Monthly Archives: September 2014

Pollination: A Sampler

If it is axiomatic that nature will allow or support “whatever works,” it is our observation that many, many different things “work” in nature. The wide diversity in floral structures and pollination strategies exemplify this propensity for variety.There are endless variations on the basic story of pollen grains making their way to the stigma of an appropriate pistil, and many are easy to see if you look.

Variation one
Go out in late winter and check out the elongating catkins of beaked hazel (Corylus cornuta) and its non-native cousin the common filbert (C. avellana). The natives tend to be more conservative, flowering later than the new arrivals, but the yellow ripe catkins of both are highly visible when deciduous plants around them are bare.

catkins

Photo by Ben Legler

In these species the vector is the wind, and the plants make use of the unobstructed passage of air before the leaves emerge. Tap a ripe catkin, and you might see a puff of pollen drift off. The female flowers, more demure but beautiful when you can find them, are on the same plants. Look for buds with clusters of tiny terminal red styles.
We call plants with separate male and female flowers on the same plant monoecious, which means “one house.”

Variation two
The willows (Salix spp.) are similar in having separate male and female flowers, though in this case they both occur in catkins and grow on separate plants; we call them dioecious (“two houses”).

Take a close look at a pussy willow in the spring, and look for either bunches of anthers or bunches of spindle-shaped pistils. Willows straddle the line between insect pollination and wind pollination, with documentation of both.

Another common dioecious plant is the tall shrub osoberry or Indian plum (Oemleria cerasiformis), one of the first insect-pollinated species to emerge in spring. Both male and female plants have clusters of small white flowers, but you have to look closely, preferably with a hand lens, to see stamens and pistils.

Or, when you can see remnants of last year’s fruiting clusters, or the miniature plums of the current year, you know that the plant is female, since only they bear fruit. Female plants appear to be less common than males.

trailing blackberry

Rubus ursinus, trailing blackberry. Photo by G.D. Carr

Trailing blackberry (Rubus ursinus) has the same trait, with only female plants producing berries. Maybe it has something to do with supply and demand; this is the only dioecious blackberry, and the most flavorful, in my opinion.

Technically we would call this “functionally” dioecious, because the male plants still have vestigial (non-functioning) pistils, and the female plants have vestigial stamens. Take a hand lens and see for yourself; it may inform where you look for berries.

Variation three
A last example of variation in pollination strategy is the dandelion (Taraxacum officinale), which does not include pollination in its reproductive strategy at all. This is surprising; it has showy flowers (any three-year old will attest) and produces abundant pollen.

seed head of dandelion

Seed head of Taraxacum officinale, dandelion. Photo by Ben Legler.

But our understanding is that dandelions have acquired the ability to produce seeds that are clones of the parental plant. We call this strategy agamospermy, which literally means “seeds without gametes” (egg and sperm cells). Apparently, in weedy situations, the benefit of maintaining the genetic characteristics of the parent may be greater than the opportunity for improvement that sexual reproduction offers.

These are just a few glimpses into variation in the pollination strategies employed by flowering plants.

Reprinted, with permission, from Douglasia