Which of the following is not a type of primary meristematic cell found in apical meristems?

A. Protoderm.

B. Procambium.

C. Ground meristem.

D. Vascular cambium.

Photo of the apical meristem of an onion root tip
Photo of the apical meristem of an onion root tip

The correct answer is D. Vascular cambium.

The tissues of the plant responsible for development are known as meristems. These meristematic cells are small in size and divide often to produce different tissues of the plant body.

There are three main types of meristems that plants have: apical, lateral and intercalary. The apical tissue is also known as primary meristematic cells because they give rise to the primary tissues and organs of the plant.

The primary meristems can be divided into three different kinds depending on what tissues they give rise to. The ground meristem differentiates to form the ground tissues of the stem and root, while the protoderm produces the cells that form the epidermal layer.

The procambium is the meristematic tissue that differentiates to produce the original primary phloem and xylem, the vascular tissue of plants.

Primary meristems are the first cells to divide to form the tissues and organs. Later, the lateral meristems can become active to produce secondary tissue.

The lateral meristems are responsible for an increase in width or girth of a plant. Vascular cambium and cork cambium are the two types of lateral meristematic tissue that can be found in some types of plants, especially in dicots.

The vascular cambium produces secondary xylem and phloem which form rings of tissue. The cork cambium produces cork cells which combine with phloem to form bark.

Intercalary meristems enable leaves to increase in length and are particularly common in monocots such as grasses.

Meristematic  tissue

The growth of the plant body depends on the activity of tissues that are meristematic. That is to say, they actively divide to produce new cells for tissue and organ development.

These cells that actively divide are known as meristems and they are generally small in size and contain many organelles. The meristematic cells are undifferentiated and can differentiate into the various tissue types needed by the plant.

Meristematic tissues are classified based on where they are located in the plant body. There are apical, lateral and intercalary meristems which are regions of tissue that are found in different parts of the plant that divide to produce various organs.

The apical meristems are the tissues located at the growing tips of the developing root and shoot of the plant. These are the primary meristematic tissues since they give rise to the primary tissues and organs. They are the meristems that produce an increase in length or height of the plant.

The lateral meristems are found in the vascular tissue and they are responsible for lateral growth leading to an increase in width or girth of the plant body.

Primary meristems are the first cells to divide and differentiate in the developing plant body. Lateral meristem activity occurs later on once the plant has developed to some extent.

Intercalary meristems

Intercalary meristems are found in the stem of the plant where leaves form at leaf nodes. They are particularly common in monocotyledonous plants such as grasses, and they enable leaves to increase in length.

Cells arranged in parallel rows within the meristem divide to bring about an increase in growth in one direction only.

The intercalary meristem is also called a basal meristem since it is at the base of the leaf. An example of the activity of this meristem is the continual growth of grass blades even after repeatedly being mowed.

Primary meristems

These meristematic cells divide to form the primary tissues of the body of the plant. There are three types of primary or apical meristems that occur in plants.

The protoderm consists of the cells that produce the outermost tissue layer, the epidermis, while the procambium gives rise to the vascular tissue of the plant. The third type of primary meristem is the ground meristem which produces all the ground tissues of the plant.

In the flowering plants, the shoot apical meristem (SAM) changes from the vegetative function to one of reproduction when flower formation is needed. A signal molecule known as florigen is responsible for this shift in the meristem when the time is right.

When most of the plant growth is at the tip of the shoot a condition of apical dominance occurs. This prevents axillary bud development from occurring and if the apical bud is removed then lateral branch formation from axillary buds occurs.

Such apical dominance is typical of gymnosperms such as coniferous trees, resulting in very tall trees being produced.

Lateral meristems

The two types of lateral meristems are the vascular cambium and cork cambium. The vascular cambium allows for the development of secondary xylem and phloem, which are both vascular tissues. The cork cambium forms the cork of plants, which really becomes the bark of trees.

These tissues divide to enable the plant to grow laterally, and to increase in width. Such secondary growth is relatively common in dicotyledonous plants but is less common in the monocots.

Secondary growth results in the formation of wood and bark, which are tissues produced from the vascular and cork cambium, respectively.

Vascular cambium is meristematic tissue that occurs between the primary xylem and primary phloem of the plant. The xylem and phloem are known as vascular tissue because they transport water and sugars through the plant body.

The annual growth rings evident in a tree trunk are actually the result of the activity of the vascular cambium. The climate influences the size of the cells that are formed each year so tree rings can give not only a record of how old a tree is but also how the climate has changed.

Cork cambium is more laterally situated compared with the vascular cambium and it is responsible for producing the cork cells which can then form bark along with the phloem cells.

References

  1. S Kerr (2018). Plant Development II: Primary and Secondary Growth. Retrieved from gatech.edu.
  2. Y Machida, H Fukaki, T Araki (2013). Plant meristems and organogenesis: the new era of plant developmental research. Plant & Cell Physiology.
  3. BR Speer (1998). Tissues. Retrieved from Berkeley.edu.
  4. J Heslop-Harrison (2019). Plant development. Retrieved from Encyclopedia Britannica.
  5. Editors of Encyclopedia Britannica (2019). Meristem. Retrieved from Encyclopedia Britannica.

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