 When you sow a seed in the soil and provide it with enough moisture and the perfect temperature, you would expect it to germinate, right? Sometimes that doesn't happen. Even if the environmental conditions are perfect, the seed does not germinate. And this condition is called seed dormancy. So why do seeds become dormant? And what is the point of their dormancy? A tree that makes seeds would ordinarily want the seeds to germinate, right? So why make it dormant at all? To understand all this, first take a look at the reasons which result in seed dormancy. The first reason is an impermeable seed coat, which means it is not permeable to water or gases. So what happens when a seed coat is not permeable to water? You know when you sow a seed, it swells up, right? Because it imbibes water. And that won't happen if the seed coat is impermeable. And a seed, just like us, needs to breathe in oxygen and give out carbon dioxide. So when a seed coat is impermeable to gases, the embryo inside the seed cannot take up oxygen or give out carbon dioxide. So it won't survive. So that seed coat, which is impermeable, will not permit the germination of the seed. So this is one reason why a seed can be dormant. Another reason for seed dormancy is a hard seed coat. See, when a seed is germinating, the radical and the plumeo, that is the root and the chute of the embryo, they need to come out of the seed coat. Now if the seed coat is really, really hard, that won't happen. The embryo won't be able to come out. So germination won't happen. So that's another reason why seeds can be dormant. Another reason for dormancy is chemical inhibitors. For example, you know about ABA, abscessic acid, right? So that's a chemical inhibitor. You know abscessic acid is a growth inhibitor, it inhibits growth. Similarly, it inhibits growth in the embryo, in the seed. So that's another way of making a seed dormant. Now let's look at the ways in which a seed overcomes dormancy. Suppose there is a desert and you saw a seed in the desert. What do you expect? Naturally, the seed won't germinate, right? Because there's not enough moisture. Now let's say there is some amount of rainfall. What happens then? Does the seed germinate? You'd expect it to. There is moisture. But turns out there are seeds which will not germinate in that much rainfall because it's not enough. Why not? Think about it. If there is just a little rainfall, the water will soon dry out in a few days. And let's say the seed germinated with that amount of rainfall and turned it to a seedling. But the seedling will not be able to grow because the amount of water is gone because it is just a little bit of rainfall. So that's not enough. So these seeds will germinate only when there is a considerable amount of rainfall. So when there is a considerable amount of rainfall, what happens? The rain washes out all the inhibitors, like abscess acid, inside the seed. And that results in germination. So considerable rainfall would mean that the soil would be moist for a long time. And that would be sufficient for the seedling to grow. Hence this mechanism. Now let's say there is a forest. And a seed falls on the floor of the forest. Now in a forest, typically there are a lot of trees and plants around. And when a seed germinates there, it might face too much competition in terms of moisture, in terms of space. And the seedling might not do well. It might die off even. So that's not a good place for a seed to germinate. So what to do? Just make the seed dormant and wait for better conditions. So what are these better conditions? Sometimes a forest fire actually works out for the best. So if there is a forest fire, most of the trees and plants will die. Now what happens is for some dormant seeds, this extreme heat, this will make the seed coat permeable. Once the seed coat is permeable, things are favorable for the seed to germinate. And it will. Now picture another condition. Let's say it is autumn and a seed is sown in the ground. And it germinates and becomes a seedling. But then winter comes, very harsh winter. The seedling can't stand those cold temperatures and dies off. So again, dormancy helps the seed here to survive. Now when will the seed germinate? It will go through prolonged cold conditions. And just like extreme heat, prolonged cold will also make the seed coat more permeable. What does it mean when a seed goes through prolonged cold conditions? That means it has passed the entirety of winter. It has gone through winter and it has entered spring. Now in the spring, the conditions are favorable and the seed is ready to grow into a seedling. So that's what it does. Now tell me another thing. Suppose you sow a seed very deep into the ground versus you sow a seed a little bit at a shallow depth close to the surface of the ground, of the soil. So which one will germinate better or germinate at all? Naturally, the seed which is not sown too deep, right? First of all, if it's sown that deep, the seedling that comes out may not be able to respire. Secondly, the seedling may not even be able to photosynthesize because it can't come out of the soil. So in this case, what happens is, whenever there is light and light means the seed is closer to the surface of the soil and not buried in the darkness. So when there is light, light inhibits AVA, abscessic acid. And that in turn will need to breaking of dormancy or seed germination. Now suppose a seed is eaten by an animal, let's say a hen. The seed will naturally pass through its digestive tract. It won't get digested completely, but what will happen is the digestive enzyme will soften the hard seed coat of the seed. So if the seed was dormant, it will no longer be dormant after it's passed through the digestive tract of the hen. So what happens in an animal digestive tract is, enzymes digest the hard coat. And as you know, that will break the dormancy because hard coat was one of the things that was preventing the seed from germinating. But how does that help? How does passing of a seed through the digestive tract of an animal help? The answer is dispersal. If you remember from lower classes, seed dispersal is very important for the species of a plant to survive. Why? Because if the seeds are not dispersed, all the seeds will tend to grow up in the same region where the parent plant was found. And all of them of course can't go because again, there may be competition for food resources and space and stuff. So the seeds need to be dispersed. They need to be spread out far and wide. And this is what happens. Let's say an animal eats a seed somewhere, but maybe wanders off somewhere else and passes off the seed that it ate. And that seed then grows there, far away from the parent plant and grows up to be a healthy plant. So we have seen the different reasons why seed dormancy is caused, why it is caused at all, and how in nature seed dormancy is broken.