The Cleistogamous Flowers of Collomia grandiflora

Fig. 1. Two photographs of Collomia grandiflora by Bruce Watts, showing the tiny cleistogamous flowers at the top of the flowering head, and the large showy chasmogamous flowers at the sides of the flowering head. Click on the photographs for larger versions.

Have you ever wondered why nearly all photographs of Collomia grandiflora only show a ring of open flowers well below the top of the flower head, and you never see a photograph showing just a few flowers at the top of the flower head?

There's a good reason for that! C. grandiflora usually has two types of flowers in its terminal head: very tiny closed, self-pollinating (cleistogamous) flowers at the top; and open, potentially outcrossing (chasmogamous) flowers at the sides. Most photographs show the absence of showy flowers at the top, but only a few also show the tips of the small closed flowers, such as the photographs in Fig. 1. See also photographs from iNaturalist showing the tiny corollas of the cleistogamous flowers: from Fred Melgert and Carla Hoegen; and from Karen Parke.

Fig. 2 shows a close-up photograph of a cleistogamous flower with the corolla both in situ, and detached from the rest of the flower.

Fig. 2. The inside of a cleistogamous flower, with the calyx split open and spread apart. Left: intact flower. Right: corolla detached from ovary. The corolla is about 1.5 mm long, and the entire calyx is about 7 mm long, the same size as the calyx surrounding the large showy flowers. Photographs by Tom Chester.

In addition, if you look closely at a population of C. grandiflora, you may be able to spot smaller plants with a single terminal head of just three to seven flowers that are all cleistogamous! See Fig. 3. Look for those plants in parts of the habitat less favorable to the development of a robust plant, such as shadier areas; areas with poorer soil; or more crowded areas. See also funastrum's iNaturalist observation of such a plant.

Fig. 3. The terminal inflorescence for two smaller plants of C. grandiflora, both in "full bloom" with closed flowers, ~3 flowers (left) and ~7 flowers (right). Photographs by Tom Chester.

C. grandiflora is one of the best studied of the ~42% of plants that have both types of flowers (Goodwillie et al 2005 Annu Rev Ecol Evol Syst 36:47-79). Producing both types of flowers is a way to hedge your flower bets. The cleistogamous flowers guarantee that some seed will be produced, but might eventually result in in-breeding depression. The chasmogamous flowers allow sexual reproduction when pollinators are available.

Ellstrand et al 1984 (Bot Gaz 145:329-333) found that in the apical head of a typical plant, the top six flowers were cleistogamous; flowers seven to 14 were mixed; and only chasmogamous flowers were produced below that. (The flowers are spiraled around the inflorescence axis, so it is easy to identify a flower by its position.) In their greenhouse studies, they found that a typical plant produces ~30 lateral branches below the apical cluster, for a total of ~300 flowers, and most of those flowers are cleistogamous. They found that the corolla length for cleistogamous flowers ranged from 2 to 7 mm; and the corolla length for chasmogamous flowers ranged from 14 to 24 mm.

Btw, C. grandiflora is an example of a cymose inflorescence, in which the top flower opens first, followed by flowers opening successively lower in the head.

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Copyright © 2020 by Tom Chester and Bruce Watts
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Last update: 18 June 2020