Escape is the aerial part of any plant. It consists of an axial part - a stem, and a lateral part - a leaf. It is the stem that performs the functions of locating the body in space and transporting substances. What structural features allow this organ to ensure the viability of plants?
What is a stem?
The stem is the axis of the shoot, its central and main 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 arrange themselves vertically in space. The developed system of conductive tissues determined the process of continuous flow of water from the soil and organic substances from photosynthetic organs.
Stem functions
But the stem is not only an organ that is the axial skeleton of plants and provides an ascending and descending flow of essential nutrients. For example, in higher spore plants, horsetails, it is chlorophyll-bearing. And in cacti it increases significantly in size, storingwater. The process of photosynthesis is also carried out by this organ, since the leaves of this plant have turned into needles in order to lose less moisture.
On each stem there are not only leaves, but also buds. These are future organs that are in their infancy. They are vegetative and generative. The former give rise to the vegetative organs of plants - leaves and shoots. The latter contain the rudiments of a generative organ that provides sexual reproduction - a flower.
External structure of the stem
On the stem, with the naked eye, it is easy to see the attachment points of the leaves or the scars that they left. They are called nodes. And the distances between them are internodes. The stem and leaf blade form an angle - the axil of the leaf. It contains the axillary (lateral) kidneys. Due to them, the escape branches. The growth in height is provided by the apical buds of the plant.
In nature, stem modifications are common. They contain the same structural elements, but visually look different. For example, the peppermint rhizome has rather elongated internodes and, being underground, keeps the plant viable longer.
Internal structure
The functions of the stem are determined by the peculiarities of its internal structure. Outside, the organ is covered with cells of the integumentary tissue. They may be alive (skin) or dead (cork). They protect the contents of the stem from mechanical damage.
Skin exists in young plants, whose age does not exceedone year. It contains special structures - stomata, due to which gas exchange occurs.
Later this living tissue is replaced by a multi-layer cork, and the stomata are replaced by small tubercles of lentils. As the plant grows, its thickness also increases. It more effectively provides a protective function, since its dead cells are empty, they contain only air. Tightly adjacent to each other, they create a serious barrier to negative environmental factors: dangerous microorganisms, dust. excessive transpiration.
The next layer is bark. It consists of sieve cells and companion cells that provide movement and storage of substances. In the same layer there are bast fibers - elements of mechanical tissue that make the stem strong. It is thanks to them that during the strongest hurricanes, tree trunks remain safe and sound.
Next is the lateral educational tissue of the stem - the cambium, due to which the stem grows in thickness, sometimes reaching a significant size. Their work is especially active in spring and summer.
The largest part of the stem is wood. The conductive elements of this part transport substances, the mechanical elements provide strength, and the main ones store the necessary substances. This layer is the widest, densest and most valuable for human use in his economic activities.
In the center is the core, large and loose cells of which perform a storage function.
Variety
The shape of the axis of the shoot and its location in space can be veryvaried. Most plants have erect stems. Well-developed mechanical tissues and a root system, which firmly holds the plant in the soil, allow them to reach for the sun. Such stems may become woody or remain herbaceous.
Crawling and clinging stems allow their owners to quickly develop new territories, crowding out other plants. They have special devices for vegetative reproduction, for example, strawberry whiskers. But ivy with the help of special trailers can grow even on a vertical and rocky surface. The climbing hop stem wraps around any support, mastering a decent area for photosynthesis.
The shape of the stem of representatives of the plant world may vary. So, in cereals it is round, and in sedges it is trihedral. Representatives of the Umbrella and Cucurbita families have a hollow stem.
What is a 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 - the cambium - leads to a thickening of the shoot axis and the formation of a stem.
Cambium cells divide annually, forming a layer of a certain thickness - annual rings. By their number, you can determine the age of the plant.
The best
The photo below shows the stem of the largest tree in the world, the 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. She is believed to be around 2500 years old.
The baobab trunk stores the most water. This tree is also the largest in the world in terms of trunk girth. But the orchid stem has a length of 0.5 mm, being the smallest.
Changes
Depending on what functions the stem performs, it can acquire new structural features, forming modifications of the stem. These include the potato tuber. It consists of a thickened stem with vegetative buds called eyes. By the way, tubers are not only underground, like the aforementioned potatoes and earthen pear - Jerusalem artichoke. Kohlrabi cabbage forms an above-ground tuber rich in valuable minerals.
Stem modifications have couch grass, physalis and lily of the valley. They are called roots. On their long internodes are the beginnings of leaves and buds, from which adventitious roots and green shoot leaves develop.
The stalk is also the bulbs of garlic, lily and tulip. Their flat and underdeveloped stem is called the bottom. The root system of such plants is fibrous, represented by a bundle of adventitious roots. From the buds located on the bottom, leaves develop. They can be of several types. So, in onions, juicy and fleshy leaves protect dry and membranous ones. And when favorable conditions occur, young green shoots grow from the buds.
Modifications also serve for the vegetativeplant reproduction. An example of this is strawberry whiskers. Cucumber tendrils help this plant to cling to a support, allowing it to take the most advantageous position in space in relation to the sun.
Thorns, wild pear, barberry and hawthorn are a means of protection. These trees are famous for their juicy fruits with bright colors, which many animals love to feast on. Sharp thorns prevent them from doing this, and the fruits remain ripening on the branches.
The stem is an organ essential for plant life. A person uses it in economic activities, makes household items from wood. Many modifications of the shoot are eaten, serve for vegetative propagation, providing an increase in the viability of many plants.
