arr] THE MECHANISMS OF FLOWERS. 291 
200. VinurNuM OpuLus, L.—The flowers are aggregated in a 
at corymb, whose marginal flowers have the petals greatly enlarged 
at the expense of the essential organs. They thus attain conspicuous- 
1ess, useless to themselves, but of the highest importance to the 
community, as Sprengel correctly explained (702, p. 159). When 
1e flowers expand (Fig. 98, 2) the anthers have already begun to 
lehisce, and the stigmas are already so far developed that if pollen 
e applied to them it adheres easily and in large amount. Shortly 
terwards the anthers become covered all round with pollen, and 
rotrude, diverging, from the flower, while the stigmas remain in 
ie base of the flower close above the ovary, whose upper surface 
‘secretes honey. The honey is fully exposed, and forms a flat, 
‘adherent layer, so that it is only attractive to flies and other 
Fic. 98,—Viburnum Opulus, L. 
‘ —Marginal flower, from above, showing rudimentary pistil and stamens (x 24). 
2.—Fertile flower, soon after expansion (x. 42). 
8.—Ditto, after removal of the anterior part of the corolla and stamens (x 4%). 
ort-lipped insects; the pollen attracts both flies and pollen- 
lecting bees; the white colour of the flowers makes them 
tractive to beetles, some of which are very destructive visitors. 
e most frequent visitors and the most efficient fertilisers are 
0 those which get most benefit from the flowers, viz. flies, 
hich by turns suck honey and feed on pollen. As each flower 
contains a very small amount of honey, the insects rapidly 
a; the corymb; and they repeatedly effect fertilisation, 
which is usually cross-fertilisation, since in each flower anthers 
d stigmas generally come in contact with different parts of the 
sect’s feet or proboscis. In absence of insects, self-fertilisation is 
not impossible, for, in spite of the divergence of the stamens, the 
igma comes to lie in many flowers immediately below an anther. 
U2 
