Control and Regulation Patterns of Growth in Plants and Animals M r G D a v i d s o n

Control and Regulation

Patterns of Growth in Plants and Animals

Mr G

Mr G

David

son

David

son

Tuesday, April 18, 2023 G Davidson 2

Patterns of Growth

Growth is more than simply getting larger or heavier or having more cells.

If we compare ourselves now to when we were babies, many changes have occurred in shape and complexity , and these differences are the result of growth.


Patterns of Growth

Growth is, however, more than simply getting larger or heavier or having more cells, we can measure height or weight at various stages to record growth patterns, but it is much more difficult to count cells.


Patterns of Growth

We can measure height or weight at various stages to record growth patterns, but it is much more difficult to count cells.

This can be done for bacteria providing conditions that are right for growth.

In this way, we can work out how fast the population grows.


Patterns of Growth

However, for larger organisms, measuring growth is not easy as our weight varies quite substantially depending of the amount of water in our bodies.

It is therefore more sensible to measure the dry mass of an organism by removing all the water.

In this way, organisms are killed at various stages of growth and their dry mass is measured.


Patterns of Growth

This involves large numbers of organisms and is not a practical method for measuring human growth but it is often used to study plant growth.

““Growth is an irreversible Growth is an irreversible increase in the dry mass of an increase in the dry mass of an organism along with an increase organism along with an increase in cell numbers.”in cell numbers.”


Growth Curves

Where the dry mass is impossible to measure, variable factors are used, e.g. height and weight.

If these are measured over a period of time, a growth curve can be drawn.

A growth curve normally takes the shape of a sigmoid curve.


Sigmoid Curve

Accelerating growth

Dry

Mas

s

Time

Rapid steady growth

Decelerating growth

No growth


Growth Curves

The curve for the birch tree is different again but still follows the same pattern, just stacking up the curves.

Although each of the graphs is different, they all show an increase in growth as well as the sigmoid shape.


Time

Gro

wth

Birch Tree Growth Curve


Growth Curves

It should be realised that one of the differences between the growth of plants and animals is that plants continue to show an increase in growth until they die, whereas in animals, growth stops on reaching adulthood.


Growth Curve of a Sunflower

Time

Dry

Mass


Growth Curves

If we compare the growth curve of an insect with a human, the shape of the curve remains, but the insect loses weight every once in a while as it sheds its exoskeleton to allow further growth.

This process of moulting is called Ecdysis.


Plant Animal

Growth continues throughout life

Growth stops at adulthood

Plants can regenerate quite

extensively

Have limited powers of regeneration

Growth only occurs at growth points

called meristems

Growth occurs all over the body

Summary of Plant and animal growth


Plant Meristems

A meristem is a specific area within an organism where a group of cells are found that can divide to form new tissue.

Cells in these regions divide by mitosis and make daughter cells identical to the parent cells.

Plants have quite specific meristematic regions.


Plant Meristems

The cells in these regions are unspecialised (i.e. they have no set functions).

These cells can differentiate into a wide range of tissues

Animals have no meristems.


Primary Growth

Primary growth only occurs at the root and shoot tips called apical meristems, which results in an increase in length of the plant by the production of new cells as well as elongation of these new cells.


Primary Growth

Once the cells have elongated, they then differentiate, which is the process of an unspecialised cell being altered and adapted to become part of the permanent tissue of the plant, e.g. into root hair, xylem or phloem etc.


Region of Mitosis

Region of Differentiation

Region of Elongation

Root Cap


Secondary Growth

Secondary growth is an increase in thickness or girth of a plant in order to support itself.

This is caused by lateral meristems – cambium and cork cambium.


Secondary GrowthThe cambium is a layer of cells found

between the xylem and phloem in the vascular bundles.

Transverse section through young tree twig

Bark

CortexPhloem

CambiumXylem

Pith

Cork Cambium


Secondary Growth

As the cambium divides, it forms a ring in the stem and makes new xylem and phloem tissue called secondary tissue.

The secondary xylem is formed inside the cambium and the secondary phloem on the outside.


Secondary Growth

Each year of growth sees a new secondary xylem which results in annual rings in trees.

The secondary phloem just gets larger each year as the girth of the tree increases.

As the stem gets thicker, there is not only a need for water transport up the plant, but also, across the stem, and this is achieved by the medullary rays.


Secondary Growth

These consist of rows of parenchyma (unspecialised cells) which come from the cambium.

The cork cambium is another ring of cells, this time, around the outside of the stem.

Its function is to make new cortex cells on the inside as well as dead cortex cells on the outside which make up the bark of the tree.


Annual Rings

If we study the cross section of a fallen tree, we see a series of annual rings.

These result from the cambium producing large xylem vessels in Spring which transport large volumes of water, which are required for the new growth.


Annual Rings


Annual Rings

One year’s growth

This tree is 13 years old


Annual Rings

In the spring the cambium is very active and produced large xylem vessels , while in Autumn much smaller xylem vessels are produced.

In winter, there is no growth, and the following year, the pattern begins again. Spring

Wood

Autumn Wood


Annual Rings

A closer study of the rings reveals that some are wider than others and this can tell us a little about the weather of the year.e.g. a wide ring indicates either warm or

wet weather when growth conditions are optimum.


Regeneration in Plants and Animals

Although an animal may stop growing, on reaching maturity, cell division doesn’t cease.

In plants and animals, new cells are needed to replace dead cells.

Regeneration is the replacement by growth of the parts of an organism which have been lost.



Plants and animals have different powers of regeneration, e.g. if you prune a hedge, it grows again

in a matter of weeks, however, if you ‘prune’ a dog, it doesn’t grow a new head.

Market gardeners use this power of regeneration to increase their supplies of certain plants by taking cuttings, grafting etc.



Tissue culturing is a fairly modern technique used to mass produce plants.

This involves the removal of a few cells and allowing them to reproduce in a tissue culture.



This means thousands of plants can be grown from one.

The main disadvantage of this is that all the new plants are genetically identical and are called clones.



Regeneration in animals is limited, but in some animals, it can be very extensive, e.g. starfish.

In mammals, however, it is restricted to healing and replacement of damaged tissue (e.g. blood, bone, etc.)

Regeneration depends of the degree of specialisation of the cells.



The more highly specialised, the lower the powers of regeneration. e.g. nerve cells are highly specialised

and cannot regenerate, however, liver cells are not very specialised which makes their regenerative powers much higher.

Documents

Control and Regulation Patterns of Growth in Plants and Animals M r G D a v i d s o n