Plant reproduction can be a bit of a mystery. Until about 1700, it was thought that plants simply sprouted seeds that would grow into clones of themselves. Now, we know that plants can undergo both sexual and asexual reproduction – that is, they are capable of both creating ‘clones’ of themselves and reproducing with another individual.
You may understand pollination as a broad concept, but are you familiar with the many different types of flowers and pollination out there? Using invasive species as examples, we’ll walk through some of the more complex aspects of plant reproduction. This blog is part of a series of posts called How Do Plants Work?
Plant Reproductive Organs
Before diving into the world of pollination, we need to go over plant reproductive organs. Like humans, plants have male and female gametophytes (i.e. egg and sperm), but in this case, it’s pollen and ovule. In humans, one individual typically produces one of the gametophytes and must find an individual that produces the other gametophyte in order to reproduce. In plants, it can be more of a game of mix-and-match.
In some plant species, gametophytes work the same way humans do. One individual produces one type of gametophyte, another individual produces the other; those plant are dioecious. In other species of plants, one individual can produce both types of gametophyte; they are monoecious. Stranger still is that this concept also applies to flowers.
Some plants have flowers which produce either male or female gametophytes, and some plants have flowers which produce both male and female gametophytes. Flowers which produce both gametophytes are called perfect flowers because they contain everything needed to reproduce, while flowers that produce only one gametophyte are called bisexual or hermaphroditic flowers.
Dioecious plants can only have hermaphroditic flowers, as one individual can only produce one of the gametes. Monoecious plants, on the other hand, can have either perfect or hermaphroditic flowers, as one individual can produce either or both gametes in its flowers.
The male gametophytes, or pollen, is produced and stored in the stamens. The stamens are comprised of the filament and the anther and generally form a whorl just within the petals. The female gametophyte, or ovule, is found in the ovary which is part of the pistil. The pistil’s tip, called the stigma, is where pollen is deposited. The pistil, if present, is always found in the center of the flower.
Pollination
Pollination can occur in a variety of ways, but the overall goal of the process is always to have pollen transported to a pistil in order to fertilize the ovule. There are two main types of pollination: self-pollination and cross-pollination.
Self-pollination occurs when the pollen and the ovule came from the same flower. In plants with perfect flowers, a pollinator isn’t even needed to transport the pollen – it can fall right onto its own pistil! This method of pollination is quick, easy, and reliable. However, using your own pollen to fertilize your flowers results in new plants that are essentially clones of the parent plant. This leads to decreased genetic diversity within the species and can lead to widespread disease and loss of resilience.
Mountain Bluet is an example of a plant that self-pollinates. Each of its flowers contains both male and female gametophytes. This ensures that it will always be able to produce viable seeds, and allows the plant to spread prolifically.
Cross-pollination occurs when pollen from one individual travels to the stamen of another flower. This method, while energy-intensive and unreliable, leads to new plants that are a mix of both of their parents’ genetics. This makes them more resilient and increases the species’ ability to evolve with changing environments. However, this method is more energy-intensive because the plant must either make itself attractive to a live pollinator or adapt to other methods of pollination.
For example, English Holly is dioecious. so it must male and female plants are needed in close proximity in order to reproduce. Because of this, these plants must always cross-pollinate. English Holly is extremely pest-resistant and has developed thick, leathery, spiky leaves that are unattractive to wildlife. These adaptations are likely due to the amount of genetic variation found in the population.
Many species have methods in place to prevent self-pollination unless it’s absolutely necessary. Some perfect flowers space out the timing of when each gametophyte matures. For example, they may release their pollen before the female gametophyte is mature enough to accept it. Other plants space apart the different gametophytes within the flower itself.
In these cases, some flowers have very long stamen and short styles, and some have very short stamen and long styles in order to switch which part of the pollinator the pollen is deposited on, and which part of the pollinator the style will touch. This is called heterostyly.
Many plants are simply self-incompatible. These plants are incapable of self-pollination: their flowers can recognize their own pollen from other individuals’, and destroy it if it comes from themselves. In this way, they only allow fertilization to occur if the pollen is genetically different from the ovule, which increases the genetic diversity of the population.
The method that a cross-pollinating plant uses in order to get pollen from stamen to pistil is called its pollination vector. Pollination vectors can be animals (bees, beetles, flies, birds, bats, mammals, lizards, etc.), wind, water, or soil movement. Plants tend to look a certain way to either attract their preferred pollinator or make their seeds more suitable for travel by wind or water. Here are some examples of invasive species’ pollination vectors:
Bee pollination
The flowers of Common Tansy are designed to attract bees. Bees prefer yellow or blue flowers that are strongly aromatic and have a good landing platform. Despite their lack of petals, The flat clusters of flowers provide ample landing space for bees.
Bird pollination
Butterfly Bush has long, tubular flowers full of nectar. These flowers attract both hummingbirds and butterflies, as well as a few other pollinators. However, despite their name, they do not support all parts of butterflies’ life cycles, and larvae cannot survive on them. Hummingbirds and butterflies prefer flowers that are pink, purple, or red, and butterflies require a landing platform, while hummingbirds do not.
Fly pollination
Milk Thistle flowers attract a variety of pollinators, including several species of flies. Flies tend to prefer visiting flowers that are purple, blue, or white and either have no odour or a nasty odour (like rotten meat). The purple or white, faintly-scented flowers of Milk Thistle are perfect for flies to pollinate.
Wind pollination
The flowers of Tree of Heaven are pale yellow-green and organized in large clusters. These plants put no energy into making their flowers attractive to pollinators. Instead, they’re designed to allow wind to carry their pollen from one tree to another. In order to increase the chances of their pollen finding another tree, they release copious amounts into the air.
Fruit Development
Following fertilization, the ovules grow into seeds within the ovary, which then develops into a fruit. Technically speaking, any structure that encloses a seed is a fruit. Fruits can be fleshy, like many of the common fruits we think of (apples, grapes, pears), or dry, like nuts and legumes.
Fruit types vary wildly, but their purpose is always to help protect and distribute the seeds. Some fruits are meant to be tasty to attract wildlife and hitch a ride in their stomachs. Other fruits are meant to be dry, to spring open and fling seeds long distances, or to simply protect them from the elements. Here are some examples of fruits seen in invasive species. This doesn’t even come close to a full list of all of the different types of fruit. Can you believe that bananas are actually berries?!
Legume
After the bright yellow flowers bloom, Scotch Broom becomes covered in green pea pod-like fruits that eventually dry out and turn black. After drying, they split open and catapult the seeds inside away from the parent plant. Legumes by definition are pods containing seeds which eventually split along a line and release their seeds.
Achene
Canada Thistle flowers, after blooming, dry out and become puffballs, similar to dandelions. Each fruit, an achene, is attached to a tuft of bristles that will catch the wind and blow them far from the parent plant. Achenes are dry fruits that contain only one seed and don’t split open.
Drupe
Himalayan Blackberries are technically aggregates of drupelets (a cluster of small drupes). Each little bead of the blackberry is a drupe – a fleshy fruit surrounding a seed or stone. These dark berries are delicious and are eaten by all sorts of wildlife. This spreads them with the wildlife far away from the parent plant.
Schizocarp
Wild Parsnip’s small yellow flowers mature into schizocarps, or small fruits that will eventually split apart into smaller segments, each with one seed inside. In this case, they will only separate into two segments. This allows Wild Parsnip to put less energy into creating fruits, but still spread them as though they were individual seeds.
References
- Taiz, L., Zeiger, E., Møller, I. M., & Murphy, A. S. (2015). Plant Physiology and Development (6th ed.). Sinauer Associates Inc.
You might also like…
Species mentioned in this post
Add Comment