WHAT IS A FLOWER? 83 
the former, and to avoid the latter, many highly inter- 
esting devices are found, materially affecting the 
structure and development of flowers. 
The essential parts of a flower, then, consist of 
stamens and carpels. Flowers consisting of no other 
parts but either or both of these are not common, but 
we may compare, for example, the rarely produced 
flowers of the Duckweeds (Lemna), in which a tiny 
group of two stamens and a carpel represents one 
flower, or, according to some views, a group of three 
flowers. More commonly the flower is much more 
composite, consisting mostly of four sets of organs, 
arranged in whorls or rings, or more rarely in close 
spirals. In the centre is a group of carpels; outside 
them—in other words, slightly lower on the stem— 
a ring, or two rings, of stamens, few or many; then a 
ring of petals, forming the corolla, usually coloured, 
leaf-like, and conspicuous; and outside of them a ring 
of sepals, forming the calyx, generally green and leaf- 
like. The main function of the calyx is protective; it 
encloses the essential organs and guards them till 
they are mature, when the flower opens and stamen 
and stigma play their parts. The calyx is usually 
tough, and often covered with hairs, or with a sticky 
substance, to keep the flower safe and ward off the 
attacks of insects or other small devourers. If we 
turn to the corolla we find a singular variety of size, 
form, and colour. To account for this, it is necessary 
to consider the means by which pollen is distributed. 
There are two chief ways in which pollen is conveyed 
from flower to flower—by means of the wind, and by 
means of flying insects. If we examine wind-pol- 
linated flowers, such as Hazel (Corylus), Scotch Fir 
