Self, Cross, and Open Pollination - What's Going On?
Self and cross pollination are two different types of open pollination. The following is a quick rundown on how plants "get busy".
Pollination is a sexy topic. Literally it pertains to the topic of plant reproduction. Unlike in the animal kingdom, however, there are a few different ways that plants “get busy” to pass along their genes.
Some plants require pollination from another plant, and some are capable of doing it for themselves. Some pollination only occurs in nature; man induces some as well. Either way, pollination requires the transfer and successful placement of pollen from the anther of one flower to the stigma of another flower on the same or a different plant.
Pollination that occurs in nature, such as by animals and insects or water and wind, is called open pollination. This is how plant reproduction has occurred (for those plants that reproduce this way) for millions of years.
This is in contrast to the “newer” practice of hybridization where plants with various traits deemed desirable by plant breeders are intentionally pollinated for commercial or other botanical purposes.
Open pollination occurs by either self-pollination, where the male element of a flower produces pollen intended to be used to pollinate a female flower on the same plant, or by cross-pollination, which occurs when the male flower of a plant is able to pollinate the female flower of another plant of the same species.
Self-pollination comes in different forms. Some species of plants produce flowers with both male and female components within the same flower (autogamy for those of you “Words With Friends” buffs).
Other species of plants produce separate male and female flowers on the same plant (geitonogamy for you old-school Scrabblers). While nature doesn’t decide upon its courses of action by consulting a “pros” and “cons” table, there are some definite advantages and disadvantages to self-pollination.
Self-pollination creates progeny that is essentially a carbon copy of the parent plant. It is a form of plant reproduction that does not unduly stress the plant or require large quantities of stored nutrients or energy.
Flower size is irrelevant in self-pollination (no snickering), as it does not need to attract the services of insect or animal passers-by. Ditto for not needing to produce a fragrance or nectar.
The downside of self-pollination is that defective genes and mutations, if there are any, are passed down from plant generation to generation. Plants that reproduce this way often produce poor-quality seeds as compared to those that cross-pollinate. This aspect is more relevant to commercial producers than the plants themselves, but does represent a biological disadvantage to species survival.
Unlike self-pollination, there is basically only one form of cross-pollination (allogamy for you Trivial Pursuit enthusiasts). The object of the game is to get the pollen produced by the anther of a male flower on one plant to the stigma of a female flower on another plant of the same or similar species. This occurs with the successful confluence of either biotic factors or abiotic players assist in the pollen transfer.
Birds, bats, insects, spiders, animals and any other living species can directly aid in the pollination by going from flower-to-flower in search of a meal, or may transfer the pollen inadvertently by depositing pollen unknowingly as they go about their usual business in the proximity of fertile flowers. Wind and water are also responsible for about 10 per cent of pollination; however, most is performed by living species.
Cross-pollination offers the advantages to a plant species of healthier offspring in general as compared to self-pollinating species. New varieties of plant species within plant genera can be and have been created by cross-pollination. Plants that reproduce by cross-pollination tend to produce larger quantities of seed as well.
There are of course some downsides to reproduction by this method. Unlike with self-pollination, reproduction is uncertain. Both sides can be ready, willing and able to reproduce but without the conduit provided by pollinators, reproduction will not occur.
Plants that cross-pollinate also use a tremendous amount of their energy stores in the production of pollen and in the creation of large, showy, fragrant flowers that are meant to attract their unknowing accomplices.
Tip: It is possible to intervene in the plant pollination process by manually helping plants along. By using a small brush or cotton swab, growers can hand-pollinate their crops. Check out The Birds & The Bees: How to Pollinate Plants to learn how this can be done.