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 body to ensure the viability of plants?
What is a stem?
Stem is the axis of escape, its central andmain 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 that isaxial skeleton of plants and providing an ascending and descending current 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 to needles to lose less moisture.
On each stalk there are not only leaves,but also the 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 seeplaces of attachment of leaves or scars, which they left. They are called nodes. And the distance between them is internodes. The stem and the lamina form an angle - the axillary of the leaf. In it there are axillary (lateral) kidneys. Due to them, the branch 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.
Functions of the stem are due to the peculiarities of itsinternal 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. It contains special structures - stomata, due to which gas exchange takes place.
Later, this living tissue is replaced by a multilayeredcork, and stomata - on small bumps of 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 tissuestalk - cambium, due to which the stem grows in thickness, sometimes reaching a considerable size. Their work is especially active in the spring and summer.
Most 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 have a storing function.
The shape of the escape axis and its location inspace 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 can be lignified or remain herbaceous.
Creeping and clinging stems allow theirowners quickly master 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 curly 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 the families Umbrella and Pumpkin have a hollow stem.
What is the trunk?
There are several life forms of plants: grass, 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.
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 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.
Depending on what functions are performedstem, it can acquire new features of the structure, forming a modification of the stem. They 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 grass, physalis andlily 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 bulbs of garlic, lilies andtulip. 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 when favorable conditions come from the kidneys, a young green shoot grows.
Modifications also serve for vegetativereproduction 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, wildpear, 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 great importancefor 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.