Carissa Fletcher

Plant structure and growth

Carissa Fletcher

Draw and label plan diagrams to showthe distribution of tissues in the stemand leaf of a dicotyledonous plant.

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Structure Function

Cuticle Waterproof layer

Epidermis Transparent but protective layer.

Xylem (vascular bundle) Support and water transport

Phloem (vasular bundle) Products of photosynthesis transported by this tissue.

Sclerenchyma These are dead cells with a large component of lignin – again for structure.

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Plant classification

All plants on earth have been classified into the following phylum;

• bryophyta,

• filicinophyta, coniferophyta and

• angiospermophyta.

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Angiosperms or flowering plants have traditionally been classified into two further groups; dicots and monocots (these

are some examples of the morphological differences).

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Explain the relationship between thedistribution of tissues in the leaf and

the functions of these tissues.

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The epidermis is the outer layer of cells covering the leaf.

The epidermis serves several functions:

1. protection against water loss, regulation of gas exchange,

2. secretion of metabolic compounds,

3. (in some species) absorption of water

Epidermis tissue contains; epidermal cells, guard cells, subsidiary cells, and epidermal hairs (trichomes).

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The epidermis is covered with pores called stomata, part of a stoma complex consisting of a pore surrounded on each side by chloroplast-containing guard cells, and two to four subsidiary cells that lack chloroplasts. The stoma complex regulates the exchange of gases and water vapour between the outside air and the interior of the leaf. Typically, the stomata are more numerous over the (lower) epidermis than the (upper) epidermis.

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- The cylindrical shape of palisade cells allows a large amount of light to be absorbed by the chloroplasts.

- Beneath the palisade mesophyll are the spongy mesophyll cells, irregularly-shaped cells that having many intercellular spaces to allow the passage of gases, such as the intake of carbon dioxide for photosynthesis to take place.

- positioned towards the upper surface of the leaf and contain the largest number of chloroplasts per cell in plants.

- they have a very large surface area in order for them to absorb more light during photosynthesis

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Midrib Many dicots also have a large

midrib. This contains the vascular bundle of xylem and phloem tissue.

VeinsA small network of veins is found

throughout the leaf. These ensure that no cell is far from a xylem phloem vessel. All cells have a constant supply of water and can have their sugars removed after photosynthesis.

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Roots and shoots

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Modifications of plant structures

A bulb is an underground vertical shoot that has modified leaves (or thickened leaf bases) that are used as food storage organs by a dormant plant.

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Tubers are various types of modified plant structures that are enlarged to store nutrients.

They are used by plants to survive the winter or dry months and provide energy and nutrients for re growth during the next growing season

They are a means of asexual reproduction

Two different groups of tubers are: stem tubers, and root tubers.

A stem tuber forms from thickened rhizomes. The tops or sides of the tuber produce shoots that grow into

typical stems and leaves and the under sides produce roots.

They tend to form at the sides of the parent plant and are

most often located near the soil surface. E.g. a potato

A tuberous root or storage root, is a modified lateral root. enlarged to function as a storage organ.

The enlarged area of the root-tuber, or storage root, can be produced at the end, or middle of a root, or involve the entire root.

It is thus different in origin but similar in function and appearance to a stem tuber. Examples of plants with notable tuberous roots include the sweet potato, cassava.

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A Tendril is a specialised stem, leaf or petiole with a threadlike shape that is used by climbing plants for support and attachment, generally by twining around whatever it touches. They can be formed from modified shoots, modified leaves, or auxiliary branches

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Plant growth and meristems

• Plants have regions of cells called meristems.

• These are undifferentiated embroyonic tissue that is plueripotent cells

• In dicotyledons there are apical meristems. These are growing tips that will form first of all a root and a shoot. There are also auxillary that form from leaf axials.

• The region 1 in the photograph on the left is the apical meristem

Meristems are analogous with STEM CELLS in humans.

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The vascular cambium is a lateral meristem (causing the diameter of the plant to grow) in the vascular tissue of plants. The vascular cambium is the source of both the secondary xylem (inwards, towards the pith) and the secondary phloem (outwards), and is located between these tissues in the stem and root.

The vascular cambium is a type of meristem - tissue consisting of embryonic (incompletely differentiated) cells from which other (more differentiated) plant tissues originate.

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AuxinsAuxins are a class of plant growth

substances or plant hormone.

Auxins have an essential role in coordination of many growth and behavioral processes in the plant life cycle.

They typically act in concert with, or in opposition to other plant hormones.

For example, the ratio of auxin to cytokinin in certain plant tissues determines initiation of root versus shoot buds.

Thus a plant can (as a whole) react to external conditions and adjust to them, without requiring a nervous system.

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Phototropism

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The role of auxin in phototropismPhototropism is directional growth in which the direction of growth is

determined by the direction of the light source. In other words, it is the growth and response to a light stimulus.

The cells on the plant that are farthest from the light have a chemical called auxin that reacts when phototropism occurs.

This causes the plant to have elongated cells on the farthest side from the light.

Phototropism in plants such as Arabidopsis thaliana is directed by blue light receptors called phototropins

Other photosensitive receptors in plants include phytochromes that sense red light.

Auxins have many roles but in this respect, auxins are responsible for expelling protons (by activating proton pumps) which decreases pH in the cells on the dark side of the plant.

This acidification of the cell wall region activates enzymes known as expansins which break bonds in the cell wall structure, making the cell walls less rigid.

In addition, the acidic environment causes disruption of hydrogen bonds in the cellulose that makes up the cell wall.

The decrease in cell wall strength causes cells to swell, exerting the mechanical pressure that drives phototropic movement.