Pregnancy is an all-or-nothing situation; a woman either is or she isn’t. But in plants, the situation is very different. Partial or incomplete fertilization occurs when some of the ovules are pollinated and some are not. Insufficient pollination limits the number of seeds that can be formed, but it can also have a large impact on the amount of fruit produced per plant, as well as the quality of fruit.
Pollination is performed by wind, gravity, rain, or animals. Some flowers can self-pollinate, but many others must be pollinated with pollen from different flowers on the same plant, or by flowers from a totally different plant.
One of the easiest places to see incomplete pollination is in corn, a wind-pollinated plant. To be completely fertilized, every single silk (flower) must be pollinated separately. Once pollination is complete, a pollen tube grows down through the silk, the ovule is fertilized, and the kernel can grow. Any silks that were not pollinated produce nothing. In the photo you can see the difference between a fully pollinated ear, and one that was only partially pollinated.
In brambles such as blackberries, raspberries, and salmonberries each of the little nubbins (technically called drupelets) needs to be pollinated separately as well. If not, the berries are irregular and small.
In trees such as apple and pear, partial pollination results in lop-sided fruit. The ovaries of these fruits are divided into compartments called carpels. When the ovules in the carpel are properly fertilized, the carpel expands and provides protection for the developing seeds. If no fertilization occurs within a carpel, the carpel stays small. Poor pollination of the trees will greatly reduce both the yield and the quality of fruit.
In many other plants, insufficient pollination results in small or misshapen fruits as you can see in the photo of cucumbers, strawberries, and blueberries. Simply stated, if there are few viable seeds inside, the plant doesn’t need to grow a big fruit to protect them while they develop—a scrawny little fruit will do.
The flesh of tomatoes expands and ripens when hormones are released by the developing seeds. If a tomato is only partially fertilized, the fruit may be irregular or part of it may never ripen.
To further complicate the process, all this pollination needs to take place during a short flowering period with proper weather. So when you think of pollination, discard the idea that it is yes or no, on or off, black or white. Pollination is a “process” with many discrete steps that demand a lot of helpers and whole bunch of luck. Click on any photo for slides.