ÐÓ°ÉÔ­´´

Knowing your dews

Question: There have been many times that I have unzipped the flap door on my
tent while still in my sleeping bag to see that a heavy dew had fallen during
the night. Being so close to the dew-laden grass, I always notice that the
individual drops occupy an apparently precarious position at the very tips of
grass blades. How do they get there and how do they stay there?

Answer: This process is called guttation. On the surface of leaves there are
stomata or pores through which water is lost by transpiration. At night, the
stomata close, causing a reduction in transpiration. Drops of water are then
forced out of the leaf through special stomata or hydathodes. These special
stomata are found along the edges of the leaves or at the tips. It is believed
that guttation is caused by high root pressure. Grasses often force water out of
the tips of their blades, as your wide-awake camping correspondent noticed.
Guttation also happens in potatoes, tomatoes and strawberries on their leaf
margins.

Frances Tobin

Manly, Queensland

Answer: The drips result from guttation. Plant roots take up inorganic ions
from the soil and transfer them to the xylem from which they can’t leak back.
Water is drawn in by osmosis, which creates positive pressure in the xylem.
Because of this pressure, xylem sap leaks from pores (the hydathodes) at the
tips of grass leaves (or directly from the cut or cropped ends of the leaves).
When the drops are large enough they fall and new ones form.

Guttation usually happens at night because, by day, water loss from the
leaves is normally sufficient to maintain a negative pressure in the xylem.
Conditions favouring guttation also favour camping: clear skies and light winds,
warming the ground by day and cooling the air (thus raising the relative
humidity) by night, with the ground moist enough to accept a tent peg.

Some useful ions are probably recovered by the hydathodes and some of the
ions in the xylem in the roots may have been recirculated by the phloem. A
similar process to guttation brings calcium to developing fruits. Its
interruption is bad news. For example, a dry atmosphere in a greenhouse by night
can prevent positive pressure building up in the xylem, and developing fruits
such as tomatoes can show calcium deficiency in the form of blossom-end rot.

John Tulett

Edinburgh

Answer: Guttation has been observed in more than 330 genera of 115 plant
families and is induced by conditions which permit root absorption but retard
transpiration. It is, therefore, more frequent at night and commonest in humid
tropical areas where the higher soil temperature favours root absorption and the
moist atmosphere retards transpiration. Among plants in temperate regions,
species of Impatiens and grasses, including cereals, readily exhibit
guttation. In the tropical plant Colocasia antiquorum 200 millilitres
of water may be exuded by a single leaf in just one day.

John Tomlinson

Stratford-on-Avon, Warwickshire

This week’s questions

Tank trap: On a recent visit to Zhaoqing, near Canton, in China, I visited
the Plum Temple. This features an optical illusion tank about four metres long
and a little under a metre across. The puzzle is in the perceived depth.

If you stand at the right-hand end, the water appears to be about 1 metre
deep, sloping away to the left end where it appears to be about 10 centimetres
deep. If you move to the left end and look to the right, then the appearance is
reversed and you are now at the deep end. If you stand in the
middle, the bottom appears to be U-shaped, with the deepest point exactly in
front of you, sloping away to become shallow at both the left and right
ends.

Although the party I was with included technical types, they could not
explain it. The temple was built in 996, so this tank has been baffling visitors
for 1004 years. Can anyone offer any explanation?

Phillip Bruce

Hong Kong

Lamp light: One afternoon I turned on an incandescent lamp and noticed that
an object’s shadow was blue. I conducted some more experiments and found that
the phenomenon only happened when sunlight was combined with the light from the
incandescent lamp. I then tried lamps of many colours, with the results shown
below. Why does the shadow change colour?

Lamp’s colour → Shadow’s colour

Yellow → Blue

Red → Green

Pink → Green

Green → Purple

Blue → Red

Xinyue Zhao

Taoyuan Primary School

Dalian, China