A Definition of Meristematic Tissue in Plants

In plant biology, the term "meristematic tissue" refers to the living tissues containing undifferentiated cells that are the building blocks of all specialized plant structures. The zone where these cells exist is known as the "meristem." This zone contains the cells that actively divide and create specialized structures such as the cambium layer, the buds of leaves and flowers, and the tips of roots and shoots. In essence, the cells within the meristematic tissues are what allow a plant to increase its length and girth. 

Meaning of the Term

The term "meristem" was coined in 1858 by Karl Wilhelm von Nägeli (1817 to 1891) in a book called Contributions to Scientific Botany. The term is adapted from the Greek word "merizein," meaning "to divide," a reference to the function of the cells in the meristematic tissue.

Characteristics of Meristematic Plant Tissue

The cells within the meristem have some unique characteristics:

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• Cells within the meristematic tissues are self-renewing, so that each time they divide, one cell remains identical to the parent while the other can specialize and become part of another plant structure. The meristematic tissue is therefore self-sustaining. 
• While other plant tissues can be made of both living and dead cells, the meristematic cells are all living and contain a large ratio of dense liquid.
• When a plant is injured, it is the undifferentiated meristematic cells that are responsible for healing the wounds through the process of becoming specialized. 

Types of Meristematic Tissue

There are three types of meristematic tissues, categorized according to where they appear in the plant: "apical" (at the tips), "intercalary" (at the middle), and "lateral" (at the sides).

The apical meristematic tissues are also known as "primary meristematic tissues," because these are what form the main body of the plant, allowing for vertical growth of stems, shoots, and roots. The primary meristem is what sends a plant's shoots reaching for the sky and the roots burrowing into the soil. 

Lateral meristems are known as "secondary meristematic tissues" because they are what is responsible for an increase in girth. The secondary meristematic tissue is what increases the diameter of tree trunks and branches, as well as the tissue that forms bark. 

Intercalary meristems occur only in plants that are monocots, a group that includes the grasses and bamboos. Intercalary tissues located at the nodes of these plants allow the stems to regrow. It is intercalary tissue that causes grass leaves to grow back so quickly after being mowed or grazed.  

Meristematic Tissue and Galls

Galls are abnormal growths occurring on the leaves, twigs, or branches of trees and other plants. They usually occur when any one of about 1500 species of insects and mites interact with meristematic tissues. 

Gall-making insects oviposit (lay their eggs) or feed on the meristematic tissues of host plants at critical moments. A gall-making wasp, for example, may lay eggs in plant tissues just as leaves are opening or shoots are lengthening. By interacting with the plant's meristematic tissue, the insect takes advantage of a period of active cell division to initiate the formation of a gall.

The walls of the gall structure are very strong, providing protection for the larvae feeding on plant tissues within. Galls can also be caused by bacteria or viruses infecting the meristematic tissues. Galls may be unsightly, even disfiguring, on stems and leaves of plants, but they rarely kill the plant.