Parenchyma is a type of simple permanent tissue that makes a major part of ground tissues in plants, where other tissues like vascular tissues are embedded. They are non-vascular and composed of simple, living and undifferentiated cells, which are modified to perform various functions. The other simple permanent tissues are:

Collenchyma
Sclerenchyma
The main characteristics of parenchyma are:

1.They are living permanent tissues that have the ability to divide at maturity and help in the regeneration and healing of wounds
2.Parenchyma cells are the foundation of a plant as reproductive cells (spores, gametes) are parenchymatous in nature
3.Single parenchyma cell of a zygote has an ability to develop into an entire plant. These cells are called “totipotent” cells
4.Parenchyma cells occur in the form of continuous masses as homogeneous parenchyma tissues e.g. in pith and cortex of stems and roots, mesophyll of leaves, the flesh of succulent fruits and in the endosperm of seeds
5.Parenchyma cells may be associated with other types of cells to form heterogeneous complex tissues such as parenchyma of xylem and phloem
6.Parenchyma cells are essential for activities like photosynthesis, storage, secretion, assimilation,
respiration, excretion and radial transport of water and solute
Structure of Parenchyma Cells
1.It is a living cell
2.It has a prominent nucleus and protoplast
3.The cells of parenchyma are isodiametric or polyhedral in shape. They may be polygonal, oval, round or elongated
4.These cells are closely packed or may have small intercellular space
5.They are made up of thin cell wall made up of cellulose, hemicellulose
6.Plasmodesmata join the cells of the parenchyma tissue
7.They have several small vacuoles. In the older parenchyma, smaller vacuoles merge to become a large central vacuole, which may accumulate anthocyanin or tannins
8.Water is abundant in the vacuoles of the parenchyma cells that act as a water reservoir
9.Storage parenchyma cells may have thick xyloglucan walls e.g. in the endosperm of the date palm. The sugar is used during germination and walls become thin
10.The parenchyma cells of flowers and fruits contain chromoplasts
11.Parenchyma cells may have a thick lignified wall that makes it difficult to differentiate it from sclerenchyma
12.Hydraulic property of cells gives the parenchyma its mechanical strength
13.Chloroplasts are present in the parenchyma cells that are specified to perform photosynthesis
14.The parenchyma cells which perform a secretory function, have dense protoplasm that is rich in ribosomes, Golgi bodies and a highly developed endoplasmic reticulum
Types of Parenchyma Cells
Parenchyma cells can be categorised based on their structure, location and functions performed. The main parenchyma tissues are:
Chlorenchyma: Cells which have chloroplast and perform photosynthesis
The mesophyll cells in leaves which differentiate into palisade and spongy cells
In the other green parts of the plants like stems, sepal etc.
Transfer Cells: They play an important role in the transport of solutes over short distances.
They have cell wall ingrowths, which greatly increase the surface area of the plasma membrane
Sucrose is transported across the membrane through a proton/sucrose co-transport mechanism
These are found in the areas of absorption and secretion in plants like nectaries, salt glands and in carnivorous plants
Vascular Parenchyma: The parenchyma cells which are associated with vascular tissues. These are of two types:
Phloem Parenchyma: It is made up of elongated, tapering and cylindrical cells having dense cytoplasm.
Plasmodesmata connections occur between the cells through pits in the walls
It stores food and other materials like resins, latex and mucilage
Absent in monocotyledons
Xylem Parenchyma: It is made up of thin-walled cells. The cell wall is made up of cellulose.
It stores food materials like starch, fats and other substances such as tannins and crystals
Radial conduction of water takes place by ray parenchymatous cells
In the water-stress condition, they help in preventing damage to tracheids and vessels