Only One Kind of Plant Cell Is Allowed to Divide

Because only meristematic cells can divide. Most plant cells lose that ability once they mature, so growth in length happens where the dividing cells are — at the tips of roots and shoots.

That is why a nail hammered into a tree trunk stays at the same height for years. This page covers everything in the ICSE Class 7 Biology chapter's first part: the levels of organisation, meristematic and permanent tissues, the simple permanent tissues, and xylem and phloem.

A multicellular body is built up in stages, each level made of the one below it:

cell, then tissue, then organ, then organ system, then organism

A cell is the basic structural and functional unit of life. A tissue is a group of similar cells with a common origin, working together to perform a specific function. An organ is made of several different tissues working together, an organ system is a group of organs performing a major function, and the organism is the complete living body.

In a plant: cells form the tissue xylem, xylem and other tissues together form the organ we call a leaf, and the leaves, stem and roots together form the shoot and root systems.

In a human body: muscle cells form muscle tissue, which forms the stomach, which is part of the digestive system.

The reason this division of labour exists is efficiency. In a single-celled organism one cell must do everything; in a multicellular body each tissue specialises, so it can do its own job far better.

The word that carries the mark is similar. A tissue is made of similar cells, while an organ contains several different tissues — which is why a leaf is an organ and not a tissue.

Meristematic tissue consists of young, actively dividing cells with thin walls, dense cytoplasm, prominent nuclei and no vacuoles or intercellular spaces. Permanent tissue is made of mature cells that have lost the power of division and taken on a fixed shape and function.

There are three meristems, named by position:

- Apical meristem — at the tips of roots and stems. It causes growth in length, called primary growth.
- Intercalary meristem — at the base of leaves or internodes, as in grasses and sugarcane. It also adds length, which is why a lawn regrows quickly after mowing and grazed grass recovers.
- Lateral meristem (cambium) — arranged in a cylinder along the sides of the stem and root. It causes growth in girth, making a tree trunk thicker year by year.

Pruning the tip of a money plant to make it bushy works because the apical meristem is removed, so the side branches take over.

The absence of vacuoles is the detail most often left out of a comparison answer, and it explains the rest: a dividing cell needs its cytoplasm packed with material, while a mature cell stores water and food in a large central vacuole instead.

These are the three simple permanent tissues — simple because each is made of only one type of cell. They differ mainly in their cell walls.

Parenchyma — thin-walled living cells of cellulose, loosely packed with large intercellular spaces. Found in the soft parts: the cortex and pith of stems, and the mesophyll of leaves. It stores food and water and fills space.

Two modified forms are examined:

- Chlorenchyma — parenchyma containing chloroplasts, found in leaves, which carries out photosynthesis.
- Aerenchyma — parenchyma with large air cavities, found in aquatic plants such as the water hyacinth and lotus, which gives buoyancy so the plant floats.

Collenchyma — living cells with walls thickened at the corners with cellulose and pectin, and little intercellular space. Found just below the epidermis of young stems and in leaf stalks. It gives flexible strength, letting a stem bend in the wind without snapping.

Sclerenchyma — dead cells with walls thickened all round by lignin, with no intercellular space. Found in the husk of a coconut, the shell of a nut, the stony layer of a guava and the veins of leaves. It gives rigid strength and hardness.

The coconut husk and the jute fibre used for gunny bags are both sclerenchyma, which is why they are tough and long-lasting.

The distinguishing test is worth fixing: sclerenchyma is dead, the other two are alive. A tissue made of dead cells can still be essential — it is the plant's structural skeleton.

Xylem conducts water and dissolved minerals upward from the roots to the leaves. Phloem conducts prepared food from the leaves to all other parts of the plant — upward and downward.

Both are complex permanent tissues, meaning each is made of more than one type of cell working together.

Xylem has four elements:

- Tracheids — dead, tapering cells that conduct water.
- Vessels — dead, long tube-like cells joined end to end, the main water pipeline.
- Xylem fibres — dead cells giving mechanical support.
- Xylem parenchyma — the only living element, which stores food.

Phloem has four elements:

- Sieve tubes — long cells with perforated end walls, through which food flows.
- Companion cells — living cells that assist the sieve tubes.
- Phloem fibres — dead cells giving support.
- Phloem parenchyma — living cells that store food.

The timber of a wooden door is mostly old xylem, and the sweet sap tapped from a palm travels in phloem.

The contrast to remember is which tissue is mostly alive. Xylem is mostly dead, with only its parenchyma living, so water moves through hollow dead tubes. Phloem is mostly living, with only its fibres dead, because moving food requires active cells. And note the direction: xylem transport is one-way upward, while phloem transport goes both ways.