Most plants in the wild are self sterile, ie. Nature dislikes incestuous propagation and prefers cross pollination, where the pollen is transferred between two genetically different plants. Cross pollination is achieved for most vegetables by an agent (called a pollinator), that physically moves the pollen. Usually this is a bee, because they are specially equipped to efficiently carry pollen; they are brawny, fuzzy, and have a static charge, so they acquire and transfer many grains of pollen.
In the case of the tomato, mankind has (perhaps unwittingly) selected plants with the defect of being self fertile. This likely came about as the plant was moved from its native area to other parts of the world without moving the native pollinators that were best equipped to cross pollinate them. So the surviving strains were the ones that had this defect.
Looking at cross pollination from the point of view of the fruit grower is utterly different from the point of view of the seed grower. With most plants, good cross pollination is very much desired, in some cases to make fruit at all, and in other cases to cause better quality fruit. Of course uncontrolled cross pollination makes the seed grower nuts, as he cannot predict exactly the characteristics of the plants grown from the seed.
Tomatoes are kind of a dream plant that can function both for the fruit grower and the seed producer. The reason is that they exhibit little “inbreeding depression.” That is, most plants, if they have any self fertility at all, will grow puny fruit and poor quality seed.
Tomatoes can be self fertilized and still be fairly vigorous.
In a discussion like this, someone always pipes up with the claim that tomatoes are “self pollinating.” This is a myth. Remember that myths are half truths. Most of the time, when you hear the myth, it is, of course, in the context of why tomatoes AREN’T self pollinating. Odd, isn’t it?
Most flowers have anthers (the male organ) that have the pollen on the outside. Anything that brushes the anther is likely to pick up some pollen. Tomato anthers are different; they are hollow tubes with the pollen grains inside. They are not released to the touch. The tomato must have help. It requires MOTION to release the pollen. Wind can accomplish this to some extent. Pollen grains are shaken loose and may land on the sticky stigma, thus accomplishing fertilization of some of the seeds.
Note that this is not self pollination, as the tomato could not do this of itself.
You can hand pollinate tomatoes by using any means of shaking the blossoms that doesn’t break plant parts. You can tap on the stem below the flower cluster with a pencil. Or you can hold an electric toothbrush against the stem. But the best tomato pollinator is a bee that “sonicates” or “buzz pollinates” the flower.
Bumblebee on a tomato blossom
Bees, most often bumblebees in eastern USA, will land on the flower, pulling it down until the anther is vertical (thus getting gravity’s help) and then vibrating their wing muscles (and their entire bodies) to shake loose the pollen. The vibration causes the flower to resonate, and many grains of pollen are shaken loose. This drops onto the bee’s belly, which is also vibrating. Some of it will stick to the bee and be available for cross pollination when she visits a flower on another plant.
Much of it will bounce right back up to the sticky stigma.
The development of the fleshy part of the fruit is stimulated by the fertilization of the seeds. Pollen grains on the stigma grow pollen tubes to the incipient seeds and fertilize them. If only a few grains of pollen are delivered, a fruit may form, but will not size up.
When enough grains of pollen are delivered to fertilize most of the incipient seeds, the fruit will be all that it can be.
If you are growing tomatoes for fruit, and not for seed saving, cross pollination is not of much concern to you. You just want good pollination to get the most and best fruit possible.
If you are growing tomatoes for seed, then you want to control the pollination more carefully. Generally, a gardener who saves seeds would want to keep an open pollinated variety pure, so you seek means to isolate the plants. This can be done by excluding bees (cages around plant or flowers), by sufficient distance between plants, or by timing bloom at different times.
Some tomato varieties are even nicer to the seed grower. The pollen is viable, even before the flower opens. If the flower gets sufficient motion (one reason why gardeners intuitively love early morning thunderstorms), it’s possible for the seeds to be fully fertilized quickly and keeping cross pollination down to a bare minimum.