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

Thursday, May 21, 2015G Davidson2 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.

Thursday, May 21, 2015G Davidson3 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.

Thursday, May 21, 2015G Davidson4 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.

Thursday, May 21, 2015G Davidson5 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.

Thursday, May 21, 2015G Davidson6 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 increase in the dry mass of an organism along with an increase in cell numbers.”

Thursday, May 21, 2015G Davidson7 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.

Thursday, May 21, 2015G Davidson8 Sigmoid Curve Accelerating growth Dry Mass Time Rapid steady growth Decelerating growth No growth

Thursday, May 21, 2015G Davidson9 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.

Thursday, May 21, 2015G Davidson10 Time Growth Birch Tree Growth Curve

Thursday, May 21, 2015G Davidson11 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.

Thursday, May 21, 2015G Davidson12 Growth Curve of a Sunflower Time Dry Mass

Thursday, May 21, 2015G Davidson13 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.

Thursday, May 21, 2015G Davidson14 PlantAnimal 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

Thursday, May 21, 2015G Davidson15 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.

Thursday, May 21, 2015G Davidson16 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.

Thursday, May 21, 2015G Davidson17 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.

Thursday, May 21, 2015G Davidson18 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.

Thursday, May 21, 2015G Davidson19 Region of Mitosis Region of Differentiation Region of Elongation Root Cap

Thursday, May 21, 2015G Davidson20 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.

Thursday, May 21, 2015G Davidson21 Secondary Growth  The cambium is a layer of cells found between the xylem and phloem in the vascular bundles. Transverse section through young tree twig Bark Cortex Phloem Cambium Xylem Pith Cork Cambium

Thursday, May 21, 2015G Davidson22 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.

Thursday, May 21, 2015G Davidson23 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.

Thursday, May 21, 2015G Davidson24 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.

Thursday, May 21, 2015G Davidson25 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.

Thursday, May 21, 2015G Davidson26 Annual Rings

Thursday, May 21, 2015G Davidson27 Annual Rings One year’s growth This tree is 13 years old

Thursday, May 21, 2015G Davidson28 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

Thursday, May 21, 2015G Davidson29 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.

Thursday, May 21, 2015G Davidson30 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.

Thursday, May 21, 2015G Davidson31 Regeneration in Plants and Animals  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.

Thursday, May 21, 2015G Davidson32 Regeneration in Plants and Animals  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.

Thursday, May 21, 2015G Davidson33 Regeneration in Plants and Animals  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.

Thursday, May 21, 2015G Davidson34 Regeneration in Plants and Animals  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.

Thursday, May 21, 2015G Davidson35 Regeneration in Plants and Animals  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.