Comment & Analysis
HOW DO SEEDS SPROUT?
his is the time of year when
seedlings start to appear. After
planting seeds, I’ll find myself going
back far too often, checking to see whether
they’re visible yet. If you keep looking,
you’ll see the seedlings pushing aside small
bits of soil and tiny stones as they grow.
They’re not just winding around obstacles –
they’re shoving them. The more you think
about it, the odder it seems. Without any
muscles, how does such a tiny plant create
enough force to push anything?
When a seed starts to germinate, the seed
case cracks open and new plant cells start
to grow. The seedling has to rely entirely
on the energy stored in the seed until it can
reach the surface and start collecting
solar energy using its leaves. But it’s got
water and nutrients, so off it goes. The trick
to the pushing is built into the structure of
every cell. Each has a cell membrane – a bag
to keep itself in – but it also has a stronger
container outside called a cell wall. The cell
wall is made of cellulose, meaning that it’s
flexible but not stretchy.
I’m currently waiting for my tomato
seedlings to appear, but they’re busy below
the ground at the moment. As well as
growing new cells, they’re pumping water
into an inflatable sac inside each cell. This
sac is called a vacuole, and it’s filled to
bursting but the cell keeps on pumping. The
reason it doesn’t burst is that it’s held in by
the cell wall. The extra water being pumped
in doesn’t make the cell bigger – instead it
boosts the pressure on the inside, making
the cell sturdy. Instead of being flimsy and
floppy, it’s now a strong brick.
The pressures that a plant cell can
generate are astonishing. In a seedling, it’s
probably only three or four times as great as
atmospheric pressure. But in some parts of
a mature plant, it can reach 20 atmospheres- almost three times the pressure in a bottle
of champagne. And it is this pressure that
shoves the soil aside.
Plants are held up by water pressure, and
we’re most aware of this when plants wilt.
When soil becomes dry in the summer,
plants may not have enough water to keep
the pressure sufficiently high. And so the
cells lose their structural strength and start to
sag. If you put wilted lettuce into water, it’ll
soon pump water back into its cells, making
it strong and crisp again.
As my seedlings are pushing themselves
through the soil, they’ve also got a piece of
armour protecting them. The first two plant
leaves, sometimes still in their seed case, are
curled up at the top of the new shoot. As
the cells in the new plant inflate and push
upwards, these two leaves take the brunt
of the push back from the surrounding
soil, forcing their way through. Once the
seedling has reached the surface,it can cast off the seed case, open up the
two new leaves, and become a self-building
solar-powered factory.
Seedlings popping out of the soil is always
exciting. But part of that excitement for me
is the appreciation of the elegant mechanism
that has pushed them out of their dark
hidey-hole. Even a tiny seedling is a
hydraulic marvel. And better still, in a few
months, there will be tomatoes! ßT
DR HELEN CZERSKI is a physicist and BBC presenter whose
MAIN ILLUSTRATION: MATT CLOUGH PORTRAIT: KATE COPELAND most recent series was Colour: The Spectrum Of Science
“in some parts of a plant, pressure can be three times that of a bottle of champagne”