
 folium, the strong odour of the plants is probably a great additional 
_ attraction ; for species of Prosopis, which are especially fond of 
