|Multicellularity in Plants
Just like animals, plants are multicellular organisms. The diagram at the top shows a small strip of plant
tissue, say from a leaf or green stem (so-called parenchyma tissue). The cells look like polygonal boxes. On
the right we have removed the lids of these 'boxes' to show the basic structure: each box or cell has a wall,
shown in green, and is filled with gelatinous cytoplasm, which has also been removed from the cells in rows 3
and 6. (The cytoplasm is bounded by a cell membrane, as in animal cells, but this membrane is inside the cell
wall which animal cells do not possess). The blue colour represents the 'glue' that holds the boxes together.
This glue layer, or cement layer, forms what is often called the 'middle lamella' or middle layer, since we have
two green walls of two adjacent cells on either side of every blue glue layer. The glue is made up primarily of a
material called pectin.
In reality plant cells are much more complicated, see the section on plant cells for more details. Note also that
these are not the real colours, although plant cells are green, their green colour is contained inside structures
called chloroplasts that sit inside the cytoplasm.
What is cellulose? Cellulose is one of the main materials used in plant cell walls, especially in green and
fleshy parts of the plant. Cellulose is an outstanding engineering material. It is very light, and yet it is immensely
strong, especially when under tension (when it is being pulled). It is made up of chains of sugar molecules
bonded together and forms rope-like strands that are glued into layers, with several layers making up the cell
wall. The tensile strength of the cell wall is equal to that of steel (1 GPa) - that is the cell wall is as strong as
Why is this arrangement so important?
It gives the plant mechanical strength. The strip of tissue above is what engineers call a cellular solid,
though they are not talking about biological cells, but rather physical cells. Polystyrene foam is another example
of a cellular solid - it is made up of tiny balls of polystyrene 'cemented' together and filled with air. This makes
the foam stiff, when you squeeze it, the 'wall' of each ball pushes on the air inside it, the air resists further
squeezing as the balls push into one another. In the case of our plant tissue, the cells contain walls of cellulose,
not polystyrene, and they are filled with water, but the mechanical principle is the same. The tissue is stiff as it
resists bending and squeezing as the cell walls give a little, but are resisted by the incompressible water inside
the cells. This works fine so long as the plant body is full of water, and what we call turgid. When a plant gets
too little water it wilts, the cells begin to deflate and the walls become limp as they have no water to push
against. In well-watered and fully turgid (stiff) plant tissue the water inside is under pressure, so the cells are
rather like a series of inflatables, and this makes them stiffer than polystyrene which contains only air.
There is a green alga called Caulerpa. (Algae are not plants, though plants probably evolved from algae, but
include plant-like creatures such as seaweeds). Caulerpa forms large seaweed fronds up to 20 metres in
length, but remarkably each organism is a single giant cell! This proves that single cells can be big. However, in
rough waters the Caulerpa has a mechanical problem - a single large cell surrounded by a single cell wall is too
flimsy, so it starts to develop wall ingrowths, partial cross-walls that form reinforcing struts of strong cell wall
material. Take this design to its logical conclusion and you split the plant body up into many cells, each with its
won wall. Indeed, in plants there are actually channels between neighbouring cells which connect them
together, so the cytoplasms of adjacent cells are continuous, so it is still a bit like one big cell really.
Pressure. In terrestrial plants the parenchyma is under pressure. The water inside each cell is pressurised, so
long as the plant is turgid, so that each cell looks a bit swollen and pushes against its neighbours, making the
structure stiff. This is what engineers call a pressurised cellular solid.
Air spaces. Such tissues are not all water in reality, if they were then the plant would drown! Plants need air to
respire and to extract carbon dioxide for photosynthesis. Some of the corners between adjacent cells contain
little spaces or channels full of air (not shown). These channels connect to form a 3D network of channels that
run throughout the plant tissues, carrying air deep inside the plant. These channels also help take up water in
roots and take up water that gets squeezed out of cells when pressure is applied to part of a plant.
Imagine a whole sheet of cells, like those shown above, now imagine them rolled into a cylinder, and then
imagine a whole series of concentric cylinders all glued together by pectin. Now we have a fleshy plant stem.
However, plants are not quite like this. Break open a green plant stem and it will be fibrous more often than not.
These fibres are contained inside tissue like that shown above. Thus, a plant contains fibrous tissues and a
ground or packing tissue. This packing tissue is the pressurised cellular solid we have discussed here and is
called parenchyma. The fibres provide additional strength and some form vessels that conduct sap and water
around the plant. In a woody plant, the tissue becomes all fibre and hardly any parenchyma remains, but all
young and green shoots contain mostly parenchyma.
In aquatic plants support is less of a problem, and parenchyma is not under so much pressure, but is more like
a series of tiny balloons filled with water (what we call hydrostats) and the air spaces are much larger, both to
allow air in from above and to give the leaves buoyancy. Such parenchyma with very large air spaces (like a
honeycomb) is called aerenchyma (think of Aero chocolate!).