Cleistogamy

​Most flowers go to great lengths to produce nectar, fragrance, and brilliantly colored petals to wantonly attract every passing pollinator. We call this behavior chasmogamy, or “open marriage”.

Monkeyflower (ghislain118) CC BY-SA 3.0

But some plants are more modest than that. They prefer their own company to that of the masses. These private individuals have evolved a unique method of self-pollination and self-fertilization called cleistogamy [klīˈstäɡəmē]. Cleistogamy is a “closed marriage” type of pollination that occurs inside a flower. These flowers only open after pollination and fertilization have occurred, if at all.

Cleistogamy is believed to have evolved in regions with harsh conditions and fewer resources. Producing petals, fragrances, and nectar is hard work for a plant. Rather than going to all the trouble to create these attractants, cleistogamous plants pollinate themselves in a highly protected process.

Most members of the Viola genus use cleistogamy, but so do barley, beans, peanuts, and peas, to one degree or another. But different plants achieve cleistogamy in different ways. And scientists are still sorting out all the things we are learning about plants.

Using the strictest definition of cleistogamy, we have flowers that pollinate themselves and never open. In this case, the sepals are often partially or completely fused, preventing the flower from opening. Each enclosed flower simply produces its own pollen which falls on the stigma and the plant pollinates and fertilizes itself. Orchids and several grass family plants use complete cleistogamy. This method is different from the normal double-fertilization that most angiosperms use to produce seeds. It is self-sufficient in the extreme, but the downside to this method is reduced genetic diversity. Complete cleistogamy can occur above ground or underground, depending on the plant. When it happens underground, as in the case of peanuts, it is called geocarpy. ​

Peanuts send their flowers underground (Kate Russell)

Other plants use a combination of cleistogamous (closed) and chasmogamous (open) methods. This group is further divided into three categories: dimorphic, induced, and preanthesis. [The word preanthesis refers to before a flower opens.] If the plant is predetermined to develop into both chasmogamous and cleistogamous flowers, we called it dimorphic cleistogamy. Very often, dimorphic cleistogamous flowers occur at different times or in different locations on the plant. American hog-peanuts (Amphicarpaea bracteata) and Canada violets (Viola canadensis) produce seeds using dimorphic cleistogamy.

American hog-peanut (Eric Hunt) SS BY-SA 4.0

If the end result is not predetermined, we call it induced cleistogamy. Flowers in this category are no different from their more open-minded stem-mates except that they do not open. Many fescues and some Impatiens are in this group. They can switch from the normal, social method of seed production to the more protected method, often after being damaged by herbivores.

Preanthesis cleistogamy is a unique reproductive method in which self-pollination occurs first within the closed bud, the bud opens, and then external pollination becomes possible. Monkeyflower (Mimulus nasutus) is one example of preanthesis cleistogamy.

Knowing if a plant is a self-pollinating or cross-pollinating variety can make the difference between having a harvest or not. The difference between cleistogamy and other forms of pollination may not be as important to your crop, but it’s a good reminder about how diverse and amazing plants can be.