The Case of
the Blown Over Golden Spray
In early
January 2017, in one of the gardens I garden in, a Golden Spray tree suffered under a barrage of wind
from the North West and ended up leaning at a 45 degree angle to the ground
facing the South East. The Golden Spray resembles a miniature pine tree without
the cones. It has no leaves (technically it does have leaves but they are so
short they form bumpy scales on the bright green stems (the stems look like
giant, soft pine needles)).
The tree is
also known as a Broom tree and it’s easy to see why, as the tree has an upright
trunk with axial branches which contain a 'spray of' of green stems resembling
pieces of straw in an olden-days house-cleaning broom. The tree is definitely
not a gymnosperm (a gymnosperm is a plant which does not produce flowers to
attract insects which then transfer the male sperm of one flower to the female
part of another flower. A gymnosperm instead uses the wind to transfer the male
sperm to the female egg). The Golden Spray produces small yellow flowers along
each green stem.
Anyway, the
interesting aspect involved the pruning.
Lopping the top third of the Golden Spray, he advised, would give the plant a
chance to upright itself (its roots were still firmly in the soil so it could
obtain nutrients and water still). He advised that reducing the weight at the
top of the tree would allow the tree to be better able to pull itself upright
again. I was not sure how the Golden Spray would achieve this. I had read that
the trees use a growth hormone referred to as auxin (not its chemical name,
just an everyday reference name). But my interest was heightened when I read
further in the book (page 108, 'Beginners Guide to Botany, Duddington, 1970) that
the growth hormone will cause a trunk which is put on an angle to turn
correctly towards the vertical position as it grows and for a root to bend
downwards towards the centre of the earth if it is manually put at an angle to
the vertical).
I decided to do
a Google search and below I have pasted the first search result when I searched
for 'how does a tree know which way is vertical'. According to the article,
which was written several years ago, some of the answer is known but some parts
of it are still unknown. I have pasted a few of the middle paragraphs below so
you, the reader, can enjoy the gist of the article.
Plants sense gravity, in essence, the way a snow
globe does. Instead of fake snow, they use particles called statoliths. In
conifers and flowering plants, the statoliths are food storage vessels called
amyloplasts. Plants synthesize and store starch (polymers of glucose, which
plants manufacture in their green parts from light, water, and carbon dioxide)
in these granules. Inside the amyloplasts of the common bean the starch
granules resemble variously sized cotton balls stuffed into a balloon. Although
amyloplasts are usually white, the amyloplasts in this carrot root appear to be
pigmented -- perhaps they have been stained:
From Blancaflor 2012, American Jounal of Botany
100:1 143-152. Click image for link.
Under normal circumstances amyloplasts do nothing
more than sit on the bottom of special gravity-sensing cells in the central
column (columella) of root caps, and in shoots next to the vascular bundles
that transport water and sugar. When a plant is knocked over, the amyloplasts
slide from what was recently the bottom of the cell onto a formerly vertical
wall, as you can see above.
This is where things get fuzzy. Somehow, this
movement is sensed and relayed to cells that secrete the growth-regulating
plant hormone auxin on the new undersides of root and shoot. The hormone has
opposite effects in the two locations, triggering growth suppression on the
underside of roots and growth enhancement on the underside of shoots. As a
result, roots veer earthward; shoots veer skyward. Once the root or shoot
reorients, the amyloplasts slide down into their original position and the
auxin equilibrium is restored.
REFERENCE
Roots Down,
Shoots Up. But How Does a Plant Know Which is Which?, Jennifer Frazer, written
April 2 2013
,https://blogs.scientificamerican.com/artful-amoeba/roots-down-shoots-up-but-how-does-a-plant-know-which-is-which/,
accessed 23rd January 2017.
