Sorry, folks, for doing this like a teacher. If you want to learn how to save seeds properly, you need enough background to not let the science get in your way. What follows is a brief description of the reproductive biology of flowering plants. I tried to limit this discussion to the most common situations, and to bring in examples from both flowers and vegetables, from annuals and perennials. Please realize that this is not designed to be an everything-for-everybody discussion. It's just an attempt to put the basics out there, in a common place, so that we can all refer to it for mutual convenience.
FEMALE (PISTIL: EGG-OVARY-STYLE-STIGMA)
To make seeds, you need to know a little bit about the reproductive structures of higher plants. It all starts with the flower. The female parts are contained in a structure known as the pistil. The egg cell (ovum) is contained in a ovary, just like in mammals. The male reproductive cell that is contained in the pollen must pass through a stalk called the style. At the point where the style is farthest away from the ovary is a specialized surface known as the stigma. Think about a flower like an Easter lily. At the base of the inside of the flower is a swollen green ovary. The style is the green stalk, while the stigma is the swelling at the end closest to the outside. The stigma is the surface on which pollen must land (or be placed) during pollination. It frequently has a different surface appearance from the style, and often appears moist and glistening. In many flowers, the stigma splits into three, or five, or even more parts. Some stigmas are feathery and fern-like in structure. These are strategies to increase the surface area, and thereby to increase the chances of trapping pollen. And, as you might expect, wind-pollinated plants need more surface area than insect-pollinated plants. Think about the differences between a lily flower (insect) and a female maize flower (one single corn silk).
MALE (STAMEN: POLLEN-ANTHER-FILAMENT)
The male parts of a flower are specialized to produce pollen. The pollen is produced in a sac-like structure called the anther. The anther is generally attached to a long, thin hair-like structure called a filament. In an insect-pollinated species, the filament usually is long enough to place the anther at the open end of the flower, where it can be in contact with pollinating insects. Pollen is made in the anther. Egg cells are made in the ovary. Pollination puts the two together. Fertilization is what happens when the sperm nucleus in the pollen merges with the egg cell.
PETALS à COROLLA, SEPALS à CALYX
The reproductive structures in higher plants are contained within flowers. Flowers have more than one petal, and the flower petals are collectively called the corolla. A flower bud is protected by green leafy structures called sepals. Collectively, all of the sepals form the calyx. Sometimes, the sepals are colored and ornamental, too. Florida dogwood "flowers", the white or pink showy part, are actually sepals, although the term bract is used. The real flower is the tiny greenish-yellow structure in the middle. Likewise, the bright red part of a holiday poinsettia is made up of colored bracts. The real flower is the yellow structure in the center.
To a botanist, a perfect flower is a hermaphrodite. That is, a perfect flower contains both male and female structures. A lily flower is perfect. A rose is perfect. An apple flower is perfect. A tomato flower is perfect.
In contrast, imperfect flowers are incomplete. An imperfect flower is either male or female. The tassel on top of a maize plant is male (and imperfect). Maize silks are female (and imperfect). Squash flowers are imperfect. Begonia flowers are imperfect.
The picture becomes a little more complicated. There is a tremendous variety of flower forms in the world.
SINGLE INTO DOUBLE
Sometimes, a mutation affecting the reproductive parts in a flower causes the reproductive parts of a flower to become petals. The normal flower is called single (because there is a single set of petals), while the mutant is double (two sets of petals). And it should be obvious that if you take all of the reproductive parts and change them into petals, the flower is now sterile. In domestication, double flower forms tend to be selected for, because they are more showy and more attractive. However, many of them do not make seed, and must be propagated by cuttings.
Flower structures are often complex, with many smaller flowers (or florets) clustered together in what I will call a "spike". Botanists have many names for different kinds of flower spikes, and there are different ways to handle the different kinds of spike structures.
COMPOSITE (RAY AND DISK)
In some very common flowers, the spike is compressed into a flat structure, so that the idea of a long spike disappears. The florets form a dense mass that may appear to be a large single flower. The best example of this is the sunflower. These kinds of flowers are called composites, because the flower is a composite of many florets. Furthermore, in composites, there are two different kinds of florets, the ones that make up the flat, edge part of the sunflower (called ray florets), and the ones the make up the central disk or head of the sunflower (called disk florets). Frequently in composites, the ray florets are imperfect and male, and the disk florets either female or perfect. Remember, seeds are only produced by female flowers. Where are the seeds in sunflowers? In the central disk. Composites complicate the picture even more. Double composite flowers are generally pollen-sterile (the stamens change to petals), but egg-fertile. So you can make seed on a double composite flower. And worse, under some conditions, some doubles are not all double, so that you can get a small amount of pollen to work with from many double composite lines. I am explaining the double phenomenon as a either-or condition. In reality, there are stable in-between states, too. You can also turn only some of the stamens in petals, leaving some as functional stamens, making pollen. These types are generally called semi-doubles.
In this common type of flower spike, individual florets are attached to a central axis. And typically, the florets open from the bottom to the top, which means that you will have fresh pollen available from the upper florets to use on the lower florets. This assumes that pollen is shed before the stigmas are receptive, and that is the case in many species with this kind of flower spike.
In this flower spike form, as in the snapdragon, there is a central axis, but rather than having individual florets attached to the axis, there are secondary branches attached to the central axis and these may branch and branch again before the individual florets are formed. The end result is a branched cluster of florets, forming a large showy bouquet. As in the snapdragon-type of spike, the lilac-type of spike also tends to open and shed pollen from the bottom up.
Carrot family species (including Queen Anne's Lace, parsley, celery, and dill) have flat flower spikes known as umbels. These disks generally consist of both fertile and sterile flowers, and the flowers open and shed pollen from the outside of the disk to the inside.
Different plant species have different kinds of flowers. Some plants bear imperfect flowers on separate plants, and are therefore either male or female. These species are called dioecious, which is Greek meaning "two houses", indicating that the male is on different plant ("house") than the female. The best garden example of this is asparagus. The female plants make berries containing seeds; the males supposedly make stronger "spears". At least, that's the logic behind 'Jersey Knight' and all those super-male varieties. Presumably, some of the energy in female plants goes into seed production, whereas all of the energy in male asparagus plants goes into the plant, which means into the root, which means into the stalk which you cut in the following spring. Some species bear perfect flowers, and these species are called monoecious (meaning "one house"). Some monoecious species bear imperfect flowers, which means one plant has two kinds of flowers, male or female. Think of a squash plant: most of the early flowers are male, some of the later ones are female.
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