Looking For a Mate – Some Plants Just Change Their Sex

Some Plants Just Face Big, Big Challenges When Looking for Romance!

The plant world has some amazing ways to make sure that progeny are successfully reproduced. One of the most interesting ways of overcoming the challenge of not having the right mate nearby is solved when some plants change their sexual orientation as the need arises. This small group of plants have a very liberating “keep all options open” way of propagating themselves. Still others overcome the big challenges of finding a suitable plant mate by excreting toxic substances to keep their unwanted suitors away. Finding the right mate can be downright challenging for some among the plant kingdom.

The most novel solution to the elusive mate syndrome has been solved by plants like the saltbush.  If the opposite sex is not nearby, they just simply change sexes and continue with their propagation process. The sexually labile, Four-Wing Saltbush (Atriplex canescens) is a perennial that can amazingly change from one sex to the other at will. This plant, native to western Texas, stands a little over three feet (one metre) tall and has attractive winged fruits. Historically, the seeds were ground and used by Native Americans as baking powder when making bread.

Hollies like this Yaupon need both a male and female plant in close proximity to produce fruit.

Hollies like this Yaupon need both a male and female plant in close proximity to produce fruit.

When cold temperatures or drought make it tough for the saltbush to find the right mate – they use their ability to change sexes. Usually this change is from female to the male sexual orientation. This unusual “switcheroo” greatly increases the odds of reproducing the species by putting more male pollen into the air. The more pollen that is spread to the wind, the better the chances of producing seed. Spreading copious amounts of pollen in the wind may be romantic for plants, but it is often very unromantic for humans. An outdoor stroll for an allergy-prone person while sneezing and peering through watery eyes is not the scene that romance novels portray. When too much wind-borne pollen is in the air blame the over-exuberant male plants.

Another group of plants have to deal with the challenges of being designated male or female in a world that makes it far easier to reproduce by having both sexes in each flower. These unusual “one sex” plants are faced with the same “finding the right partner” challenges inherent in the human world. Called dioecious plants, they must have a member of the opposite sex nearby to produce viable fruit. Imagine being a dioecious plant and not being about to move in order to look for a partner of the opposite sex.

Literally the Greek word “dioecious” means “two in separate houses.”  Dioecious hollies (Ilex) are grown for their attractive fruit and should be planted in proportions of more females to males for the best show. If space is an issue in the garden, fewer males than females are needed to provide good pollination. The ratio is usually about one male for every eight female holly shrubs planted nearby. Nursery growers have made the task of determining sexes of hollies easier by giving some of their cultivars male and female gender based names – such as China Girl (I. ‘Mesog’) and China Boy (I. ‘Mesdob’) or Blue Prince (I. x meserveae cv.) and Blue Princess (I. x meserveae ‘Conapri’).

Success! Osage Orange Fruit

Success! Osage Orange Fruit

Osage orange is another plant that is dioecious and has male and female flowers on separate trees. The female trees have small green flowers that open in June, just after the leaves unfold. The male flowers are on terminal leaf spurs of the previous season’s growth and are 1 inch (2.5 centimetres) long in a raceme shape. Their pollen is light and travels with the wind. If a female tree is not within pollen range, they will not produce viable fruit. Female trees may look like they are producing their cannonball-like fruit well, but there may not be many viable seeds inside.

Other plants that have separate male and female plants are the tree of heaven, sago palm, persimmon, and many poplars. Even some herbaceous plants have this unusual arrangement. Spinach and asparagus have male and female flowers on separate plants, but they are both equally edible. For this reason they are grown from seed and no attention is made to their gender unless gardeners want to do some propagation at home and save their own non-hybrid seed.

Dioecious plants have even played a part in ancient Arabic wars. The date palm, a valuable agricultural crop in Arab countries has both male and female plants. Long ago fighting tribes would sneak into an enemy’s grove and destroy all the male trees. Without male pollen for fertilization no crop would be produced. This method of destruction, removing the less numerous male trees, was far less work than damaging the numerous female date palm trees.

Ginkgo Leaves

Ginkgo Leaves

The separation of plant sexes does have some advantages for gardeners, though. One of the slower plants to reach maturity, the ginkgo or maidenhair tree takes more than twenty years before it flowers. Since this tree is believed to be from the dinosaur era which dates back 270 million years, waiting for it to bloom so the sex can be determined is not that long in planet Earth time, but very long in a gardener’s time (about 20 years). The sex of a ginkgo tree is important to know because female trees have plum-like fruit that turn yellow when they are ripe. It’s the ripe flesh on the outside of the seed that is undesirable to many people because it contains butanoic acid and smells like vomit or rancid butter. There’s nothing romantic about a ripe ginkgo fruit smell. For plants that have objectionable smelling fruit like the ginkgo, growing only males means that human olfactory senses are not under assault when the female produces fruit. The one big disadvantage for the ginkgo is that it is almost impossible to tell the difference between male and female plants until they bloom – and then only under a microscope. Therefore gardeners must trust nursery labels, purchase a male clone variety or plant several of each sex in order to ensure that they have good odds that one of each is present.

The love for a ginkgo tree comes from its other features. Running a finger over the ridges of the fan-shaped leaf, looking at the divergent branching outline against sky, putting a leaf in a book to press (or finding one in an old book), or walking through the brilliant yellow fallen leaves can all conjure up sentimental feelings for this ancient tree. Who needs red roses and chocolates to feel romance and love? All it takes is a ginkgo tree (and a ginkgo may sometimes last a lot longer too).

 

Camouflaged and deadly offensive ginkgo fruit among the recently fallen ginkgo leaves.

Camouflaged and deadly offensive ginkgo fruit among the recently fallen ginkgo leaves.

Another obstacle to plant propagation occurs in the fruit tree world. Often more than one variety of a fruit tree is needed to really produce good fruit. This happens not because they are dioecious but because they are self-unfruitful. Many apples, pears and plums require pollen from another variety to have adequate pollination and good fruiting. Since these fruit trees produce pollen that is transported by winged insects other problems also come into play. The weather must be nice enough for bees to forage and the corresponding pollinating varieties must also be in bloom at the same time. To help overcome these problems, some nurseries are producing dwarf apple trees that have several varieties grafted on one plant. Now bees need only travel a few feet to perform their cross-pollination. This also solves the need to buy multiple plants if garden space is limited.

Some plants solve their reproduction challenges by chasing other plants away. Label this group of plants “anti-social” and think of them as being destined to have a solitary life without the company of others. Most of these plants are in the walnut family. This group poisons the soil around themselves so other plants stay away. Walnuts secrete a substance called juglone from their roots, decomposing leaves, and from the husks that surround their fruit. The secretion is toxic to the majority of the temperate plant world. When mature, their far-reaching roots create a neighbor-free zone that could double or triple the canopy area of the walnut. This is not a tree that is friendly to others. Luckily the fruit that it produces is highly prized by small, mobile animals that will transport it to remote locations and new communities.

Ginkgo images originally published by Wikipedia at http://en.wikipedia.org/wiki/Ginkgo_biloba and used under the Creative Commons Attribution/Share-Alike license.
Osage Orange image originally published by Wikipedia at http://en.wikipedia.org/wiki/Maclura_pomifera and used under the Creative Commons Attribution/Share-Alike license.

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