When we start putting cells together, we are creating a tissue. Surprisingly, all organs of a plant are made up of only three types of tissues. First of them is the epidermal tissue, which is positioned on the outside of the plant where it provides protection both from foreign objects coming in and from water evaporation going out. Its cells fit very tightly together, leaving no air spaces in between and isolating tissues on the inside.
With the epidermal tissue, there are certain types of specialized structures. One of those would be the trichomes, a hairlike structure, which extends from various types of leaves and they have specific functions too. For example, in spice plants such as basil, they contain chemicals which give it the well-known taste and smell. In the epidermal tissues of roots, we can see another structure called root hairs, which function is to increase the volume of the roots so it can absorb more water.
The tissues on the inside, protected by the epidermal tissues, are called ground tissues and it creates the main mass of the plant. It carries out the photosynthesis and helps to support the plant. The third type of tissue is a vascular tissue, which connects organs together. The specific organizations of all of these tissues are what gives each organ its unique shape.
Since we have already kind of talked about the principles of ground tissues in previous article, we are now going to take a deeper look at the vascular tissues. There are constructed mainly from two types of cells. The first is called a xylem, which is responsible for a transportation of water, while the second one, called a phloem, is responsible for a transportation of sugars.
The xylem cells form long tubes which transfer water from the roots up to the shoots. They can do this thanks to the fact that they are empty on the inside and open at the top and bottom. This way it can be perceived as a system of simple irrigation tubes. These cells, once they are able to maintain this function, are dead thanks to this transformation, which is programmed in its DNA.
The structure of the phloem is much more complex than the structure of xylem. In fact, xylem is comprised of two cells: one large cell called a sieve tube member and a smaller cell called a companion cell. The sieve tube member is a cell wall with the protoplast that is empty. It contains proteins for the transportation or storage of proteins. On the top and the bottom are open pores which connect individual cells so that the sugar can travel through the body of the plant. The companion cell is much smaller, but it is a complete cell with all organelles, a nucleus, etc. Its job is to provide the sieve tube member with all proteins, energy, and membranes required for its proper functioning. The two cells are connected together by numerous plasmodesmata along the common wall.
To better understand the process of xylem cells creation we need to look into a cytokinesis, a process of a cell division. To begin the division, a nucleus replicates itself and migrates to the opposite side of the cell, an action called mitosis. Once the mitosis is finished, a new cell wall is formed in the cytoplasm between the two nuclei, forming two daughter cells, each with its own separate nucleus. The position where this wall occurs determines the structure of these two cells since they are bound to each other and can’t longer migrate.
This division can be in either in the same cell layer, resulting in an elongation of this layer, or it can be divided up and down, making a new layer. The style of this determines the final structure of the tissue. The former method is called an anticlinal cell division, the latter is called a periclinal cell division. We can see the layer elongation (anticlinal cell division) mainly in a growth of a leaf, while the periclinal division is typical primarily for stem parts, which creates thicker tissues and increases the volume of the plant.
Let’s get back to the phloem. A cell that is going to become the phloem has a given set of instructions. Firstly, it has to go through the structured cell division. In this case, it is an imperfect division, where it results in one larger cell and one smaller cell. Then the larger cell goes through a partial apoptosis, where its nucleus and vacuole break down, leaving the cell with its cell membrane and cell wall, being still a partially viable cell. The second smaller cell becomes the companion cell, which provides the larger cell with all of its needs. This is the periclinal cell division since it creates cells next to each other. It is genetically programmed and necessary for the proper development of the phloem.
There is one more type of cell. They are embryonic cells called a meristem and they have a potential to form any other cell: the ground tissue, xylem, companion cell or the sieve member cell. The determinator in this decision is its positioning within the plant and the timing of its development. These cells can be found primarily at the tips, an apical meristem, found at the very tips of roots and shoots. That is the main area of the plant where the cell division and differentiation actually happens, and it is the place where the plant is growing from into both directions. The meristem cells are not differentiated until later in their development when they gain their final purpose in the complete plant physiology.