Which tissue acts as a filter on the water absorbed by root hairs?

A. Endodermis.

B. Vascular tissue.

C. Cortex.

E. Epidermis.

Diagram showing the location of the endodermis in Solanum tuberosum
Diagram showing the location of the endodermis in Solanum tuberosum

The correct answer is A. Endodermis

The root of a plant is very important in enabling the plant to remain firmly attached in the soil and to obtain water and dissolved minerals. While dicots tend to have a main root (a tap root), monocots have a fibrous root system in which several branching roots are present.

During the germination of the seed, the root is formed from the radicle, and the primary meristem gives rise to all the different tissues of the root. The procambium produces the vascular tissue, the ground meristem produces the cortex and the protoderm gives rise to the epidermis.

Epidermal cells are thin-walled and often form elongated root hairs to take up more minerals and water from the soil. The water moves through the parenchyma cells until it reaches the endodermis.

The endodermis contains cells that have a waxy layer known as the Casparian strip which directs the flow of water through the cells and into the vascular cylinder.

The vascular tissue consists of xylem and phloem which is at the center of the root where it forms the stele and is slightly differently arranged depending on if the plant is a monocot or dicot.

There are also many adaptations of roots to enable plants to take up water from the atmosphere, to parasitize other plants, or to store vast quantities of food.

The root

The plant root is important in providing support and anchoring the plant into the soil. It also absorbs water and minerals which are needed for the plant to stay alive.

Dicotyledenous plants have a tap root system in which there is one main root with a few smaller lateral roots. Monocotyledons, by comparison, have a fibrous root system in which there are several roots all about the same size and length.

The taproot system enables nutrients and water to be absorbed from deep under the surface of the soil, while a fibrous root system provides a large surface area close to the soil surface.

The root develops from the radicle of the seed and it has a gravitropic response, growing downwards into the soil.

There is an apical meristem present that consists of actively dividing cells that divide to give rise to all the tissues of the developing root. The protoderm produces the epidermis, procambium, the vascular tissue and ground meristem, the ground tissue or cortex of the root.

Epidermis to endodermis

The epidermal cells of the root have very thin walls and they often develop into extensions known as root hairs. These hairs increase the surface area for absorption and uptake of water and nutrients from the soil.

Various ions in the epidermal cells help to establish an osmotic gradient so that water can move into the cells by osmosis. The dissolved minerals and water that enter through the root hairs move through the cortex of the root.

The cortex consists of parenchyma cells that provide support for the cell and are also rather thin-walled. Food is stored in these cells after being formed by photosynthesis in other cells of the plant.

The innermost layer of cells found in the root cortex is known as the endodermis. These cells all have a layer of manly suberin and sometimes lignin material in the cell wall known as the Casparian strip.  This layer also blocks pathogens and toxins from entering the vascular cells of the root.

The Casparian strip is believed to control how the water moves through the cells and into the root vascular tissue. It forces the water to move through the plasma membrane and not along the sides and around the cells.

Vascular tissue

The vascular tissue consists of xylem and phloem cells which transport water, dissolved nutrients and sugars. Xylem cells form long tubes that transport water up the plant where the molecules can be used in photosynthesis.

Phloem cells transport sugars produced during photosynthesis to different regions of the plant body. The phloem tissue is also made up of elongated cells arranged in tubes.

The vascular tissue forms a cylinder in the center of the root. There is also a layer of cells surrounding this tissue which is called the pericycle, and which produces lateral roots for the plant.

In dicots, the vascular cells often form a cross shape while in monocots the cells are arranged in a ring. In a mature plant, secondary tissue is often produced from the lateral meristem. This is usually in the form of secondary xylem and phloem.

Root adaptations

There are many different types of root adaptations present in the plant kingdom, which have allowed plants to colonize an assortment of different habitats and climatic conditions.

In some plants, the taproot (main root formed from the radicle), has become modified to store food.  Turnips, beets, and carrots are good examples of a root that has become modified for food storage.

Some parasitic plants have haustoria, which are modified roots that can embed into another plant and enter the vascular tissue. In this way, the parasite is able to directly get nutrients from its host.

Pneumatophores are specialized roots that are formed in mangroves. These plants grow in estuary muds which are high in salt and low in oxygen. Thus, these lateral roots, the pneumatophores, are designed to allow the mangrove to take up more oxygen.

Epiphytic plants such as orchids produce aerial roots that are able to attach to tree branches or even rock surfaces. Orchid roots also develop spongy tissue to help take in moisture for the plant.

Some plants have stems which produce adventitious roots. These provide added support for the plant as it grows larger and bigger.

References

  1. Editors of Encyclopedia Britannica (2019). Cortex. Retrieved from Encyclopedia Britannica.
  2. Editors of Encyclopedia Britannica (2019). Root. Retrieved from Encyclopedia Britannica.
  3. MJ Farabee (2007). Plant anatomy. Retrieved from estrellamountain.edu.
  4. PH Raven, RF Evert, SE Eichhorn (1987). Biology of Plants. Worth Publishers.
  5. S Naseer, Y Lee, Y, et al. (2012). Casparian strip diffusion barrier in Arabidopsis is made of a lignin polymer without suberin. Proceedings of the National Academy of Sciences.

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