All plant cells are characterized by several common features, such as chloroplasts, a cell wall, and a large vacuole. These features also differentiate plant cells from animal cells. In addition, there is a number of specialized types of plant cells that are found only in vascular plants. These include parenchyma cells, collenchyma cells, and sclerenchyma cells.
1. Parenchyma cells
The term parenchyma is derived from the Greek word para, which means “beside”, and en + chein, meaning “to pour in”. Parenchyma cells are the biochemistry machines of the plant. They are usually described as the typical plant cell, because they are not very specialized. These cells are commonly found in leaves, roots, and stems, usually having a spherical shape with only primary cell walls and highly functional cytoplasm.
Parenchyma cells play an important role in food storage, photosynthesis, and aerobic respiration, meaning that they synthesize and store organic products in the plant and are the place where most of the plant’s metabolism takes place. These cells are alive at maturity and are responsible for a wide range of biochemical processes. Most parenchyma cells have the ability to differentiate into other cell types under special conditions
2. Collenchyma cells
The term collenchyma is derived from the Greek word kola, meaning “glue”. Collenchyma cells have a support function in plants, particularly in young plants. They help to support plants while not restraining growth due to their lack of secondary walls and the absence of a hardening agent in their primary walls. This means that their main function is to provide support for parts of the plant that are still growing, such as the stem.
Collenchyma cells form strands or continuous cylinders just below the surfaces of stems or leaf stalks. They are similar to parenchyma cells in the sense that they are alive at maturity. To better understand what collenchyma cells do, imagine a stretchable support, without elastic snap-back.
3. Sclerenchyma cells
The term sclerenchyma is derived from the Greek word skleros, which means “hard”. Sclerenchyma cells are the ones providing rigid support for the plant, because they have a hardening agent and are much more rigid. These cells develop an extensive secondary wall that is invested with lignin (the main chemical component of wood), making it extremely hard. Besides support, sclerenchyma cells have other functions, which include discouraging herbivory, and conduction.
There are two types of sclerenchyma cells – fiber and sclereid. Fiber cells are long, slender cells that usually form strands or bundles. Sclereid, also known as stone cells, occur singly or in groups and have various forms. Most sclerenchyma cells are dead cells at maturity. Sclerenchyma cells cannot survive for long because lignin also makes the wall waterproof, therefore making it impossible for the cells to exchange materials long enough for active metabolism.
These are the main and most important types of specialized plant cells. Other specialized plant cell types include xylem, phloem, and epidermis cells.