Sex life of plants (part 1)

 The sex life of plants is important to gardeners because it is at the heart of what our gardens are about — flowers, fruit and diversity.

The ultimate thing to understand about plants and sex is that plants are rooted. From this predicament stems the many strange variations of plant sex. Imagine you are blind, stuck in the ground, having to procreate with other individuals blind, stuck in the ground and possibly miles away. This is no simple boy-meets-girl scenario.

Plants generally need an accomplice and a cunning plan to perform the act. It is these accomplices rather than their prospective “mates” that are the focus of most plants' sexual displays and strategies. The sex life of plants is quite mercenary, it is all about rewards and payment for services rendered. Sometimes, as with certain orchids, the payment is in forged notes.

We can thank Federico Delpino for the concept of pollination syndrome that describes the various go-betweens used by plants to get fertilised. Federico came up with a suite of fabulous words to drop at your next garden soirée.

I think ‘psychophily’ tops the list, would you have guessed butterfly pollination? ‘Anemophily’ is wind pollination. Plants that employ this strategy along with those that adopt ‘hydrophily’ (water pollination) don't have to waste resources on wooing an insensitive element. No need for bright flowers or sweet scent. They do have filaments that catch the wind; think corn silks and lots and lots of pollen. Anemophily is the bane of the asthmatic; avoid these plants if you suffer allergies.

Other plants use animal vectors and they have to try a little harder to “put themselves out there”, offering rewards of nectar, oils or even perceived sexual gratification. We are talking phalaenophily (moths), myophily/sapromyophily (flies), cantharophily (beetles), chiropterophily (bats), ornithophily (birds) and melittophily (bees).

Out of the vast array of possible food crops we only exploit a tiny percent as staples. We seem to have domesticated the plants with the least complicated and most generalist sex lives.

Wind pollinated grasses and fruit pollinated by the, until recently, ever reliable bee dominate. Basically farmers don't want plants reliant on very specific vectors for sex. You can't easily produce a crop or transfer seeds around if you are having to also wrangle a rare wasp or beetle.

For gardeners, describing the sex parts of plants gives us a language to talk about the structure of flowers. It is what plant taxonomists examine to categorise and describe plant species. Sex is confusing especially when botanist use terms like: ‘Androgynomonoecious’. Like any good sex education 101 class, I find it best to start with a simple diagram like the one below.

Man bits

Stamens are the male parts of a flower, they come in all shapes and sizes. Next time you run into a hanging basket at grandma’s have a second look at a fuchsia and admire its relatively huge stamens thrusting out from its skirt. At the tip of the stamen's filament is the anther that produces and holds the pollen.  The structure of pollen is unique for each species.  It is a key that allows plants to accept only pollen from their own species.  Although plants are sometimes less choosy and that is how interspecies, even intergenera, hybrids can occur.

Lady bits

The female parts of a flower tend to take centre stage with the male bits arranged at the periphery. Combined, the female parts of a flower are called the pistil.

This is basically composed of:

• The stigma that receives the pollen and defends against pollen from other species.
• The style is the tube that leads to the ovary. Pollen does not travel down this tube but rather thrusts a pollen tube through it to the ovary.
• The ovary is where the fertilisation occurs. Within are arranged the eggs or ovules. There may be one or many and there arrangement varies from the single avocado pip to the row of peas in pod. As the ovules are fertilised and develop the male bits and the bright petals wither and fall away.

Other paraphernalia

Accompanying the sex parts of flowers are the structures of their ad campaign. Petals brightly coloured, sign posted with landing strips or in the case of the bee orchid (Ophrys apifera) shaped like a female bee. Other petal like structures are the tepals that we see around tulip flowers and bracts that surround bougainvillea flowers. The other enticement and potential trap flowers offer is the nectary that offers sustenance in the form of nectar or sometimes oils.

Now to confuse things the classic flower diagram with male and female parts is of only one type of flower — the “perfect” flower. And let me tell you that perfect flower is a bisexual hermaphrodite. The morphology of plant sex is rainbow coloured. At the most basic there are monoecious plants with male and female flowers on the same plant and dioecious plants with male and female on different plants.

Beyond this there are those with male, female and bisexual flowers on the same plant and those that change their sex throughout their life. All this matters to the gardener because you may find either the male or female flower more attractive. The male flowers of the dioecious Garrya elliptica are longer and considered more attractive than the female tassels. When seeking fruit from a dioecious species like kiwi fruit it is essential that you have a Mr and Mrs Hayward.

A species being dioecious largely evolved to promote outbreeding. The genetic advantages of outbreeding, in preventing degenerative mutation and promoting diversity and adaptability, are fairly clear. In human society we have the incest taboo. In the amoral world of plants evolution has produced a set of mechanisms that limit self-fertilisation. Some use the timing of the opening and maturing of the male and female parts or flowers to limit selfing.

The bee pollinated foxglove has an elegant method. Bees are attracted to the tubular flowers whose spots provide landing beacons. Bees work their way up the stem from the fully opened lower flowers to the progressively closed newer flowers at the top, then fly off to the next flower spike. In the flowers the stamens open first revealing pollen. As the flower ages it opens further, the stamen withers and the stigma becomes prominent. So as the bee travels up the spike it starts in the fully open “female” flowers, progresses through the “males” at the top then flies off laden with pollen to deposit on the “females” at the bottom of the next spike.

“I don’t want anybody else” wailed Chrissy Amphlett and a small group of plants who, because of their environment or lack of vectors, have to take care of business themselves. While an easy option, self-fertilising within a single flower has ramifications that lead to reduced genetic diversity, limiting a species adaptation to change. Some flowers, like sweet violet, hedge their bets and produce two sorts of flowers. You will have noticed this. An early spring flush of dull, closed, selfing flowers that bloom below the leaves. Then a showier blooming that sticks above the leaves once warmer weather brings bees into the picture. To make the most of the earlier blooming I have cut back violet leaves before flower buds form to reveal the flowers below for an early display.

The environmental factors can skew the sex of plant progeny. In saltbushes (Atriplex sp.) female plants dominate on more fertile alluvial sites and more male plants are produced toughing it out on drier slopes. The advantage is obvious, more seed produced to fall on fertile ground.

Hopefully this rough sketch of plant sex has whetted your appetite to delve into more research of your own. Although be careful what your search engine brings up. Even a basic understanding gives us tools to describe and identify the plants in our care. And perhaps to see them not as inert ingredients we use to make a garden but as living bizarre beings in their own right.