Pollination is often a complex interaction between the plant and a pollinator species. In Nature, this pollinator is often an insect, but can also be a bird or a mammal. If you hand-pollinate, for whatever reason, you are taking the place of this natural pollinator. In many flowers, there are "honeypots" called nectaries at the base of the inside of the flower, where the filaments, and the petals all join together at the base of the ovary. To get to these nectaries, a pollinating insect (like a honeybee) must go through a gauntlet of other structures. Typically, the first flower part passed is the stigma, and pollen clinging to the insect can get transferred from the insect to the stigma. The insect travels deeper into the flower, gets a drink of nectar, and backs out of the flower to leave. As it does so, it brushes by the pollen-laden anthers, and gets covered in pollen. It leaves the flower, and takes this load of pollen to the next flower, thus creating cross-pollination.
Pollination is an interesting event, because the pollen grain acts just like a seed. When a grain of pollen lands on a receptive stigma, it germinates much like a seed does, and a root-like structure called a pollen tube grows down the style from the stigma to the ovary. What is interesting is that there is a biochemical conversation that occurs on the surface of the stigma between the pollen grain and the stigma. Only the right kind of pollen can actually germinate, and only the right kind of pollen can grow down the style and into the ovary. Controlling pollination is all about sexual reproduction in plants, and at a cellular level. The goal of sexual reproduction in plants is to make seeds. This requires that the number of sets of chromosomes in the plant cell gets reduced from two sets (normal cells) to one set (reproductive cells). This reduction process is known as meiosis.
Meiosis enables the number of chromosome sets to stay the same when the sperm and egg merge in fertilization to form an embryo. Every normal cell has two sets. The sperm has one set; the egg has one set. One plus one equals two. Why is this important? Well, it keeps the amount of DNA constant in each cell of each species. Meaning reproduction keeps things running smoothly. The "two become one becomes two" process also sets up a system where genes get mixed up and interchanged with each generation, and that is what fuels the evolutionary process. It is also important because meiosis is a process that occurs as part of pollen development and maturation. The timing of meiosis in pollen development determines how long pollen can survive, and that determines whether or not pollen can be collected and stored. If you can store pollen, making pollinations becomes much easier: you gather pollen when you can, and make pollinations when you want. If not, you need a source of fresh pollen each and every day that you want to make a pollination. The final division in meiosis can occur before pollination, or after pollination.
If before, then the pollen grain has three nuclei (known as trinucleate), is very short-lived and is generally not able to be successfully stored. Species which tend to depend on the wind for pollination (like grasses) tend to be trinucleate at pollination. Trinucleate pollen tends to be light and dry. Trinucleate pollen does NOT store very well, or for very long. Species in which that final division occurs after pollination have two nuclei at pollination and are called binucleate. Species which tend to depend on animals for pollination are frequently binucleate. Binucleate pollen tends to be heavy and sticky. Binucleate pollen can successfully be stored cold and dry for years. These rules of thumb work for many species, but not all. If you do not know for sure, the botanical literature will usually describe the pollination ecology of a species.
How easy is it to save seeds? The answer is "It depends". And it depends on whether a species is an inbreeder or an outbreeder.
INBREEDERS SELF-POLLINATORS (INDIVIDUAL OR GROUP)
Inbreeders (IB) tend to be self-pollinators, or tend to grow in isolated non-clonal colonies so that pollination occurs within a small number of individuals. This promotes inbreeding. In an IB, anthesis often occurs before the flower opens, or just after the flower opens, so that self-pollination occurs at a high rate. IBs tend to have perfect flowers, although not all species producing perfect flowers are IBs. If a species having perfect flowers is not an IB, there must be some mechanism in place to minimize self-pollination. These mechanisms can make your job as pollinator more difficult. For instance, apples have perfect flowers, but a single isolated apple tree will generally not set fruit. It needs a pollinator tree. Why? Because self-pollen is generally ineffective in apple: the pollen from one tree will not fertilize egg cells on that same tree. This phenomenon is called self-incompatibility. IB species are self-compatible. Apple is not an IB species.
IB species require little if any isolation to ensure that seed produced is pure. Examples of commonly-grown IB species are: legumes, tomatoes, impatiens, bell peppers, eggplant, and lettuce.
Outbreeders (OB) are not IB! OB species tend to be cross-pollinated, and often are very resistant to inbreeding, both from a practical sense and from a genetic sense. In many OB species, OB is assured by self-incompatibility, or by having the male and female flower borne on different parts of the plant, or by producing pollen either before or after the stigma is receptive, or by placing the anther is a position where a pollinating animal is most likely to touch the anther only after the animal is likely to not cause self-pollination.
WHEN IS POLLEN SHED?
The timing of anthesis is a fundamental way to avoid self-pollination. Most commonly, pollen is shed before the stigma is receptive: this phenomenon is called protandry, and species which exhibit protandry are known as protandrous. The prefix "prot" indicating "before", or "first", and "andr" indicating "male". So what happens in a protandrous species? The flower opens, pollen is shed, and after all of the self-pollen is shed and gone, the stigma becomes receptive to pollen. Since there is no self-pollen available, the only pollen that can cause pollination is cross-pollen, thus ensuring cross-pollination.
FLOWER STRUCTURE CAN AVOID SELF-POLLINATION
OB species can bear perfect or imperfect flowers. In a sense, imperfect flowers make cross-pollination easy, while perfect flowers make cross-pollination difficult. But Cross-pollination can also be assured by the very structure of the plant. Consider the flowering maize (corn) plant: the tassels at the top are the male flower, while the female flower is the silk (and every silk is attached to a single ovary, each of which becomes a seed. The ear is therefore a form of flower spike!). Maize is somewhat protandrous, since much of the pollen is shed before the silks are receptive. However, the later-opening tassels often shed pollen when the silks are first receptive. But the later-opening tassels are higher up the plant, and therefore, pollen from the later-opening tassels tends to be wind-blown away from the plant. The separation of the male and female maize flowers encourages cross-pollination.
ECOLOGY: THE INTERACTION OF ANIMAL AND PLANT
The ecology of pollination also encourages cross-pollination in some species. There is a tremendous literature about specific plants requiring specific animals for pollination. Indeed, there are insects whose tongues are the exact length to reach the nectaries of a certain species of flower. Without the flowers, the insects would become extinct; without the insects, the plants would become extinct. This animal-plant interaction, in combination with the timing of anthesis, and the structure of the flower, can encourage cross-pollination.
But many plants also have a back-up mechanism to ensure that cross-pollination occurs. In many OB species with perfect flowers, including the common Brassica family (cabbages, etc.), pollen from one plant will not germinate or grow on the stigma of that same plant. This rejection of self-pollen is a biochemical phenomenon known as self-incompatibility, and has been studied for decades, largely in the cabbage family because of its global economic importance.
OB species require either isolation, or hand-pollination (with protection taken against cross-pollination or cross-contamination both before and after hand-pollination). How much isolation is required between plantings of the same SPECIES? The real answer is "It depends". For a wind-pollinated OB species, a quarter mile downwind is probably as close as you can risk. But less than a thousand feet, side-to-side, if the wind is constant from one direction, is probably OK. For an insect- or animal-pollinated species, the distance is generally much smaller. For general security, a few hundred feet between plantings will be OK. You can also grow even closer plantings if there are vertical barriers in between. Plants on opposite sides of a house will generally not cross-pollinate, for instance, and home garden seed savers will frequently use pole beans or maize to isolate varieties, and with good success. Realize that "good success" may not mean 100% pure. The only way to be absolutely sure is to only grow one variety of each species for which you are trying to save seed, and that means one variety IN BLOOM. You can be saving seed from broccoli and growing cabbage in the same garden, because the cabbage will not be in flower until next year. Examples of OB species are the Brassicas (broccoli, Brussels sprouts, cabbage, cauliflower, kale), maize, carrots, onions, composites (sunflowers, marigolds), petunias.
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