Form 1:
Nutrition in Plants and Animals

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Leaf structure

External Leaf structure

Externally, the leaf has a petiole through which it attaches to the leaf branch or stem, lamina- the broad flat surface, margin- the outline and the leaf apex.

Image:External leaf structure

The size of a leaf depends on its environment. Plants in arid areas have small sized leaves with some leaves reduced to needle like shape. This helps reduce the rate of water loss in such plants.

However, the plants in areas of water abundance have broad leaves to enable them lose the excess water.

Internal leaf structure.

a) Cuticle

This is the outermost layer of the leaf.

It is a thin non-cellular, waxy, transparent and waterproof layers that coats the upper and lower leaf surfaces.

Functions of the cuticle

a) Being waterproof, it minimizes water loss from the leaf cells to the environment through transpiration and evaporation.

b) It protects the inner leaf tissues from mechanical damage.

c) It prevents entry of pathogenic microorganisms into the leaf.

Image:Internal leaf structure

b) Epidermis

This is the outermost one cell thick layer covering upper and lower leaf surfaces. Its cells are flattened and lack chloroplasts.

Functions of the epidermis:

a) It protects the leaf from mechanical damage.

b) It also protects the leaf from entry of disease-causing micro-organisms.

c) It secretes the cuticle.

There are many small pores on the epidermis known as stomata (singular-stoma) through which exchange of materials occur. The opening and closing of the stomata is controlled by the guard cells. Each stoma is controlled by two guard cells.

The guard cells have chloroplasts and are bean shaped. They have thicker inner cell wall and thinner outer cell wall.

Image:Guard Cell

Adaptations of the guard cells

They have differentially thicker walls to enable them bulge as they draw water through osmosis from the neighboring cells making them to open the stomata.

They contain chloroplasts that manufacture sugars which increase osmotic pressure of the guard cells. As they draw water through osmosis, they bulge making the stomata to open.

c) Palisade mesophyll

This is the chief photosynthetic tissue in plants. Its cells are regular in shape.

Its cells contain numerous chloroplasts for photosynthesis.

Their close packing and location just below the epidermis enables them to trap maximum sunlight for photosynthesis.

Location of palisade layer on the upper surface explains why upper leaf surfaces are greener than the lower surfaces.

Image:palisade tissue

d) Spongy mesophyll layer

This layer contains loosely arranged irregular cells. This leaves large airspaces between the cells which permits free circulation of gases carbon (IV) oxide and oxygen into the photosynthetic cells. Spongy mesophyll cells contain fewer chloroplasts compared to palisade cells.

e) Vascular bundle/tissue

This is found in the midrib and leaf veins. Vascular bundle is made of phloem and xylem tissues.

Xylem tissues conduct water and some dissolved mineral salts from the roots to other plant parts while phloem translocates manufactured food materials from photosynthetic areas to other plant parts.

Chloroplast

This is the organelle in which photosynthesis takes place. It is an oval shaped double membrane bound organelle.

Internally, it is made up of membranes called lamellae suspended in a fluid filled matrix called stroma.

Lamellae forms stacks at intervals called grana (singular-granum). Chlorophyll molecules are contained in the grana.

Within the stroma, fat droplets, lipid droplets and starch grains are found.

The stroma contains enzymes and forms the site where light independent reactions take place.


 
Image:Chloroplast
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