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Absorption of Water and Minerals Salts

   

   

Absorption of Water and Minerals Salts

Transport is the movement of substances such as water, nutrients, respiratory gases and waste products of metabolism from one part of an organism to another, or out of the body. Animals take in food materials necessary for their metabolic processes. These food materials need to be moved to the cells where they are required. The animals give out waste products from the various metabolic activities. These need to be moved to organs of excretion for elimination from the body.
Small organisms such as amoeba have a large surface area to volume ratio, hence transport is by simple diffusion. On the other hand many multicellular organisms such as man have a small surface area to volume ratio hence materials have to move over long distances to cells located deep within the organism. Such organisms need an elaborate transport system.
Roots play an important role in transport. Different plants have different types of roots.
A fibrous root system
Buttress root
Prop roots
Mangroves: Have breathing roots that sprout above the water in which they grow.
Breathing roots of mangroves
Roots
In addition to transport, roots also have the following functions:
Anchorage,
that is, they hold the plant firmly in the soil.
Storage, for example in root tubers such as carrot and arrow root. Others store water for the plant.
Gaseous exchange -
Absorption of water and mineral salts.
The graphic on next plate shows a generalised longitudinal section through a root tip of a dicot. Apical meristem cells are located behind the root cap and actively divide to give rise to new cells that are responsible for the elongation of the root. The root cap consists of simple parenchyma cells that protect the delicate apical meristem cells from mechanical damage.

You might have observed the following differences:
The monocot root has a pith at center.Phloem and xylem alternate around the pith.

The dicot root lacks a pith. Xylem forms a star shape and phloem occur between extentions of xylem.
The root cap consists of simple parenchyma cells that protect the delicate apical meristem cells from mechanical damage. Behind the root cap is the apical meristem cells which actively divide to give rise to new cells that are responsible for growth and elongation of the root.
Internal Structure of Roots
In a dicotyledonous root the vascular tissue has the xylem at the centre with phloem occurring between the extensions of the xylem.
In a dicotyledonous root the vascular tissue has the xylem at the centre with phloem occurring between the extensions of the xylem.

Notice the following parts:

Piliferous layer: Cells of this layer are thin walled for efficient passage of water and mineral salts. On maturity it is replaced by a less permeable epidermis.

Cortex: is made of loosely parked thin walled parenchyma cells. Acts as a storage tissue. Water molecules pass through it to the xylem.

Endodermis: Is made of a single layer of cells surrounding the vascular bundle that is made of xylem and phloem. The endodermis controls the amount of water entering the xylem.

Pericycle: Is made of a single layer of cells giving rise to lateral roots.
Vascular bundles: Occupy the central position. Each bundle consists of xylem and phloem.

Root hairs: microscopic outgrowths on the piliferous layer. These are thin walled. They are numerous in number, hence provide a large surface area for absorption of water and mineral salts.
Note that the monocotyledonous root has pith at the centre and that the phloem and xylem alternate around the pith.
The piliferous layer cells are thin walled for efficient passage of water and mineral salts. On maturity the piliferous layer is replaced by a less permeable epidermis.
Endodermis: This is a single layer of cells surrounding the vascular bundles. It controls the amount of water entering the xylem.

Pericycle: This is a single layer of cells giving rise to lateral roots.Vascular bundles: These occupy the central position.Each bundle consists of xylem and phloem tissues.

Root hairs

These are microscopic outgrowths on the piliferous layer.

Thy are thin walled and numerous, hence provide a large surface area for absorption of water and mineral salts.
The illustration shows the structure of a root hair cell.
The Stem
Sugar cane
Cactus
A woody stem

In this lesson, you will learn about the structure and role of the stem. The illustrations below show transverse sections through dicot and monocot stems. Compare the monocot and dicot stems shown.
You might have noticed that in dicots a pith and cambium are present. Vascular bundles arranged in a circle.In monocot stems vascular bundles are scattered and pith and cambium are absent.

 

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