As in animals, plants have separate transport mechanisms that are responsible for the delivery of nutrients to individual cells and tissues. Today we will discuss the structural features of the conductive tissue of plants.
What it is?
Conducting tissues are called those along which the movement of nutrient solutions necessary for the growth and development of the plant organism occurs. The reason for their occurrence is the release of the first plants to land. From the root to the leaves, as you might guess, an upward flow of solutions of salts and other nutrients moves. Accordingly, the downward current flows in the opposite direction.
The upward transport is carried out by means of vessels in the wood tissue (xylem), while the downward delivery is carried out by means of sieve structures in the bark forehead (phloem). In general, the xylem form resembles that of animal vessels. Their cells are elongated, have a pronounced oblong shape. What other structural features of the conductive tissue of plants are there?
What are they like?
You should know that there are primary and secondary tissues of this type. Let's give their standard classification, since the visibility of the material improves its absorption. So, here is the simplest structure of the conductive tissue of plants, presented in the form of a table.
Plant tissue groups Simple | All cells in this tissue group are almost identical in both shape and structure. |
Difficult | Cells have a common origin, but differ significantly in their structure and their functions. |
As you can already understand, xylem and phloem are complex species, since due to their heterogeneous structure they are able to perform such a wide range of functions.
The main structural elements of xylem and phloem Conductive fabric | Structural elements |
Conductive structures | Mechanical elements | Stocking fabrics |
Xylem | Tracheids, standard vessels | Wood fiber | Wood fiber parenchyma |
Phloem | Sieve tubes, satellite cells | Bast cells and fibrous structures | Bast type parenchyma |
As you can see, the structure of the conductive tissue of plants does not differ in any supernatural complexity. In any case, it is much simpler than in cells of higher mammals.
Xylem. Conductive elements
The most ancient elements of the entire conductive system are tracheids. So-called cells of a specific shape, with characteristic, pointed ends. It was from them that the usual fibers of wood tissue subsequently came from. They have a woody wall of considerable thickness. The shape of the tracheid can be very different:
- Annular.
- Spiraling.
- In the form of dots.
- Spore-like.
It should be remembered that along the way solutions of nutrients are filtered through multiple pores, and therefore their speed of movement is quite low. These important structural features of the conductive tissue of plants are often forgotten.
Which plants can occur this structural element?
Tracheids can be found in almost all higher sporophytes. Most of the lower gymnosperms also have these structural elements in their structure, and even in them they play a very important role. The fact is that the strong walls of the tracheids, which we already wrote about above, allow them to perform not only a directly conducting function, but also to be a supporting, mechanical structure. These are the most important structural features of the conductive tissue of plants, on which a lot depends.
Often, they alone are the only supporting structure that gives the plant body the necessary strength. It is curious, but all (!) Conifers in the wood completely lack some special mechanical fabrics, and the strength is ensured solely due to the tracheids we discuss. The length of these amazing conductive elements can range from a few millimeters to a couple of centimeters.
In general, he studies these structural features of the conductive tissue of plants in Grade 5 of any secondary school, but often the question of the longest vessels in plants confuses even students of biological faculties.
Vascular characteristics
They are a very characteristic element in the xylem of angiosperms. They look like long and hollow tubes. Each of them is formed as a result of the fusion of elongated cells according to the “joint to joint” scheme. Each cell is called a member of the vessel, which in its functional structure repeats that for the tracheid. We note, however, that the segments are much wider and shorter than them.
What category of students should know these structural features of the conductive tissue of plants? Grade 5, which began to go through botany and the structure of the plant organism, can already navigate the simplest questions of this topic.
Vessel formation process
The xylem that first appears in the process of plant development is called primary. Its laying occurs in the roots and tops of young shoots. In this case, the divided xylem vascular segments grow at the distal ends of the prokambial cords. The vessel itself appears after their confluence, due to the destruction of the internal partitions. You can verify this if you look at their section through a microscope: rims are stored inside, which are just the same and are the remains of a destroyed partition.
Let's recall thanks to which structural elements a conductive plant tissue is formed, and which of them are in the root of the plant:
- Epidermal membrane.
- Bark.
- Protoderm, which constantly updates the layers above.
- The apical meristem, which is the main zone of plant root growth.
- Root cap protects more delicate tissue from damage.
- Familiar tissues are located inside the root: xylem and phloem.
- They are formed, respectively, from protofloem and protoxylem.
- Endodermis.
Protoxylem (that is, the first vessels formed in the plant) appears at the very top of all young axial organs. The formation occurs directly under the meristem layer, that is, where the surrounding vessels of the cells continue to grow and stretch intensively. It should be noted that even the mature vessels of protoxylem do not at all lose their ability to stretch, since their walls have not yet undergone stiffness.
As a rule, conductive tissues of flowering plants undergo such compaction quite early, since the stem needs to maintain a sufficiently massive and vulnerable flower.
Remember what is responsible for the solidification process? Lignin. But it is just deposited in the walls of the "blanks" of vessels either in a spiral or in a ring-shaped direction. This position of its layers does not prevent the vessel from stretching. At the same time, this lignin provides a very decent strength of young vessels in the plant, which prevents their destruction during mechanical stress.
That is why conductive plant tissue is so important. The picture that is available on the pages of this article will certainly help you better understand this issue, as it clearly demonstrates the main components of the mentioned fabric.
Metaxilema formation
In the process of growth, new vessels appear, which are much earlier subjected to the process of woodiness. When their formation ends in the mature parts of the plant, the process of metaxilema growth is completed. How should the structure of the conductive tissue of plants be considered in a school biology course? Grade 5, as a rule, is limited only by the fact that vessels exist in plant tissue . Further study is included in the curriculum for older students.
At the same time, the first vessels formed from protoxylem are first stretched and then completely destroyed. Mature structural formations that arose from metaxilema are not capable of stretching and growth in principle. In fact, these are dead, very rigid and hollow tubes.
It is easy to consider the biological feasibility of the process in this direction. If these vessels appeared immediately, they would greatly interfere with the formation of all surrounding tissues. As with tracheids, thickening of the walls of blood vessels can be divided into the following groups (depending on their shape):
- Ring-shaped.
- Spiral-shaped.
- Staircase form.
- Mesh.
- Porous.
We draw your attention to the fact that long and hollow xylem tubes with sufficient mechanical strength are an ideal system for delivering water and solutions of mineral salts over long distances. The movement of fluid along their cavities is not hindered, there is practically no loss of water and nutrients. What else are the structural features of the conductive tissue of plants? Biology (6th grade of a secondary educational institution) also considers the mutual conductivity of the walls with xyl. Let’s explain.
Being similar in this respect to tracheids, xylems allow water to flow through the pores in the walls. Since there is a lot of lignin in them, they have high mechanical strength, and therefore do not deform, in addition, there is almost no risk of rupture under the pressure of the nutrient fluid. However, we have already talked about the highest importance of this distinctive feature of xylem, due to which the wood of many species of trees is characterized by high strength and elasticity.
It is the strong and at the same time resilient xylem that the ancient ships owe their strength. The invisible but strong conductive plant tissue provided high resistance to long pine masts, which rarely broke even in the most severe storms.
Conductive Phloem Structures
Consider the conductive matter that is present in the tissues of the phloem.
Firstly, sieve structures. The material of their occurrence is Procambius, localized in the primary phloem. Note that with the growth of the tissues surrounding it, the protofloem quickly stretches, after which part of its structures die off and completely ceases to function. Metafloem ends its ripening after (!) After the plant stops growing.
Other features
So what else should you know about the structural features of the conductive tissue of plants? Grade 7 of a comprehensive school should study, in addition to all of the above, also the characteristics of sieve structures, as well as their companion cells. Let's write this question in a little more detail.
Members of sieve structures have a particularly characteristic structure. Firstly, they have extremely thin cell walls, which contain quite a lot of cellulose and pectin. By this, they strongly resemble parenchyma cells. Important! Unlike the latter, upon maturation, the nucleus completely dies in these cells, and the cytoplasm “dries out”, being distributed in a thin layer on the inside of the cell membrane. Oddly enough, but they remain alive, but at the same time dependent on satellite cells (resembles the relationship of neurons and astrocytes in the brain of animals).
Of course, class 6 usually does not consider these structural features of the conductive tissue of plants, but it is useful to know them. At least in order to imagine the essence of the processes taking place in the plant body.
Sieve tubes and companion cells
So. The members of the sieve structure form a single whole, being closely interconnected. A companion cell is unique in its cytoplasm: it is extremely dense, it contains a huge number of mitochondria and ribosomes. You could guess that they provide food not only to the “companion”, but also to the sieve-like segment. If a satellite cell dies for some reason, the entire structure that is associated with it dies.
The sieve tubes themselves can be easily distinguished by the sieve plates available in their composition. Even when using a weak light microscope, they can be easily seen. It arises in the place where the joint of the end ends of the two segments was formed. It is logical that these plates are located exactly along the growth of these same segments.
Types of Conductive Beams
Are there any other structural features of the conductive tissue of plants? Biology considers as such some aspects of the structure of conducting beams, which we will briefly discuss.
In any higher plant you can find the mentioned structures. They are a specific type of strand located in the roots, young shoots and other parts that are constantly growing. The composition of these bundles includes vessels and the mechanical supporting elements already discussed by us previously. Each such structural unit consists of two parts:
- Wood department. It consists of vessels and stiff fibers.
- Bast site. It consists of sieve structures and bast fibers.
Very often, a protective layer is formed around the bundles, which consists of live or dead parenchymal cells. In addition, according to their structure, they are divided into two types:
- Full - contain xylem and phloem.
- Incomplete - only one of these tissues is included in their structure.
Lotova classification of conductive beams
Currently, the standard Lotov classification, which divides the conducting bundles into the following varieties, is quite common:
- Closed, collateral type.
- Closed, bicollateral species.
- Concentric type - xylem is located outside.
- A variation of the previous species in which the xylem is inside.
- Radial beams.
In general, this is almost all the information that you should know when studying the conductive tissues of a plant as part of the school curriculum.