188 BOTANY 
Petals and sepals together form the perianth, petals 
alone the corolla. Petals are chiefly to attract insects 
but may also be protective. They may be apopetalous 
(free) or synpetalous (united). A regular flower is 
actinomorphic, an irregular one zygomorphic. [lowers 
attract insects by their size, shape, colour, nectar, and 
perfume. Petals may protect the pollen by arching 
over it or by closing. 
Essential Parts.—The stamens form the androecium, 
the carpels the gynecium. The stamens may be united 
or free. The stamen consists of filament or stalk and 
anther with pollen sacs containing pollen. Dehiscence 
may be introrse (inwards) or extrorse (outwards). 
The gynecium.—The term pistil and gynccium 
are synonymous where there is only one carpel, or the 
carpels are united. When there is a number of free 
earpels each carpel is a separate pistil. Each pistil has 
an ovary containing ovules and a style lifting up the 
stigma or receptive part. The gynecium may be 
syncarpous or apocarpous. The ovary may be one or 
many celled. Placentation of the ovules may be 
parietal, axile, free central, basal, or suspended. 
Pollination takes place when the pollen grain 
reaches the stigma; fertilization when the pollen 
nucleus reaches the egg-cell of the ovule. 
Nectaries are gland-like structures that secrete 
nectar. Honey guides may show the way to these. 
Cohesion refers to the union of the members of the 
same whorl; adhesion to the union of members of 
different whorls. The normal arrangement is hypo- 
gynous, with calyx lowest on the receptacle, corolla 
next, stamens next, and carpels next. In a perigynous 
flower the petals and stamens are lifted up on the 
receptacle, which forms a cup round the ovary. In an 
epigynous flower the sepals, petals and stamens are on 
top of the ovary. Stamens are sometimes epipetalous. 
