Hey everyone! At this point we have accomplished pollen transfer and fertilization, which means we are ready for the zygote and then the embryo to begin developing for seed dispersal and seed germination. So that's what we'll look at in this video.
Following fertilization, we have a diploid zygote, which is the beginning of the sporophyte generation. The zygote is going to begin a little bit of cell division right away. The first cell division will be unequal, so there is going to be one smaller cell and one larger cell as a result of unequal cytokinesis.
The smaller daughter cell will eventually develop into the embryo. The larger daughter cell is termed the suspenser, and the suspenser will function to link the embryo to its food supply in the seed during this period of early embryonic development. Although the suspensor is the larger cell, it's actually mostly empty and contains basically just a big vacuole.
The majority of the cytoplasm in fact goes to the smaller cell, which is the embryo, since that's the one that's really going to divide a lot to develop into the embryo. This initial unequal division will end up persisting as a body axis. throughout the entire rest of the lifespan of the plant. So the differentiation between the shoot system, which will grow upward, and the root system, which will grow downward, begins immediately as a result of this unequal cell division. The vertical axis of the plant is set up based on which direction becomes the embryo cell and which direction becomes the suspensor cell.
That axis itself is set up by the location of sperm entry, just the same way that it was when we learned about animal development. So depending on where sperm enters, we're going to get this unequal cell division, and that vertical axis may further be modified during embryonic development by environmental signals such as gravity or sunlight, because of course you want to make sure in a plant that you are growing vertically and not sideways. So the...
Embryo, the smaller daughter cell, is going to grow upward towards the shoot system, whereas the suspensor is going to develop into the root system. As the embryo develops, that will lead to this vertical axis as well as the establishment of the apical meristems. Remember that the meristems are areas of active division that allow the plant to continue growing upwards.
and downwards, and we're going to have one to grow up and one to grow down. So when we establish this vertical axis, we also have to establish the apical meristems. So the embryo is going to divide upwards a little bit, and one of the first structures that you can see appear is the cotyledons, those seed leaves that we talked about, which will function to help the embryo utilize its food supply in the seed.
We will continue from this stage to differentiate into three embryonic tissue systems, which will further differentiate into the three tissue systems that we see in the adult plant. You can think of these as being quite similar to the germ layers that we learned about in the animals, the endoderm, mesoderm, and ectoderm. Those are tissues present in the embryo in layers.
These tissue systems are also present in the embryo in layers. which will persist to the adult plant. The outermost layer of tissue is termed the protoderm. The protoderm is similar to the ectoderm, so it is going to develop into the dermal tissue of the adult plant, including the epidermis. Just interior to the protoderm is an embryonic tissue called the ground meristem.
The ground meristem will further differentiate into the adult into ground tissue, which is kind of like the space filler. of the plant. In the center of the plant we have our third embryonic tissue system, which is the procambium.
The procambium will develop in the adult into the vascular tissue, which is very often in the center of stems and things like that. So procambium in the middle, protoderm around the outside, and ground meristem in between. Notice that as these tissue systems are developing, you can see the cotyledons are elongating and The embryo is getting bigger. The suspensor is going to shrink a little bit as the cotyledons get larger.
And the food supply in the seed will also start to shrink because it is being used up as the embryo divides and develops. So when you look at this mature embryo here, you can see that it has its shoot axis and root axis. It has its three tissue systems developed. It has a shoot apical meristem. just in between the two cotyledons, and it has a root apical meristem right at the border of the suspensor.
So all of the tissue systems are pretty much in place, in the right location, and ready to continue developing. At this point, the embryo is going to enter dormancy. While all this embryo development is going on, while we are developing our three tissue systems and the apicomerostomes and the cotyledons, there are some changes going on around the embryo as well.
The food supply within the seed will develop a little bit and then get used up by the embryo. The seed coat will develop around the outside of the seed. and then around the outside of the whole seed, the fruit will also develop and mature.
So test your memory and see if you can remember where did each of these three things come from originally, and what are their functions. Once the seed is mature with its tissue layers and its apical meristems and its cotyledons, and the fruit has matured around the seed, we are ready for seed dispersal. At that point, the seed is going to enter dormancy. It will shut down its metabolism, stop growing, and nearly all of the water will be removed from the seed. So it has the embryo and it has a food supply, but it doesn't really contain any water, and it's going to go dormant and remain dormant for potentially a long period of time.
Once the seed disperses to a new location, it can then germinate to develop into a new sporophyte. The embryo will emerge from dormancy, and begin growing out of the seed. This is triggered by environmental conditions.
The type of environmental conditions that trigger seed germination will vary by species, so it can be dependent on temperature, on water availability, on sunlight. In some cases, a seed will not germinate until it has been through a long period of cold temperatures to make sure that you wait out the winter. Other seeds might be triggered to germinate by fire, which indicates that old plant material has been cleared away and there is space for a new baby plant to grow. Other seeds will be triggered to germinate after passing through exposure to digestive chemicals in the digestive system of an animal because that indicates that the fruit has been consumed and they have been dispersed to a new location. So there's usually a wide range of environmental conditions you have to have just the right triggers in order for seed germination to begin.
When the seed germinates, the first thing that will happen is it will restart its metabolism and begin taking on water because there was no water in the seed, so it has to acquire water as quickly as possible. Bringing water in creates a lot of osmotic pressure which will cause the seed coat to burst. So you take on water in order to break out of the seed coat. The first thing that is going to emerge from the seed coat is the radical which is an embryonic root. The radical will emerge first and start growing downwards towards the soil.
Plants do have a way of detecting gravity, which we'll learn about a little bit next time, and so we're going to make sure that the root system grows downward in the radical. After the radical, the cotyledon may appear if it is a eudicot, and you can see the cotyledon kind of pops out of the seed. The cotyledon may photosynthesize like a leaf or it may not depending on the species.
It could just wither. Eventually, the shoot system will grow some real leaves that definitely will begin photosynthesizing, and at that point the embryo is no longer dependent on the food supply within the seeds. So the radical grows first downwards, and then the shoot system grows upward with a cotyledon and then eventually real leaves. So you can see that we are differentiating into the below-ground tissues, the roots, and the above-ground tissues, the shoots.
and that access will be maintained for the life of the plant. Here's a comparison of seed germination in eudicots and monocots. For a eudicot, which is what we have just been looking at, you can see the radical emerges first, pops out of the seed coat, and then there is kind of a hooked stem that's going to emerge, which is termed the hypocotyl, and the hypocotyl will then pop the cotyledons out of the seed above ground. The cotyledons may photosynthesize or may not.
but from the cotyledons you will continue growing the shoot system which will develop eventually real photosynthetic leaves. For monocots there are going to be a few modifications to seed germination because monocots are adapted for a drier environment. In those types of habitats the soil is rougher and more abrasive and the embryo is actually quite fragile at this stage.
Seed germination is one of the most vulnerable portions of the life cycle of a seed plant because these embryonic tissues, the embryonic root, the embryonic shoot are very delicate and very fragile. And so this is the period in the life when the embryo is actually at the greatest risk. In the seed it was very well protected, but during germination, it's a little bit vulnerable. So for the monocots, we have to have some special hardened structures on the seed to protect it from abrasive soil. The radical will still emerge first, but it will be protected by a sheath called the coleorhiza so that it can penetrate through the soil.
The stem will be the next thing to emerge, and it is also protected by a sheath called the coleoptile. So since the soil is more harsh for a monocot, The radical is protected by a sheath, the stem is protected by a sheath. And the cotyledons will actually not emerge from the seed, so the first leaves that you see will be true photosynthetic leaves that will begin growing. At this point, once the seed has germinated and has its root system developing and its shoot system developing, it's just going to continue growing into a mature sporophyte.
So that concludes the life cycle of the plant.