the orchid family there is only one anther and this divides into two pollinia,
sticky masses of thousands of pollen grains which are dispersed as a unit. The
filaments vary, wind-pollinated flowers often having long flexible ones, and a
few plants bearing wide filaments acting as the main attractive organs for insects.
Some are sensitive to touch, contracting on contact with insects.Carpels Theovariesare usually oblong shaped but some are spherical or disc-like and
they may be separate or a few, often three or five, fused together. The ovaries
may be situated on the surface of the flower, known as superiorovaries, or may
be buried in the receptacle with only the style and stigma projecting, in which
case they are known as inferior. There are intermediates.
Astylemay or may not be present, and, where the ovaries are fused, there is
only one; it is up to several centimeters long in some bulbous plants. The style is
usually solid with some central transmitting tissuethrough which the pollen
tubeswill grow after the pollen grains have germinated on the stigma, but in
some it is hollow and pollen tubes grow over the inner surface. The stigmain
animal-pollinated plants is usually club-shaped, or with two-to-five lobes, some-
times one lobe per ovary. The surface may be dry or sticky (Topic H3). In wind-
pollinated flowers the stigma is often lobed and feathery with a large surface
area, frequently with no style.
Each ovary may have one ovuleor several, up to a few thousand in the
orchids. The ovules are usually attached to the side of the ovary nearest the
center of the flower. They have a central embryo sacwhich includes the egg,
surrounded by a nucellusof parenchyma cells and with usually two protective
integumentsaround that. A few plant families have lost one or both integu-
ments while others have four. There is a small opening in the integuments, the
micropyle, through which the pollen tubes penetrate. The ovule is usually
turned through 180° with the micropyle facing the stalk, known as anatropous,
but a few species have upright ovules, ones turned to the side or even turned
through 360° (Fig. 4).
Nectar Nectaris secreted by many flowers and is collected as a food by animal visitors.
It is mainly a solution of glucose,fructoseandsucrosein varying proportions
depending in part on which are the main pollinators, the more specialist bee,
butterfly or bird-pollinated flowers having high sucrose content. The concentra-
tion ranges from 15% sugar by weight to 80%or even crystalline sugar
depending on flower form and weather conditions. It may also contain small
quantities of amino acidsand other substances that may be important nutrients
for some pollinators such as flies. It is secreted in various different parts of a
flower, most commonly at the base of the petals, or in a specialized spurat the
base of a petal or petal tube in some flowers. In other flowers it is on the recep-
44 Section D – Reproductive anatomy
Integuments
Nucellus
Embryo sac
Micropyle(a) (b) (c) (d)Fig. 4. Ovules: (a) anatropous orientation (the most widespread type); (b)–(d) other types.