I spend far too much time here...I know that, you know that...but I've got to get my revision done for my next exams somehow. So, in between answering forums and stuff I'm revising. This is my way of revising...but anyone who's going to do/is doing OCR's Growth, Development and Reproduction module is perfectly welcome to read through these revision notes...as long as you don't mind the occasional rambling or distracted thought.

I'm revising just from the text book at the moment, but as soon as I sort out my class notes (which verges on the impossible, because for some reason I never bother to revise using them..) then I'll fill in any missing details...I promise.

Growth and Development

There are seven fundamental things that all life does...

nutrition, growth, respiration, reproduction, excretion, sensitivity, and for some, locomotion.

Growth and reproduction are the two things that are studied in this module. Just thought I'd let you know, just incase you hadn't figured it out by now. Growth is usually accompanied by development, a change in form which is genetically programmed and may be modified by the environment (hence it's place in the title of the module).

The most simple and straightforward answer to this is that growth is an increase in size, but this is not always true. Consider...

• a zygote can divide to form a ball of cells with no increase in size or volume. This would appear to be growth without an increase in size or is it just development and not actually growth?
• a plant organ, such as a potato tuber, may increase in size, as measuered by mass or volume, by taking up water via osmosis. This would seem to be an increase in size without growth. This could easily be reversed if the tuber lost water.

So the question is still, "What is growth?" Well, the answer is...

...growth is a permanent increase in the dry mass of a living material.

By specifying dry mass and a permanent (and therefore irreversible) increase the short-term fluctuations in water content that are very common in plants, can be ignored. Mass is the best indicator of growth, but even the defintion of growth as being a permanent increase in dry mass, can have it's problems. Normally, growth and development go together, but in the early development of a zygote there is an increase in the cell numbers by cleavage, but there is no increase in mass or volume. Thus, according to the definition, a zygote can develop without growing. A germinating seedling shows a net loss in dry mass until it starts ot photosynthesise. By this time, much development has taken place,(eg primary root and shoot development) accompanied by an increase in cell numbers anf fresh mass. Should the definition specify that new dry mass must be living material? In some organisms, a significant amount of dry mass is of non-living origin, eg xylem vessels. Can the addition of non-living mass be regarded as true growth?

In a multicellular organism, growth usually involves and increase in size as a result of cell division and a consequent increase in cell numbers. As the cells mature, they tend to specialise and become adapted for different functions. This is known as differentiation. Thus, growth and developmant typically involves three separate processes, cell division, cell enlargement, and cell differentiation. The whole series of changes is controlled and regulated by interactioins between the genes and the environment.

Unlike animals, where growth occurs throught the body, plant growth is confined to specific regions called meristems. The three processes of cell division, sell enlargement and cell differentiation are well illustrated by meristems because the processes are separated in time and space. This is particularly clear in the root tips, which are responsible for the lengths of the roots. An example of growth in plants being separated in time and space can be found here. There is a time difference between cell division and cell differentiation, so the younger, newly divided cells are found at the tips, but the slightly more mature and differentiated ones can be found further back. These three phases of growth also occur in animals, but they are not in different locations, and so are not as obvious.

The zone of cell division in the root tip is a constant source of new cells whilst the root is growing. In the region of cell enlargement, the cells take up water by osmosis, synthesising new materials, and often becoming much longer. In the zone of cell differentiation, the cells become specialised for their different functions and develop specialised structures. For example, phloem contains long, living tubes (sieve tubes), which carry organic solutes such as sugars. Parenchyma is a type of packing tissue which may contain chlorophyll. Epidermal cells have a protective function, and roots can grow root hairs to increase the surface area available for water absorption. All of these cells contain identical DNA, and therefore identical sets of genes. Differentiation must therefore involve the switching on or off of particular genes in different cells at different times. For example, the cells whic are destined to become xylem vessels, must synthesise lignin. The enzymes required for this are coded for by the DNA. Each type of cell has its own unique range of enzymeswhich control its activity. The whole genetically programmed sequence of events which unfolds is known as development. The genetic program responds to environmental stimuli, such as gravity, light and moisture. This means that development is a progressive series of changes which are a product of genes and environment. The root tip, therefore, illustrates the general principals which apply to all multicellular organisms.