Stem is what? Plant stem: structure, functions

Escape - the aerial part of any plant. It consists of the axial part - the stem, and the side - sheet. It is the stem that serves as the location of the organism in space and the transport of substances. What features of the structure allow this organ to ensure the viability of plants?

What is a stem?

The stem is the axis of the shoot, its central and basic part. In the process of evolution, it arose as a factor in the adaptation of plants to the terrestrial habitat. Thanks to the appearance of mechanical tissues, plant organisms were able to be located vertically in space. The developed system of conducting tissues caused the process of continuous water supply from soil and organic substances from photosynthetic organs.

Functions of the stem

But the stem is not only the organ, which is the axial skeleton of plants and provides the ascending and descending currents of the necessary nutrients. For example, in higher spore plants, horsetails, it is chlorophylliferous. And in cacti significantly increases in size, storing water. The process of photosynthesis is also carried out by this organ, as the leaves of this plant have turned into needles to lose less moisture.

On each stalk there are not only leaves, but also kidneys. These are the future organs that are in their infancy. They are vegetative and generative. The first give rise to the vegetative organs of plants - leaves and shoots. The second contain the rudiments of the generative organ, which ensures sexual reproduction - the flower.

External structure of the stem

On the stem, with the naked eye, it's easy to see the places where the leaves are attached or the scars they left behind. They are called nodes. And the distance between them is internodes. The stem and leaf plate form an angle - the axillary of the leaf. In it there are axillary (lateral) kidneys. Due to them, the shoot branches. Growth in height is provided by the apical buds of the plant.

In nature, often there are modifications of the stem. They contain the same elements of the structure, but visually look different. For example, the rhizome of peppermint has sufficiently long internodes and, being underground, keeps the plant viable longer.

Internal structure

Functions of the stem are due to the peculiarities of its internal structure. Outside the body cover the cells of the cover tissue. They can be alive (peel) or dead (cork). They protect the contents of the stem from mechanical damage.

Peel exists in young plants, the age of which does not exceed one year. In it there are special structures - stomata, due to which gas exchange takes place.

Later this living tissue is replaced by a multilayer cork, and the stomata is replaced by small ligatures of the lenticulus. As the plant grows, its thickness also increases. It effectively provides a protective function, since its dead cells are empty, only air is in them. Densely adhering to each other, they create a serious barrier to negative environmental factors: dangerous microorganisms, dust. Excessive transpiration.

The next layer is represented by bark. It consists of sitoid cells and companion cells that provide movement and storage of substances. In the same layer are bast fibers - elements of mechanical tissue, making the stem strong. It is thanks to them during the strongest hurricanes tree trunks remain unharmed.

Next is the lateral educational tissue of the stem - cambium, due to which the stem grows in thickness, sometimes reaching considerable dimensions. Particularly intensified is their work in the spring and summer.

The most part of the stem is wood. Conducting elements of this part transport substances, mechanical - provide strength, and the main - store the necessary substances. This layer is the widest, dense and most valuable for human use in its economic activities.

In the center there is a core, large and loose cells of which perform a storing function.

Diversity

The shape of the escape axis and its location in space can be very diverse. Most plants have erect stems. They are attracted to the sun by well-developed mechanical tissues and the root system, which firmly holds the plant in the soil. Such stems may be lignified or remain herbaceous.

Creeping and clinging stems allow its owners to quickly develop new territories, displacing other plants. They have special devices for vegetative reproduction, for example, strawberry mustache. But ivy with the help of special trailers can grow even on a vertical and stony surface. The curling stalk of hops entwines any support, mastering a decent area for photosynthesis.

The shape of the stem of the representatives of the plant world can vary. Thus, in cereals it is rounded, and in sedge-grasses it is triangular. Representatives of families Umbrella and Pumpkin have a hollow stem.

What is the trunk?

There are several life forms of plants: grasses, shrubs and trees. The latter are distinguished by the presence of one well-developed stem. Intensive cell division of the lateral educational tissue - cambium - leads to thickening of the shoot axis and the formation of the trunk.

Cambium cells are divided annually, forming a layer of a certain thickness - annual rings. By their number, you can determine the age of the plant.

The best

On the photo below - the stalk of the largest tree in the world - sequoia. The trunk of this representative of the plant world can reach a height of more than 80 m.

Sequoia General Sherman is also a long-liver. Presumably her age is about 2500 years.

Most of all the water is stored by the baobab trunk. This tree is also the largest in the world in the girth of the trunk. But the stem of the orchid has a length of 0.5 mm, being the smallest.

Modifications

Depending on what functions the stalk performs, it can acquire new features of the structure, forming a modification of the stem. These include a potato tuber. It consists of a thickened stem with vegetative buds, which are called ocelli. By the way, tubers are not only underground, as with the aforementioned potato and ground pear - Jerusalem artichoke. Cabbage kohlrabi forms an overground tuber, rich in valuable minerals.

Modifications of the stem are wheatgrass, physalis and lily of the valley. They are called rhizomes. On their long internodes there are rudiments of leaves and kidneys, from which the accessory roots and green leaves of shoots develop.

The stem is also the bulb of garlic, lily and tulip. Their flat and underdeveloped stalk is called the Donets. The root system of such plants is friable, represented by a bundle of accessory roots. From the kidneys located on the bottom, the leaves develop. They can be of several kinds. So, onion juicy and fleshy leaves protect dry and filmy. And with the advent of favorable conditions, a young green shoot grows from the kidneys.

Modifications also serve for vegetative propagation of plants. An example of this is a strawberry mustache. Antennae of cucumber help this plant to cling to the support, allowing to occupy the most favorable position in space in relation to the sun.

The means of protection are thorns, wild pear, barberry and hawthorn. These trees are famous for their juicy fruits with a bright color, which many animals like to eat. Sharp thorns do not allow them to do this, and the fruits remain ripening on the branches.

Stem is an organ of vital importance for plant life. Man uses it in economic activities, from wood makes household items. Many modifications of the shoot are used for food, serve for vegetative reproduction, providing an increase in the viability of many plants.