Seeds - Online Tutor, Practice Problems & Exam Prep

The journey of a plant begins with fertilization, leading to the formation of a zygote that divides into apical and basal cells. The apical cell develops into the plant embryo, while the basal cell forms the suspensor. Germination follows, initiated by imbibition, where water absorption breaks the seed coat. Monocots and eudicots sprout differently, with eudicots pushing cotyledons above soil and monocots emerging straight. Seed dispersal occurs through wind, water, or animals, and fruits, which develop from ovaries, aid in this process, showcasing various types like simple, aggregate, and multiple fruits.

Embryogenesis

How does a little plant seed turn into a great big tree? Well, all great things have small beginnings, and plants are no exception. The development of a seed begins with fertilization. The pollen grain will deposit sperm into the ovule on the flower, and this results in embryogenesis, which is the development of the seed, and, more importantly, the plant embryo within the seed. Now, after fertilization, what you're left with is a zygote. And this zygote is going to divide into two daughter cells known as the apical cell and the basal cell. So here we have our zygote, and it is going to divide into an apical cell and this basal cell. Now, the apical cell is going to form a mass of cells that will eventually basically become the plant. This cell mass will differentiate into those primary meristems we talked about: the protoderm, ground meristem, and procambium. And you can see the apical cell's division as it continues along to this structure, and eventually just, you know, it's growing into this mass of cells, which will eventually form the plant embryo that you can see behind my noodle here. Now, the basal cell forms a structure called the suspensor that contributes to support structures for the embryo. Only one cell in the suspensor will actually contribute to the plant embryo. So here, we have our suspensor.

Now these embryonic plants have embryonic leaves, we call them cotyledons, and plants will actually be differentiated into monocots and dicots depending on how many cotyledons they have. Monocots have one cotyledon. Right? Mono means one. And eudicots have two cotyledons, di meaning two. And if you're curious, the 'u' part here means true, so these are saying basically, two true leaves or cotyledons. Now the embryonic plant has some other structures that we should go over. These include the hypocotyl, which you can see here, let me jump out of the way, that is basically the embryonic stem of the plant. So it's this portion here. It also will have a radicle, which is an embryonic root. And here, you can see our radicle.

Now, looking at these two seeds over on the left, you'll notice that these are differentiated as monocot and dicot. So here we have a monocot seed, and here we have a eudicot seed. You can tell based on the cotyledons, this one has just one, this guy has two, and you can see the other seed parts, including the seed coat, that protective outer layer, the endosperm, which is going to be the nutrients that the embryo feeds on, and also, we'll have this structure here called the epicotyl, and that is something that some plants have. It's kind of like an embryonic stem, but it's an embryonic stem that extends beyond the cotyledons. So, when our plant sprouts, this becomes more readily apparent. And we'll actually talk about sprouting when we flip the page.

Germination

Once our plant embryo is formed and our seed is all set, it still has to somehow turn into a fully formed plant. This begins with germination, which is the process by which a plant forms from a seed. And it usually occurs after water is absorbed by the seed, through a process known as imbibition. Now, the seed coat, as we've said before, is that protective outer layer, and it forms from the integument, which you might recall as a structure found in the ovule. Now that outer layer is tough, and while sometimes imbibition will be able to cause enough swelling that it can break the seed coat open and sort of free the plant to develop, some seed coats are actually too thick to absorb water and require some type of physical penetration in order to germinate. In some cases, this could actually require being, you know, for example, chewed up and digested a little by an animal or something before the seed can actually germinate.

Now, the way a seed will sprout depends on whether the plant is a monocot or a eudicot. See, eudicots will have a hypocotyl that curves and grows towards the surface, and essentially pushes the cotyledons out of the soil. So you can kinda see that happening in this diagram here, where this hypocotyl will be curved, and it will grow up, and push its way out of the soil, and once it's out of the soil the cotyledons will actually sprout out of this seed. Now, in monocots this happens a little differently. Monocots will actually push their shoots straight up through the soil, whereas in the case of eudicots, the hypocotyl, that embryonic stem, is curved, and you can almost think of this as like a process of the plant arching its way out of the ground. In monocots, this is more of a straight shot. And because of that, monocots actually have to have these protective structures that help their emerging roots and shoots. And we call those the coleorhiza and coleoptile. The coleorhiza is the first structure to actually emerge from the seed, let me hop out of the way here, you can see it here, and it is protecting the radicle, and you might, I hope, you caught the spelling mistake in this figure, radicle is supposed to be spelled like that. I just liked the spelling mistake too much to take it out of the figure, because this is basically spelled like, you know, like radical dudes. So, anyhow, the coleorhiza protects the radicle, and the coleoptile protects the cotyledons, which you can see as these little structures here inside the coleoptile.

Now, hopefully, as you can see in this image here, here's our seed, it's going to stay where it is. And from here, its shoots are going to push up, and its roots are going to push down. So whereas the eudicot, again, did that curving, pushing out of the soil. Right? This is going to have the seed stay where it is, and it's going to push its shoots and roots out from that point. Now, how do these seeds get where they need to go? Well, seed dispersal is the technical term for the transport of seeds away from the parent plant, and there is a number of ways this can happen. You can have wind disperse your seeds, water, also animals, as we can see with this dog here that is covered in burrs, which are seed pods for plants. And oh, here also you can see these little plumes from a dandelion. Here is the seed, and this tuft on top is to help the seed be dispersed by wind. Now, seeds don't germinate right away when they land in dirt. They actually can enter a period of dormancy and will actually wait to germinate until the conditions are favorable. And in the case of some seeds, this could be years of dormancy before germination. Alright. Let's turn the page.

Fruit

One way to encourage animals to disperse your seeds, aside from having them stick to your fur, is by putting them in fruit, which, of course, is delicious, and we animals love to eat. So fruits are technically these seed-bearing structures that form from the ovary after flowering. Now, as you can see in this diagram here of a peach, or it really could be any stone fruit, you have your seed on the inside there, with its endosperm, embryo, and seed coat, familiar structures to us. And then surrounding it in this area we have what's called the pericarp, and this is the part of the fruit that we eat, and this is what ripens from the ovary and will surround the seeds in a sweet delicious coating.

Now, as I'm sure you're aware there are many different types of fruit and they take all sorts of shapes and sizes, and in fact, we call a lot of fruits vegetables, even though they're technically fruits. For example, zucchini, technically a fruit. Right? It's got that ripened ovarian walls surrounding seeds. Not that sweet, which is why we call it a vegetable, but technically it is a fruit. And, of course, amongst fruits that we think of as fruits, there's a ton of variation. And what we'll see is that these are actually due to different types of development.

So you have simple fruits, and these are fruits like cherries. They develop from a single flower with a single carpel, or fused carpels, so a single pistil or a single carpel. And, these will have, you know, a simple structure like a cherry, that ovarian wall surrounding the little seed in there. And this forms from, again, one flower with its single pistil, or single carpel.

Now you can also have aggregate fruits, which develop from single flowers that have multiple carpels. And an example of this is the raspberry. So even though this develops from a single flower, all of these little morsels in the raspberry form from separate carpels, and they each have a seed in them. Those are again called aggregate fruits. We also have multiple fruits, which develop from multiple clustered flowers, and a nice example of this is the pineapple, which, you can kind of see in its structure how it would develop from clustered together flowers.

Lastly, we have accessory fruits, which develop from ovary tissues, as well as tissues outside the carpel. And a nice example of this is a strawberry, which has its seeds on the outside, and has its delicious fruity meat, inside the outer coating of seeds. So, many different types of fruits result from different types of flowers. And that's all I have for this lesson. I'll see you guys next time.

Here’s what students ask on this topic:

How does a seed develop into a plant?

A seed develops into a plant through a series of stages starting with fertilization. The pollen grain deposits sperm into the ovule, leading to the formation of a zygote. This zygote divides into an apical cell and a basal cell. The apical cell forms the plant embryo, while the basal cell forms the suspensor, which supports the embryo. The seed then undergoes germination, where it absorbs water through imbibition, causing the seed coat to break. The plant embryo then grows, with monocots and eudicots sprouting differently. Monocots push their shoots straight up, while eudicots curve their hypocotyl to push out of the soil.

What are the differences between monocots and dicots?

Monocots and dicots differ primarily in their cotyledons. Monocots have one cotyledon, while dicots (eudicots) have two. This difference affects their germination and growth. Monocots push their shoots straight up through the soil, protected by structures like the coleoptile. Eudicots, on the other hand, have a curved hypocotyl that pushes the cotyledons out of the soil. Additionally, monocots typically have parallel leaf veins, while dicots have a branched vein pattern. These differences are crucial for identifying and understanding the growth patterns of various plants.

What is seed dispersal and why is it important?

Seed dispersal is the process of transporting seeds away from the parent plant. It is crucial for reducing competition for resources among seedlings and increasing the chances of species survival. Seeds can be dispersed by wind, water, and animals. For example, dandelion seeds have tufts that allow them to be carried by the wind, while burrs stick to animal fur. Some seeds are enclosed in fruits, which are eaten by animals, and the seeds are later excreted in different locations. Effective seed dispersal ensures that plants can colonize new areas and maintain genetic diversity.

How do fruits aid in seed dispersal?

Fruits aid in seed dispersal by attracting animals to eat them. The seeds are often encased in a sweet, nutritious pericarp, which animals consume. After digestion, the seeds are excreted in new locations, facilitating plant spread. Fruits can be classified into different types based on their development: simple fruits (e.g., cherries) develop from a single flower with one carpel, aggregate fruits (e.g., raspberries) from a single flower with multiple carpels, multiple fruits (e.g., pineapples) from clustered flowers, and accessory fruits (e.g., strawberries) from ovary and other tissues. This diversity in fruit types enhances the effectiveness of seed dispersal.

What is the role of the seed coat in germination?

The seed coat plays a protective role in germination. It forms from the integument of the ovule and serves as a tough outer layer that shields the seed from physical damage and pathogens. During germination, the seed absorbs water through imbibition, causing the seed coat to swell and eventually break open, allowing the embryo to emerge. In some seeds, the coat is too thick to absorb water directly and requires physical penetration, such as being chewed by animals, to initiate germination. The seed coat ensures that the seed remains viable until conditions are favorable for growth.