Phloem is a special type of plant tissue. Translated from Greek, this term means “bark”. It is also often called the bast. Phloem is a tissue due to which the transfer of nutrients to the organs of plants occurs. What structure does it have? How does nutrient transport occur? What is the difference from xylem?
Conductive plant tissue: xylem and phloem
Conducting tissue is needed to transfer minerals and water to different parts of the plant. It consists of two types of complex tissues - phloem and xylem.
Xylem is also called wood, and phloem - bast. They, as a rule, are in close proximity to each other and form conducting bundles (also called vascular fibrous). According to the mutual arrangement of phloem and xylem, several types of conductive tissues are distinguished:
- Collaterial (tissues are adjacent to each other and are located equidistant from the axial part of the plant organ).
- Bicollaterial (xylem is surrounded by two sections of the phloem).
- Concentric (when the xylem surrounds the phloem and vice versa).
- Radial (when phloem and xylem alternate in radius).
Phloem structure
Plant phloem is a special type of conductive tissue that is necessary for the transfer of nutrients resulting from photosynthesis to the organs of the plant where they are used. According to the type of origin, it is divided into the following types:
- primary (differentiated from procambium);
- secondary (formed from cambium).
Their main difference is that in the primary phloem there are no heart-shaped tubes. However, their cellular composition is identical.
The phloem consists of the following cell types:
- sieve (provide the main transport of substances and do not have cell nuclei);
- sclerenchymal (serve for support);
- parenchymal (perform the function of near radial transport).
The main feature of sieve cells is the presence of special pores in the cell walls. Their origin is still unclear. The channels of the sieve elements are lined with callose (polysaccharide), which can accumulate in them. Callose can clog the channels of these cells, for example, when the plant is in a dormant phase in the winter.
Phloem Transport
A phloem is a tissue along which concentrated solutions of hydrocarbons (mostly sucrose) formed as a result of photosynthesis move. In addition, assimilates and metabolites are transferred, but in a lower concentration. The transfer rate of substances reaches several tens of centimeters per hour.
The transfer of substances is carried out from organs where nutrients are actively formed, to those parts of plants where they are used or stored. Active transfer of substances occurs to the roots, shoots, forming leaves, reproductive organs, tubers, bulbs, rhizomes.
As a result of experiments, scientists found that transport is carried out from donor organs to those parts of plants that are located closest to them. In addition, the transfer of substances is two-way. Therefore, the plant at different periods of the growing season can accumulate nutrients or spend them.
Phloem: functions
Photosynthesis is carried out in leaf chloroplasts with the participation of sunlight. Its products, water and other solutions of minerals absorbed by the roots of plants are necessary for the functioning of absolutely all cells. A phloem is a tissue that provides their translocation. Solutions flow through the sieve elements from areas with high hydrostatic pressure to areas of low value. Therefore, the main function of the phloem is transport.
The difference between phloem and xylem
Despite the fact that xylem and phloem perform similar functions and are in close proximity, they have differences. The movement of substances in the xylem occurs from the root to the leaves. Moreover, the cells forming this type of tissue are vascular elements, tracheids, fibers and wood parenchyma. Xylem is necessary for the transfer of water along with dissolved nutrients.
So, phloem is a type of conductive tissue of a plant. It serves to transfer nutrients from those organs of the plant where they are actively formed, to those parts where they are stored or consumed. The phloem is represented by three types of cells - sieve, sclerenchymal and parenchymal. The main transport function is performed by sieve cells with special pores that do not have nuclei.
The transfer of substances can be carried out in two directions, and its speed sometimes reaches several tens of centimeters per hour. Another conductive tissue similar in function to the phloem is xylem. But their main difference lies in the fact that the xylem carries water in only one direction (from roots to shoots) with minerals dissolved in it.