All plants can be divided into spore and seed. Spores include mosses, club mosses, ferns and horsetails. Their life cycle is divided into sporophyte and gametophyte. The sporophyte reproduces asexually by producing spores. The gametophyte is characterized by sexual reproduction, in which the plant forms gametes - sex cells - male and female. When they unite, a zygote is formed, from which a new individual grows, which, in turn, will already form spores. In seed plants, everything is more complicated, since they form seeds from a zygote.
What is this?
A seed is a special multicellular structure that a plant needs to reproduce. They are studied by the science of plants - botany, which includes biology. The structure of seeds can be complex and depends on the department and class to which the plant belongs.
Classification of seed plants
All of them are divided into two departments: gymnosperms and angiosperms. The determining factor in separation isthe structure of the seeds, namely the presence or absence of additional protection in it.
Gymnosperms
This department consists of about 700 kinds of plants. They are divided into four classes: conifers, ginkgos, cycads and gnetos.
Greatoid class
It is represented by three families: coniferous, gnetovyh and velvichivyh. The last family consists of a single species - Velvichia amazing. The gnetaceae family is represented by about 40 species of gnetum, and the conifers are represented by 67 species of conifers, or ephedra, including Rough Conifer, Mountain Ephedra, and others.
Ginkgo
Only one type of plant belongs to it - Ginkgo biloba. This is a relic organism that has been preserved since the Permian period.
Class cycads
It consists of a family of the same name, which includes 90 plant species. These include, among others, the comb-shaped cycad, the drooping cycad, the Tuara cycad, and others.
Conifers
This is the most numerous class of gymnosperms. Previously, this class was divided into three orders, representatives of two of which are now extinct. Today, conifers consist of one order - pine. It, in turn, includes seven families: pine, yew, araucaria, cypress, podocarp, sciadopitis and capitate.
Angiosperms department
These plants are more numerous than gymnosperms. This is the dominant department in our time. It is divided into two large classes: monocots and dicots. The decisive factor in this division was the structure of the seeds.plants.
Monocots
This class is represented by 60 families, including lilies, onions and cereals. In total, this class has about 60 thousand plant species.
Dicot class
Consists of about 350 families. The most famous of these are cruciferous, rosaceae, legumes, Asteraceae and nightshades.
Structure of seeds of gymnosperms
Let's consider the seeds of conifers, ginkgos, cycads and gnetoids. These are the first plants to have evolved a seed.
Its external structure provides for the presence of a dense peel. It may have additional outgrowths that contribute to better protection and seed distribution. For example, pine seeds have wing-like appendages that help them spread.
Since gymnosperms have no fruit, their peel has a complex structure. So, in cycads and ginkgos, it consists of three layers. The topmost is called the sarcotesta. It is soft and fleshy. The middle layer is the hardest, and it protects the seed. It's called sclerotesta. The inner layer by the time the seed ripens becomes membranous, it is called endotest. Most of these seeds are spread by animals that eat the tasty, fleshy sarco pasta without damaging the hard sarco pasta. As you can see, the seed coat of gymnosperms is practically an analog of the fruit of angiosperms.
It contains the germ and endosperm.
A germ is essentially a small plant. It has a germinal root anda shoot consisting of a stem, leaflets (their number may vary) and an apical bud.
Endosperm is the nutrients needed for seed to germinate.
The structure of monocot seeds
In angiosperms, the structure of seeds is slightly more complicated than in gymnosperms. In addition, they are additionally protected by the fetus. A striking example of monocotyledonous plants is cereals. Therefore, consider the structure of the wheat seed. They, like the seeds of gymnosperms, are built from a peel, endosperm and an embryo consisting of a root, a leaf and a kidney, however, they also contain a cotyledon (in this case one). The cotyledon is a thick leaf, which, when the seed germinates, becomes the first leaf. Cereal, including wheat, is not a seed, but a fruit (caryopsis), consisting of a seed and a pericarp, which is tightly fused with the peel. Most of the internal space of the monocot seed is occupied by the endosperm - a combination of nutrients (starch, fats, proteins, etc.). The cotyledon separates the embryo from the endosperm.
The structure of the seeds of all monocots resembles the structure of a wheat seed. But there are some exceptions. For example, there is no endosperm in arrowhead seeds, and the nutritive chemical compounds necessary for germination are already in the embryo itself. And in onions and lilies of the valley, the endosperm is located around the embryo.
Dipartite
The structure of a dicot seed is in many ways similar to that of monocots. However, they also have differences. The main difference between the structure of seedsmonocotyledonous and dicotyledonous plants, is the number of cotyledons. The plants under consideration now have two of them. They are located on both sides of the embryo. The stem, root and bud are located between the cotyledons.
As a typical example, we can take the structure of bean seeds. This is a typical representative of the dicotyledonous class, belonging to the legume family. The structure of bean seeds provides for the presence of a thick shiny peel that reliably protects the embryo. There is a scar on the concave side of the seed. This is the place to which the seed stalk is attached, which is necessary for connecting the ovule with the wall of the ovary. Next to it is a small hole - the seed entrance. The structure of bean seeds also provides for the presence of nutrients in the cotyledons. This is seen in many dicotyledonous plants, so the seeds of many of them do not contain endosperm at all.
However, there are dicotyledonous plants whose embryos receive organic chemical compounds for germination only from the endosperm. These are, for example, lilac, sweet pepper, linden, poppy. There are plants whose seeds contain nutrients both in the endosperm and in the cotyledons. This, for example, ash.
Additional protection of angiosperms seed
This is a fruit. It serves to preserve the seed from mechanical and thermal damage. In addition, it is necessary to ensure the distribution of seeds over long distances.
Fruits are simple and complex. Simple ones are single fruits, and complex ones are collected from several fused fruits. Complexfruits are also called apocarps.
The fruit of angiosperms is formed from the ovary of a flower. The remaining parts of it in most cases fade, but sometimes additional shells can form from them.
What is formed from the ovary is called the pericarp. It consists of three shells: endocarp, mesocarp and exocarp, or epicarp. The first layer is the inner, the second is the middle, and the third is the outer. These three layers are easy to identify with the naked eye. For example, consider the fruit of a peach. Its skin is the exocarp, the pulp is the mesocarp, and the woody shell, which reliably protects the only seed in the fruit, is the endocarp. Everything is similar in an apple: the skin is the exocarp, the flesh is the mesocarp, and the transparent plates surrounding the seeds are the exocarp. Basically, in all fruits, the mesocarp is represented by pulp, but there are exceptions. For example, in citrus fruits, the exocarp is the skin, the mesocarp is the white or yellowish layer between the skin and the pulp, and the pulp is the endocarp.
Spreading seeds
This is very important for plants, because in this way they can spread over as large an area as possible. Seeds, especially flowering plants, are able to spread much further than spores. This is one of the significant advantages of seed plants over spore plants.
There are four main types of seed dispersal:
- by air;
- on the water;
- using animals;
- with the help of people.
Depending ontype of distribution, the seeds and their fruits have various additional adaptations, for example, dandelion parachutes for air flight, clinging burdock needles for spreading on animal hair, etc. help from animals and people.
What is the advantage of seeds over spores?
Firstly, this structure has a great chance of germination, as it has enough nutrients in the form of endosperm and skin, with which the seed can survive adverse conditions and germinate later.
Also, they don't need water to spread, as is the case with spores. They are also able to spread much further than spores, which ensures the development of new territories by gymnosperms and angiosperms.
And the third advantage is that seeds, unlike spores, are the result of sexual reproduction, which makes it possible to diversify the plant genotype and ensure their better adaptation to environmental conditions.
Conclusion: table
| monocots | dipartite | gymnosperms |
| one cotyledon | two cotyledons | a few cotyledons (from 2 to 18) |
| peel,germ, endosperm | ||
| there is fruit around the seed | eating fruit | no fruit |
Now you know how seeds are arranged, why they are needed and why they are better than the argument.
