Plant reproduction
The flower and flower function
A complete flower has four types of parts.
- The sepals are on the outside of the flower. Together the sepals are called the "calyx". The sepals protect the developing flower in the bud.
- The petals are the next set of parts as you move toward the inside of the flower. Together, the petals are called the "corolla". The petals are usually brightly colored and serve to attract pollinators (animals that will carry pollen from one flower to another).
- The stamens are the next set of parts as you move toward the inside of the flower. They consist of two parts: the anthers, which produce the pollen, and the filaments (the stalks that hold up the anthers).
- The pistil(s) are the inner-most set of structures. We recognize three parts of a pistil: the stigma (the sticky tip, where the pollen lands), the style, and the ovary (the inflated part at the base that contains "ovules", discussed below). A flower often only has one pistil, but some species have several pistils in one flower.
[Variation in flower structure will be covered in greater detail in a later lesson.]
The pollen grain is a small multi-cellular structure that carries the sperm of the plant.
The ovules, little balls inside an ovary of a pistil, are very complex little structures that contain eggs.
Pollination is the transfer of the pollen grain from the anther of a flower to the stigma of a pistil.
Fertilization is the joining of the sperm (carried by the pollen grain) to an egg (contained in an ovule). To get the sperm from the pollen grain on a stigma to an egg in an ovule, a pollen grain grows a long pollen tube down the style of a pistil until it reaches an ovule, where it delivers sperm to the egg in the ovule.
Pollination
Plants often use animals to pollinate their flowers. Some species do not. Oak trees and grass species are examples of plants whose flowers do not have colorful petals (or any petals), and rely on wind to carry their pollen.
Some plants (not all) have traits that encourage transfer of pollen from one flower to another (rather from the anthers of a flower to the pistil of the same flower).
In some cases, the pistil and anthers mature at different times, preventing self-pollination.
In some cases, the flower is constructed to avoid pollinating its own flower. Video clips of changes in a passion flower and a discussion of how these changes discourage self-pollination may be viewed on the Plants in Motion website: http://plantsinmotion.bio.indiana.edu/plantmotion/flowers/passionflower/passion.html
Monkey flowers, many of which are native to southern California, have flat, sensitive stigmas that close after being touched. Below is a video clip of this phenomenon from YouTube (http://youtu.be/O2RPIMoz7Mg). Speculate on how this action affects self-pollination.
Not all flowers prevent self-fertilization. Sometimes self-fertilization is beneficial, allowing a plant to produce seed even when it is the only member of its species within a hundred miles.
Some plant species (not all) require very specific pollinators to carry their pollen from one plant to another. This can create major conservation problems and/or economic problems when pollinators go extinct or disappear from an area. Hand-pollination is a common practice for increasing seed production of rare plant species, especially ones with low or no seed production in the wild.
Fruits and seeds
After the egg in an ovule is fertilized...
...the ovule eventually becomes a seed,
...the fertilized egg [the zygote, a term you might not use] grows into the embryo (baby plant) inside the seed, and
...the ovary that contained the ovule(s) becomes the fruit that contains the seed(s)
Fruit types and seed dispersal
In general, the function of fruits is to help disperse seeds (spread them away from the parent plant).
[Reflect: Compare a plant that disperses its seed away from itself to a plant that does not, and drops its seed directly at the base of its stem. Which plant's offspring would likely grow best? Which plant's descendents would be more widespread after a few generations?]
Fruits may be sweet and juicy, and attract animals to eat the fruit and scatter the seed.
Fruits may be dry and prickly or sticky, and use animals for dispersal by sticking to their fur or socks.
Fruits may mechanically eject seeds, squirting them or flinging them as the fruit splits.
Some fruits have wings or parachute-like structures to help them sail away on the wind.
[Note that the seeds, themselves, may have barbs, parachutes, wings, fleshy attachments, and other features that help in their own dispersal. You should be able to distinguish a fruit from a seed so that if a bright student asks "So where is the fruit on this plant?", you know.]
Videos of squirting cucumbers, exposive dehiscence, wind dispersal, and other methods of dispersal are available on the web. Examples:
http://www.gettyimages.com/detail/video/high-speed-exploding-himalayan-balsam-pods-stock-video-footage/143278384
Although there are many technical terms for different types of fruits, you should recognize two categories of fruits:
(1) fruits that contain several seeds and split to release those seeds when the seeds are mature, vs.
(2) fruits that contain only one seed and do not release the seed. Species that have this type of fruit disperse the entire fruit, rather than dispersing the seeds from the fruit. Acorns, sunflower "seeds", and grass "seeds" are examples of this type of fruit. When the seeds of these species germinate, the embryo has to grow through both the seed coat and the fruit wall.
[Different types of fruits will be discussed in greater detail in a later lesson.]
What is in a seed?
The outer layer of the seed is called the seed coat.
Inside the seed is an embryo and a food supply for the embryo. Good diagrams of the parts of a seed and the parts of the embryo can be found in the Arizona Master Gardner's Manual (below; at http://ag.arizona.edu/pubs/garden/mg/botany/seeds.html). The embryo has a young root (often called a "radicle", a shoot tip with young leaves (often called a "plumule", because it looks like a bunch of little feathers or plumes), one or two cotyledons (also called seed leaves). The section of the seedling axis just below the cotyledons is called the "hypocotyl" ("hypo" meaning "below'), and the section of the seedling axis just above the seedling axis is called the "epicotyl" ("epi" meaning "on top of"). New roots and new shoots with leaves will be produced throughout the life of the plant. However, the cotyledons, the epicotyl, and the hypocotyl are only produced once: in the embryo.
Note that in some seeds, like the seeds of beans and peas, there is no endosperm in a mature seed. During development, the embryo has absorbed all of the "food", or endosperm, into its body and stored it in its cotyledons (or "seed leaves"). Other seeds, like the pepper and tobacco seeds still have a food supply outside the embryo when the seed is mature.
Seed germination
When a seed absorbs water, it may germinate. Seeds also need favorable temperatures and oxygen to germinate; they need energy to grow, and they burn a lot of "food" to get that energy. (You can say that seeds sprout, but using the term "sprout" to refer to germination can get confusing: we also say that stems resprout from the top of the root when talking about the way some plants recover from fire, we talk about potatoes sprouting, and we talk about new branches sprouting from the trunk of a tree, etc. If you are discussing different ways plants react to the environment, using a general term like "sprout" to refer to different processes can really confuse the discussion.)
Some seeds also need light to germinate; some require darkness. Exposure to light tells a seed it is close to the soil surface. Being in the dark tells the seed it is well buried. Some plants need other treatments or signals from the environment before they will germinate. Some plants need several weeks of cold, moist conditons; some need exposure to smoke; some need exposure to heat, and some need exposure to either acid or something abrasive that weakens their seed coat. Most of these signals tell a seed that it is now in a time or place that is favorable for seedling survival. Having experienced cold, moist conditions tells the seed that it has gone through a winter, and it is probably spring. Exposure to smoke or heat tells the seed that their has been a fire, and that most of the large, competing, adult plants aren't so large and competitive anymore. Exposure to acid tells the seed that it has passed through the gut of an animal, and that it has probably now been dispersed well away from the parent plant. (Stomach acid is very acidic...it "burns" when you throw up.) Seeds that require acid, heat, or abrasion to germinate have hard seed coats that prevent the seed from absorbing water until they are weakened.
All seeds need water, oxygen, and favorable temperatures to germinate. Those that require special, additional conditions to germinate may lie dormant, or sleeping, in the soil until they receive those cues. Some species do not produced dormant seeds, some produce seeds that can lie dormant for 2-3 years before they die, and others produce seeds that can lie dormant for many decades before they die.
Seed germination begins with the seed absorbing water, swelling, and growing its root (called the "radicle" in the embryo) out into the soil. Next, either the hypocotyl (the section of the seedling axis under the cotyledons), or the epicotyl (the section of the seedling axis above the cotyledons) elongates. What happens to the cotyledons when this happens?
After epicotyl or hypocotyl elongation, the root and shoot grow from the tips, as they do in an adult plant.
One of the videos from the Plants in Motion website, shows corn germinating. In corn, the cotyledon remains below ground, in the seed coat, absorbing endosperm from the seed as the seedling germinates and grows. (http://plantsinmotion.bio.indiana.edu/plantmotion/earlygrowth/germination/corn/corngerm.html)
Another video from the Plants in Motion website shows sunflower seeds germinating. In this species, the hypocotyl elongates, boosting the cotyledons above ground. The tall stalk of the seedling you see in this video is the hypocotyl, and the "leaves" are really the cotyledons. The stem tip and the leaves is produces is very small at this stage, and hidden between the cotyledons. Link to video at http://plantsinmotion.bio.indiana.edu/plantmotion/earlygrowth/germination/sunflower/sun.html.
The video you saw previously, on shoot growth and leaf production (http://plantsinmotion.bio.indiana.edu/plantmotion/vegetative/rosette/rosette.html), shows the difference between the cotyldons (the first two leaf-like structures) and the true leaves produced by the shoot tip. Notice that the cotyledons have a slightly different shape from the true leaves. (This is most obvious if you play the video to see the cotyledons expand to their final size.)
After germination, the plant continues to grow, eventually flower, produce seeds, and the cycle continues.