Carrot leaf simple or compound. External structure of the leaf. Why carrot leaves may dry out

Due to the extremely high level of their existence and structure, several types of reproduction and postembryonic development have formed, which pass on genes to the offspring and continue to ensure the survival of the species.

The process of reproduction is one of the important properties of organisms and is divided into two ways: asexual and sexual.

The sexual method is used by animals with a complex body structure, such as crustaceans, and, basically, all vertebrates.

Animals have two mechanisms of fertilization: external and internal.

External fertilization

One of them is external fertilization, in which eggs and sperm fuse outside the animal’s body. For example, this method of fertilization is used by fish and amphibians. This type of fertilization is called spawning and occurs in an aquatic environment. Accordingly, sperm need water to swim to the laid eggs, and eggs in turn need water to prevent drying out. Most aquatic invertebrates, most fish, and some amphibians use external fertilization. These animals are releasing a large number of sperm and eggs, since they suffer significant losses of gametes in water. Therefore, fish simply need to spawn a large amount of eggs. Thus, female perch lay 200-300 thousand eggs, and female cod lay up to 10 million. In addition, courtship behavior in some species leads to the simultaneous release of gametes, which helps provide the egg with sperm.

Gametes, or sex cells, - reproductive cells that have a haploid (single) set of chromosomes and participate in gametic, in particular sexual, reproduction. When two gametes fuse during the sexual process, a zygote is formed, which develops into an individual (or group of individuals) with the hereditary characteristics of both parental organisms that produced the gametes. Wikipedia

Paying attention to goldfish breeding It should be noted that single-sex populations occur here (there are usually no males). The development of eggs of this species occurs after the sperm of a completely different species of fish (carp, goldfish, tench) penetrates them. But in this case, standard fertilization does not occur. In this case, the sperm is only an irritant that awakens the egg to development.

This type of external fertilization, or rather spawning, also includes seahorses. Like no one else, they mate in a bewitchingly romantic way and dance until the female places her eggs in the male’s special pouch. It turns out that the seahorse is a conscious male who becomes pregnant and bears its offspring. After the male gives birth, he leaves his young to develop and fend for themselves.

Internal fertilization

Another example of sexual reproduction is internal fertilization, in which the male injects sperm into the female's reproductive tract, where the eggs are fertilized. This fertilization is an adaptation to life on land because it reduces the loss of gametes that occurs during external fertilization. Sperm are equipped with a fluid (sperm) that provides an aqueous environment inside the male's body. Mating and reproductive readiness are coordinated and controlled by hormones so that sperm and eggs come together at the appropriate time.

After internal fertilization, most and all reptiles lay eggs, which are surrounded by a tough membrane or shell. Their eggs have four membranes: amnion, allantois, yolk sac and chorion. The amnion contains the fluid surrounding the embryo; The allantois stores the embryo's urinary waste and contains blood vessels that bring oxygen to the embryos and take away carbon dioxide. The gall sac holds the stored food, and the chorion surrounds the embryo and other membranes. In birds and reptiles, the embryo matures outside the body and is protected by a membrane.

Basically, everyone (cows, yaks, hippos, rabbits, dogs and many others) use internal fertilization, but there are exceptions - such as and, which lay eggs.

Sexual reproduction has its “advantages”: the resulting individuals carry the characteristics of both parents and this species of animal will not disappear; they adapt better to their environment.

Also found among animals parthenogenesis is a single form of sexual reproduction, during which the embryo develops from a single germ cell without any fertilization. Such reproduction is usually characteristic of insects, some crustaceans and worms.

Asexual reproduction is a process in which the next generation develops from somatic cells without the participation of reproductive cells - gametes. This type of propagation is usually used in less complex organisms.

For example, an amoeba is divided into two parts. This type of asexual reproduction is called binary fission. It's very fast and effective method for bacteria and similar types of cells to create offspring.

Dioecious animals

There are many dioecious animals. But among the lower ones there are many species that have both male and female sex glands. These animals are called hermaphrodites. These include many flatworms: liver flukes, bovine tapeworms, pork tapeworms and others.

After fertilization, a series of developmental stages occur during which the primary germ layers are established and reorganized to form the embryo. During this process, animal tissues begin to specialize and organize into organs and organ systems, determining their future morphology and physiology.

Development- This is the process of formation of the body, closely related to growth. There are two types of animal development: direct and indirect or with reincarnation.

Direct type of development- this is the development of daughter organisms very similar to adults. This includes arachnids, reptiles, birds, mammals, and worms.

Indirect type of development- this is the development in which a larva is created, which differs from the body of an adult individual in both external and internal structure, the nature of movement, and feeding. This includes insects, amphibians, and coelenterates. In the case of indirect development, larvae and adults live in different conditions and therefore do not compete with each other for territory and food. Due to this, a species can have a large number of individuals. For example, in butterflies, larvae eat plant leaves, while adults feed on flower nectar. The toad larva feeds on single-celled algae, while adult toads feed on insects and their larvae. Accordingly, the indirect type of development gives the body significant advantages.

Each animal has its own life cycle with its own stages of development. Exists simple And complex cycle. The complex life cycle is characterized by alternation of generations (one generation of the liver fluke reproduces sexually, the other asexually) or is associated with the reincarnation of the organism. For example, grasshoppers have a simple cycle: egg - larva - adult insect. But butterflies have a complex life cycle: egg – larva – pupa – adult.

Bark beetle larva

Larvae often constitute a life stage that is used for feeding or dispersal. In many species, the larval stage is the longest, and the adult stage is a short stage solely for reproduction. For example, silkworm moths have adults without mouthparts and cannot feed. And the larvae must eat enough to survive and eventually mate. Indeed, most female moths, once they emerge from their pupa, fly only once to lay their eggs. Then they die.

Many animals have regeneration– restoration of lost body parts. The smallest part of a hydra can give rise to a new organism. Among chordates, regeneration is best developed in amphibians, slightly less so in reptiles (they can renew fallen tails). In other animals, this function remains at the level of wound healing.

Each animal is characterized by its own individual development with such stages:

— embryonic (from fertilization to birth);

- immature;

- sexually mature adult;

- aging and death.

The role of Homeobox (Hox) genes in animal development

WITH early XIX centuries, scientists have noted that many animals, from simple to complex, have similar embryonic morphology and development. It is surprising that the human embryo and the frog embryo, at a certain stage of embryonic development, appear remarkably similar. For a long time, scientists did not understand why so many animal species looked the same during embryonic development, but became completely different when they matured. At the end of the 20th century, a certain class of genes was discovered that dictate the direction of development. These genes, which determine the structure of animals, are called "homeotic genes." They contain DNA sequences called homeoboxes with specific sequences called Hox genes. This family of genes is responsible for determining the overall body plan: the number

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Asexual reproduction.

In the animal world, there are a number of methods of reproduction, for example, direct fission or budding, characteristic of lower invertebrates, as well as parthenogenesis, observed even in higher vertebrates. It is quite obvious that asexual reproduction is the simplest and least energy-intensive way to increase the number of individuals. However, for some reason, in the process of evolution a complex process of sexual reproduction arose, associated with many problems and conventions.

Sexual reproduction.

1. Conjugation. Sexual reproduction appears in animals already at the lowest levels of the evolutionary ladder. Thus, even in the simplest unicellular microorganisms - ciliates, which reproduce by direct division, so-called conjugation is observed, which is a kind of analogue of the sexual process. In the process of conjugation, two ciliates seem to grow together for a time during which they exchange hereditary information. The ciliates then separate and then each continues to divide on its own.

2. Parthenogenesis. A number of invertebrates, as well as many vertebrates, exhibit a phenomenon called parthenogenesis, in which females lay eggs or give birth to live young without the participation of males. Only females hatch from these eggs or are born in a similar way, and it is interesting that in nature there are entire local populations of similar species. A similar population of rock lizards was discovered in Armenia. In other places of its habitat, this very common species reproduces in the usual way.

Parthenogenesis under experimental conditions is possible even in mammals. To do this, it is necessary to somehow stimulate the unfertilized egg to divide, which is feasible in different ways, for example, simply by injecting a microneedle.

3. Hermaphroditism. A number of invertebrate animals exhibit hermaphroditism, in which each individual has both male and female gonads. Among the most well-known animals that are hermaphrodites are, for example, earthworms, leeches and many species of snails. However, despite the fact that in such animals each individual produces both eggs and sperm, they mate with each other, producing a mutual exchange of sex cells. In large, shell-less mollusks Aplysia or sea hares living in the coastal zone of the sea, up to 10-12 individuals can simultaneously take part in the fertilization process, playing both the roles of males and the roles of females.

However, despite the presence of both types of germ cells in hermaphrodite animals, they extremely rarely resort to self-fertilization, and on the contrary, they usually strive to mate with other, preferably unrelated, individuals.

4. Gender change. Some animals, including a number of fish species, change sex with age. Thus, among swordtail fish, well known to aquarists, it is quite common for middle-aged females to transform into males. At the same time, they grow xiphoid process on the tail, present only in males, and they begin to exhibit typical sexual behavior, successfully fertilizing females. A similar phenomenon is observed in small sea cadet fish that live in the coastal zone of the Black Sea. In some species of fish, the reverse process is also observed: the transformation of males into females.

5. Alternation of sexual and asexual types of reproduction. In invertebrates such as coelenterates, worms different types, echinoderms, there are both sexual and asexual reproduction in parallel. Many invertebrates are characterized by alternating sexual and asexual generations. For example, coral polyps, which grow very actively due to vegetative reproduction, periodically produce a huge number of motile male and female germ cells. Fertilization occurs in the water column. From the resulting zygotes, mobile larvae develop, which settle on a suitable substrate and give rise to new colonies of corals. All attached forms reproduce in a similar way: sponges, bryozoans, hydroid polyps, etc.

In some species, parthenogenesis alternates with normal sexual reproduction. For example, aphids reproduce throughout the summer by viviparity, in which unfertilized females give birth to females only. Young female aphids begin giving birth to subsequent females when they are just a few hours old.

Thus, during the summer, a huge series of asexual generations passes. Zoologists have calculated that if all the offspring of one aphid survived, then in one summer they could cover the entire globe with a continuous blanket. Fortunately, since many other animals feed on aphids, this does not happen. It would seem that this method of propagating aphids is very successful and does not require anything better. However, in the fall, many aphids lay eggs, which hatch into males that fertilize a relatively small number of females.

While most aphids die in the fall, having completed their life cycle, fertilized females successfully survive the winter and lay eggs in the spring. Winged aphids hatch from the eggs, they fly away in different directions, and, having settled on suitable food plants, they lose their wings and begin to reproduce by parthenogenesis. It is obvious that for successful survival in continental climates, aphids need an expansion of the range of variability in the genome, provided by sexual reproduction.

Thus, the main significance of sexual reproduction is not simply to increase the number of individuals, but to expand the gene pool, which further contributes to natural selection.

Animals, like plants, reproduce asexually and sexually. The reproductive characteristics of multicellular animals are largely determined by their living conditions.

Asexual reproduction

Asexual reproduction is characteristic of the most simply structured animals, for example, hydra. In summer, under favorable conditions, it reproduces by budding. A protrusion of the body wall is formed on the body of the hydra - a nochule, which grows, forming a mouth and the rudiments of tentacles at the top. The bud at the base unties, falls to the bottom and begins independent life. Sometimes budding proceeds so vigorously that even before the first bud breaks off, 2-3 others manage to form on the hydra.

Sexual reproduction

Many animals, including hydra, reproduce sexually before the onset of cold weather. This process begins with the formation of sex cells - gametes. Male reproductive cells of animals are sperm. They are small and mobile, unlike sperm. Sperm are formed in males in the male gonads - the testes. Female reproductive cells, eggs, are immobile and have a large supply of nutrients. They are formed in females in the female gonads - the ovaries. During sexual reproduction, the fusion of two sex cells occurs - the egg and the sperm. This process is called fertilization. From the resulting cell, the zygote, a new organism is formed, combining the characteristics of both parents.

The advantage of sexual reproduction over asexual reproduction is that the offspring are similar to their parents and at the same time differ slightly from them and from each other. This allows organisms of the same species to better adapt to changing environmental conditions.

In most fish and amphibians (frogs, toads, newts), eggs are fertilized by sperm in the aquatic environment surrounding them. This type of fertilization is called external. If cell fertilization occurs inside the female's body, it is called internal. Internal fertilization is typical for insects, reptiles, birds, and mammals.

Development of a new organism

The development of a new organism can occur in the external environment or inside the mother’s body. Many animals lay eggs in places where there are conditions for their development. Insects - where there is food for the larva emerging from the egg (on leaves, in flower buds, next to fruits).

Reptiles usually place eggs in sand, soil or rotting leaves. Under the influence of ambient temperature or as a result of the release of heat from rotting plant debris, embryos develop in the eggs.

Lizards, turtles, crocodiles, and birds lay fertilized eggs with a large supply of nutrients. The eggs of lizards and pits are covered on the outside with a leathery shell, while the eggs of crocodiles, turtles and birds have a calcareous shell.

All these animals are called oviparous. In mammals, the embryo from the zygote develops in a special organ of the female - the uterus. This development is called intrauterine. The embryo is well protected from adverse environmental influences. This ensures high survival rate of mammalian offspring. Fully formed cubs are born. Such animals are called viviparous.

Sexual reproduction involves the formation of new individuals not from parts of the parent organism, as in asexual reproduction, but from a zygote formed by the fusion of male and female reproductive cells. Sexual reproduction in nature occurs in most species and has advantages over asexual reproduction, since it involves the unification of the hereditary material of the parent organisms.

Gametes

Sex cells, or gametes, differ in structure from other cells in the body.

Gametes have a halved amount of hereditary information (the number of chromosomes). This is achieved through meiosis, a special type of division characteristic of developing gametes.

In plants, the organs in which the development of germ cells (gametogenesis) occur are called gametangia. The plant itself on which gametes develop is called a gametophyte.

Female gametes are called eggs, and male gametes are called sperm or spermatozoa (if there is a flagellum).

Rice. 1. Sex cells.

In male animals, gametes develop in gonads called testes, and in female animals - ovaries.

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Gametes from different types vary in size and ability to move. In mammals and humans, eggs are large and immobile, while sperm are small and motile.

Fertilization

Mature gametes can combine with gametes of the other sex. This process is called fertilization and in different animals it occurs in two forms:

• external fertilization , occurring outside the body (amphibians, fish);
• internal when gametes meet inside the female's body.

The fertilized egg (zygote) has full set chromosomes, where half is obtained from the father and half from the mother.

Dioecy and hermaphroditism

In some species of plants and animals, both male and female gametes develop in the body of one individual. Such species are called hermaphrodites.

Examples of hermaphroditic species are:

• sea ​​bass;
• large pond snail;
• earthworm;
• bull tapeworm.

If a species has separate male and female female organisms, as happens in most cases, they say that these animals are dioecious.

When male and female organisms of the same species have noticeable differences in external structure or color, the species is said to be characterized by sexual dimorphism.

Rice. 2. Sexual dimorphism.

Types of sexual reproduction

In addition to sexual reproduction itself, with the fusion of germ cells there are other types:

• parthenogenesis;
• fusion of single-celled organisms;
• conjugation.

In parthenogenesis, offspring develop from unfertilized eggs.

Parthenogenesis occurs in

• ants;
• aphids;
• bees;
• crucian carp, etc.

During parthenogenesis, there is no exchange of hereditary material and all offspring are similar to the maternal organism.

Conjugation is sexual reproduction without the formation of germ cells. Characteristic, for example, of algae. Cells of different individuals grow together for a while and exchange genetic material.

In unicellular algae, fusion of entire parent cells occurs, followed by division into 4 cells.

Rice. 3. Sexual reproduction of algae.

In plants, sexual reproduction is usually combined with vegetative reproduction. For example, onions are usually propagated by shoots - bulbs, but onions are also characterized by sexual reproduction, they bloom and form seeds after pollination.

Vegetative propagation is faster and is important for gaining territory.

What have we learned?

Sexual reproduction is carried out in several ways and has its pros and cons. The importance of sexual reproduction of organisms lies in increasing their ability to adapt to environmental conditions. Its disadvantage is its dependence on the presence of individuals of both sexes and its slower speed than that of the vegetative one.

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This method of reproduction is more common in the animal world.

The essence of this process comes down to the fact that two sex cells, which are called Gametes, fuse to form a one-celled zygote (i.e., a fertilized egg), which then, through successive divisions, in turn turns into a fully developed adult.

The organs in which gametes are formed are called gonads. Almost all animals that reproduce sexually are characterized by two types of gametes: the ovum, or EGGS - large gametes with a large supply of nutrients (mainly fat, some proteins, minerals and a full range of vitamins) necessary for the development of the zygote, and spermatozoa, or sperm - very small in size and much more numerous gametes that are able to actively move with the help of a flagellum. Sperm contain only very small amounts of nutrients. Eggs and sperm are usually produced in different types of gonads. An adult animal can be either male (from the testes, which produce sperm), or female (from the ovary, which produces eggs).

However, there are exceptions to this rule. In many invertebrate animals, especially in the type of flatworms, ovaries and testes can be in the same animal, which in this case is called hermaphrodites (Fig. ...). Hermaphrodites are also known among vertebrates. For example, individuals of some marine bony fish alternate between different roles during the mating season: one of the partners behaves like a real female and produces eggs, while the other plays the role of a male and produces sperm. In the next breeding cycle, they can change roles, but are not capable of self-fertilization. Hermaphroditism does not occur in reptiles, birds and mammals.
In other cases, the egg does not fuse with the sperm, but nevertheless goes through all stages of development to the stage of an adult animal. This process, called Parthenogenesis (i.e. reproduction without fertilization), is widespread among invertebrate animals, and can be artificially induced even in mammals. However, in many animals that are characterized by parthenogenetic reproduction, this process alternates from time to time with sexual reproduction, probably in order to increase viability. This is exactly what happens in the background: during the summer they reproduce many times parthenogenetically, and at the end of it, as a result of sexual reproduction, they give rise to the last, overwintering generations. Moreover, parthenogenetic descendants are always of the same sex, often female. In the background, only females are born through parthenogenesis. Males are very rare, with the exception of the penultimate generation of the season.

Since the gonads lie relatively deep in the animal's body, gametes can enter environment in two ways: either first getting into the body cavities, and then through special openings (pores) in the walls of the body - out or through special channels - the genital ducts. For example, in annelids, gametes are excreted first into the body cavity and then out. In the paired gonads of mollusks and arthropods there are well-developed reproductive ducts leading outward (Fig. ...).

The paired gonads of vertebrates are located near the inner surface of the body wall, usually under the abdominal lining. In some mammals, the testes lie in the scrotum, which is formed by a protrusion abdominal cavity and the walls of the body and protrudes beyond the body. Normal development of sperm in such animals occurs only when the testicular tissue is cooled by several degrees to equal the normal body temperature of mammals, which is 36-38 ° C. This is why the scrotum is located outside the body and is usually poorly protected by hair.

Spermatozoa of vertebrate animals are excreted through canals, which are often combined with the canals of the excretory system. In most amphibians, reptiles and birds, excretory canals, genital ducts and anal hole intestinal tract open into a common, extremely complexly divided chamber, which, as you know, is called the cloaca.

In male mammals, the excretory and reproductive tracts are also usually united by a common urogenital canal, which comes from Bladder and then receives the genital ducts. In female mammals, the excretory canal usually opens into the anterior cavity, which is also the exit of the vagina. Thus, there is also a merging of the two paths here.

That part of the reproductive tract that serves to remove eggs from the ovaries to the outside is called the oviduct. Of course, the oviduct is lined with glands that form the protective shells of the egg. In animals whose babies develop in the female's body, the elongated part of the oviduct, where fetal development occurs, is called the UTERUS.