ULASSIFICATION 193 
The inflorescence is a capitulum, but often the capi- 
tula are again grouped in racemes, corymbs, etc. Each 
capitulum is surrounded by an involucre of bracts. 
Sometimes the florets are all alike and perfect, they 
may be all tubular or all ligulate, more commonly, how- 
ever, there is a distinction into a “disc” of regular 
florets and a “vay” of zygomorphic florets. The 
flower may be perfect or imperfect. The Calyx is 
usually a pappus of hairs, sometimes it is absent or re- 
presented by small teeth. The Corolla is gamopetalous, 
of 5 petals, ligulate or tubular, epigynous. Androecium, 
stamens 5,epipetalous. Anthers introrse, syngenesious. 
Gynoecium, syncarpous of 2 carpels. Ovary inferior, 1- 
celled, containing 1 erect basal ovule. Fruit an achene, 
often crowned with a pappus. 
The Pollination Mechanism in this order has al- 
ready been considered (Chap. VI). It is singularly 
effective, since if cross-pollination fails, self-pollina- 
tion can always take place, and even those flowers, such 
as Senecio and Sonchus, which get few insect visitors, 
can always set seed and still get an occasional chance 
of cross-pollination. 
The Dispersal of Seeds.—In most cases the fruit is 
surmounted by a pappus of hairs which enables it to be 
blown away by the wind, but occasionally the calyx is 
hooked and causes the fruit to adhere to animals—e.g. 
Bidens (Black Jack). In Arctium the bracts are hooked 
and so the whole head of fruit is carried away. In. 
Xanthium the involucre becomes hard and woody and 
covered with hooks which enable it to cling to the wool 
of animals; it has proved so troublesome a weed in this 
13 
