-
Asked by viciascience to Alison, Jean-Paul, Karolina, Matt, Rachael, Samantha, Vicky on 31 Dec 2013.
-
Samantha Hughes answered on 31 Dec 2013:
Interesting question! Some trees, like the giant redwoods in USA live for an average of 300-500 years (some are as old as 2000 years!) and will continue to grow during their life but as they grow upwards, they loose their lower branches. However, there is no relationship between growth (i.e. height) and age of the tree.
-
Comments
Ben commented on :
Hi, just browsing through the site (I participated in a previous IAS event) and saw this question that I thought I’d be able to answer it. The answer is related to the nature of our stem cells. Stem cells are essentially the cells in our bodies with the ability to develop into all other types of cell. Humans, like all animals, have a define body plan which grows to a certain size and stays roughly that size for life (except many human waistbands seem to expand with age). This is because our stem cells differentiate into other cells and then remain that way. Once our bodies have grown to adult size there is simply no more potential to grow. Your bones can no longer grow because something called the epiphyseal plate develops at either end which prevents further elongation.
Plants are different. They maintain a pool of undifferentiated stem cells at the growing tips of the plant (shoots, roots etc’) which is called a meristem. Meristems constantly divide, creating new cells to form the plant’s organs like branches, leaves and flowers whilst maintaining a constant supply of more undifferentiated cells.
In evolutionary terms this is probably down to the fact that, as a rule, animals can move when they don’t like their environment so don’t need to continuously grow. Plants can’t. If they run out of water or nutrients, their root may need to grow a bit more to find some. If they’re not getting enough sun, then they need to grow a bit taller. If something is eating them then their cell walls need to be a bit tougher etc’. This is why we call plant growth ‘plastic’. Because it is not fixed and they develop throughout their whole life.
viciascience commented on :
Thanks Ben, very helpful.
I am interested in how all this growth is controlled. I can see that our height does not increase after adolescence and that long bone growth therefore ceases, but other tissues do keep ‘growing’ to replace the cells that die, such as skin, epithelium. So somehow, some stem cells stop and others keep dividing.
Similarly in plants. Just looking at the sycamore across the road, it is no longer increasing in height, but the complexity of branches is increasing and I guess more lignified cells are being made in the trunk for support. So the apical meristem is no longer active in the stem tip, but lateral meristems are now more active, and cambium must remain active. Without some sort of central control system in a plant it is hard to see how these changes are regulated.
Penny commented on :
I just wanted to add another perspective – part of the reason some organisms have evolved to particular size parameters is basically down to mechanical constraint. A static organism (if there are no other limitations) can grow larger than a moving one as it only needs to support itself in an unmoving state. Most land animals can only reach a certain size because their body shape is adapted for that. E.g. Elephants need extremely thick legs that support directly from underneath. If you made an ant the size of an elephant it would collapse under its own weight. In fact, many sea creatures don’t have a built-in stop and can get larger as they get older – because they have the support of water around them they don’t have this limitation. This is one theory of why some animals age and some don’t. Land animals have to have this stoppage of growth and one outcome of this may be ageing. Many sea creatures that don’t stop growing also don’t show signs of ageing. My point (although I’m not a plant scientist) is that trees presumably have mechanical limits too, such as how much turgour pressure they can generate and maintain. So one simple reason for stoppage of vertical growth might just be that the tree has reached a height limit to which it can transport nutrients and water. The repartitioning of these nutrients laterally might be one way the growth is regulated.
Lynne commented on :
I think your question on control of growth is very important. We know that mammalian cells have very tight controls on division which are important in blocking cancer (p53 that Alison demonstrated in lecture 3 is one of the control proteins). We also know that as soon as human skin cells touch each other ( when growing in the lab – and presumably also in our bodies) they turn off their division programme – it is called contact inhibition and is controlled by a whole network of proteins signalling from the surface of the cell through to the nucleus to regulate gene expression and the cell division cycle. The same set of control proteins increases as cells age so that old cells permanently lose the ability to divide and become ‘senescent’.
viciascience commented on :
Thats really interesting. So control of division and growth happens at a local level in humans with additional control exerted by the endocrine system. So plant senescence could be under only local control, and explain why leaves fall off the shaded side of a tree before they fall off next to a street light?
Ben commented on :
I’m sure you’re aware that plant’s have a set of hormones, much like humans, which regulate things like growth, development and responses to environmental stimuli so senescence isn’t under local only control.
What you’re seeing with the trees you’re observing is variations on a phenomenon called apical dominance and is under the control of a hormone called Auxin, which is produced at the apical meristem. What essentially happens is that one apical meristem will dominate others and cause its particular shoot to grow faster. Now, whilst theoretically all apical meristems are indeterminate there are instances where they will cease to exist. In this instance plants will usually initiate a new apical meristem and this is how brancing occurs in trees. So the initial apical meristem from the germinating seed is no longer active but several new apical meristems may be. Complex branching patterns occur when apical dominance is incomplete. When apical dominance is complete, that’s when you’ll see the main shoot incresing in height.
Regarding a control system, there’s a fairly well understood network of transcription factors active in the meristem and the cells surrounding the meristem to maintain its boundaries and identity. Relative local levels of expression of these transcription factors tune the development of lateral organs like branches and leaves. Secondary meristems (which produce the lignified stems of woody plants) are a different kettle of fish entirely and I must admit I’m not as well read on them as I should be.