The vascular tissue in plants are a major, continuous and unified system in a plant right from the root tip to the upper-most leaf. It's importance cannot be more justified as it performs the function of conduction of matter from one end of the plant to the other end and vice-versa. To state it in simple terms, its functioning is similar to that of the circulatory system in animals.
The vascular tissue, unlike the simple tissues (such as the parenchyma, sclerenchyma and collenchyma), are complex tissues made up of vascular bundles. These vascular bundles are made up of different kinds of cells are are hence heterogenous in composition. The complex tissues in plants comprise of the xylem and the phloem.
The xylem and phloem, based on their function, occupy specific positions in the plant. To begin with, the xylem is concerned with the absorption of water and minerals from the soil. Hence it carries water and minerals from the root to the leaves via the intermediate plant organs. The phloem, on the other hand, conducts food material throughout the plant. So in essence, the phloem carries processed food from the leaves to the various other parts of the plant, up to the roots. Thus it can be seen that the xylem and phloem are a two-way transporting system for the materials of sustenance of the plant.
The xylem and phloem are structurally also very complex. While most other tissues are a collection of a single type of cells (parenchyma), the xylem and phloem each are very complex. The xylem is made up of four different types of cells, namely the tracheids, vessels, xylem fibers and xylem parenchyma. Like the xylem, the phloem too is composed of different kinds of cells. They are the sieve tubes, sieve cells, companion cells, phloem fibres and phloem parenchyma.
• Vascular bundles in action:
In the above description, it can be seen that vascular tissue are complex. However, based on the way they are positioned and function, the vascular bundles are classified into two groups: Conjoint and Radial Bundles. The characteristic features of these tissues can be observed by studying the cross-section of a plant part.
a) Conjoint Bundles:
Conjoint bundles can be seen in the leaves and stems of plants. In this case, the xylem and phloem are placed side by side along a linear plane and run parallel to each other. Conjoint bundles are of three kinds: Collateral (xylem ring inside and phloem ring outside the xylem), Bicollateral (where a ring each of phloem is seen both inner to and outside the xylem ring) and Concentric (where one vascular tissue encircles the other).
b) Radial Bundles:
According to the structure of radial bundles, the xylem and phloem occur at different radii along the plant and are not in a circular or ring-like fashion.
Much importance has been given to the vascular tissue in plants as they are the prime tissue for conduction of material. They are akin to the circulatory system in animals. Since water and minerals are needed for the full development of the plant, it becomes inevitable that these tissues are formed well and function efficiently. Besides, the xylem tissue is made up of strong, fibrous tissue that adds to the support of the plant. Similarly, the phloem is very important and it serves to provide all plant organs with the necessary food. By the process of photosynthesis in the leaves, the phloem carries the prepared food around. Hence again, it becomes necessary that the phloem functions well. When the phloem does not function well, the plant growth is stunted and wood inferior. This affects the economic and trade value of wood that is used for human need.
By understanding the structure and function of the vascular tissue in plants, it can be seen that they are of prime importance not only to the plant but also to human development as a whole. Without proper wood, our homes would be unsteady and unsafe, and the quality of the food consumed too would be inferior. The vascular tissue in plants are responsible for a balanced interaction between the animal and plant kingdom at large.