L. Hughes and M. Westoby of Macquarie University in New South Wales
say that ants disperse and protect eggs because they look and smell like
plant seeds. ‘It is a striking example of evolutionary convergence between
the plant and animal kingdoms,’ say the two biologists.

Seeds of the castor oil plant have a knob-like appendage known as an
elaiosome, which is rich in lipids and attracts many species of ants. The
ants carry the seeds back to their nest to eat but may discard them so that
they germinate.

The eggs of many species of stick insect have a similar appendage known
as a capitulum. Its function has long been a mystery. Biologists were aware
that ants took the eggs of stick insects back to their nests, but until
now assumed they were doing so simply to eat them.

Hughes and Westoby set out to see if the capitulum made an egg more
attractive to ants. They placed eggs with and without capitula close to
ants’ nests, and found that eggs with capitula were removed much faster
(Functional Ecology, vol 6, p 642).

The two biologists wondered whether the capitula were simply acting
as ‘handles’, making the eggs easy for ants to carry. To test this, they
presented the ants with eggs whose capitula had been removed and replaced
with a silicone appendage. The researchers found that these eggs were removed
no faster than those without capitula. Hughes and Westoby concluded that
the similarity of capitula eliaosomes encourages ants to remove the eggs.

There are several reasons why a stick insect’s eggs might mimic seeds.
The Australian biologists found that buried eggs were less likely to be
infested by a particular parasitic wingless wasp than those that remained
on the surface. Also, many stick insects take a long time to develop (up
to three years), so eggs left on the ground for long periods may dry out
or be eaten by vertebrates.

Although a proportion of the seeds carried into the ants’ nest are eaten,
‘at least some eggs survive’, say the researchers. ‘Large numbers of intact
eggs have been found in active ants’ nests.’

The emerging nymphs could also find themselves in a perilous environment.
But the researchers say many ant species move their nests several times
a year, making it quite likely that a nest will be inactive by the time
the nymph appears.

Ralf Kneer and Meinhart Zenk at the University of Munich have shown
that a class of compounds called phytochelatins are involved in detoxifying
heavy metals in plants. Phytochelatins are peptides – short chains of amino
acids. They trap, or ‘chelate’, heavy metal ions to form an inactive ‘complex’.

Plants are particularly vulnerable to poisoning by environmental agents
such as heavy metals because they are immobile. They must therefore protect
themselves from poisons by biochemical means. Nobody knows precisely how
heavy metal ions manage to kill plant cells, but the ions may incapacitate
enzymes, which have metal-sensitive regions on their surfaces.

Kneer and Zenk set out to discover how important it is for a plant cell
to keep its cytoplasm free of heavy metals. They added cadmium chloride,
a heavy metal salt, to cultured plant cells in ‘sub-lethal’ concentrations.
The plants were of a tropical variety called Rauvolfia serpentina. The two
biologists then carried out analysis of the cultures.

They found that of the cadmium that penetrated the cell walls, none
was detectable in the form of free metal ions. The vast amount of the metal
had been complexed by phytochelatins.

Kneer and Zenk next set out to find out to what extent the chelation
of the metal is protecting sensitive enzymes in plants. To do this, they
compared the effect of free metal and metal-phytochelatin complex on several
important enzymes involved in plant metabolism.

The researchers found that the plant enzymes could tolerate between
10 and 1000 as much heavy metal when it was in a metal-phytochealtin complex
rather than simply in the form of free ions.

Lastly, Kneer and Zenk compared the chelating efficiency of the phytochelatins
with that of two known biological chelators, glutathione and citrate. This
was done by attempting to ‘resuscitate’ an enzyme that had been inactivated
by cadmium.

The researchers found that phytochelatin revived the enzyme completely,
while glutathione was less than a thousandth as active. Citrate was completely
useless in this regard.

Kneer and Zenk conclude: ‘All the evidence so far available supports
the fact that the phytochelatin system is responsible for the sole and immediate
complexation and therefore inactivation of toxic heavy metals entering the
plant cell.’ But it is still not clear what happens to the complex once
it has formed and how it is stored in the plant cell over the long term
(Phytochemistry, vol 31, p 2663).

Yoshio Okahata and his colleagues, at the Tokyo Institute of Technology,
have built an ingenious piece of equipment, based on a quartz crystal microbalance,
which can weigh nanogram quantities of lipids. They were moved to attempt
such a weighing feat by the observation that odorous molecules are generally
more soluble in organic solvents, such as fats and oils, than in water (Bulletin
of the Chemical Society of Japan, vol 63, p 3082).

ÐÓ°ÉÔ­´´s believe that molecules of an odorous substance meeting the
membranes of specialised olfactory cells somehow trigger electrical signals
to the brain, and this is the foundation of smell perception. The precise
molecular mechanisms involved remain largely unknown but they may involve
lipid bilayers, which form the backbone of cell membranes and dissolve hydrophobic
substances.

Okahata and his colleagues coated their balance with an artificial lipid
bilayer, enclosed the balance in a sealed chamber and injected vapours of
a variety of odorous substances, including commercial perfumes.

When, for example, they injected into the chamber a saturated vapour
of a substance called B-ionine, the balance registered 760 nanograms. This
was the total amount of B-ionine that the lipid bilayer absorbed.

The researchers obtained similar results when they repeated their experiment,
this time coating the balance with membranes obtained from human olfactory
cells. They also found that when humans judged an odour to be more intense,
the microbalance registered a greater weight.

Okahata and his colleagues suggest that the first step in our perception
of smells occurs when odorous substances are absorbed by the lipid bilayer
of a cell. At this stage, no special receptor proteins are involved. They
say that the more soluble a substance is in the lipid matrix of olfactory
cells, the more intense the smell we perceive.

According to Okahata and his colleagues, the lipid-coated quartz crystal
micro-balance is both physically stable and reusable. ‘It will provide a
new sensor system to determine the intensity of odorants and perfumes,’
they say.

Ali Khan and Julian Vincent of the University of Reading have carried
out a study of the biomechanical properties of apples. They find that when
an apple is lying on its side, it is unable to undergo elastic deformation
– that is, it does not spring back into shape when it is squeezed. The researchers
say that the core prevents the fruit expanding laterally. In this orientation,
it bruises even if compressed only a small amount.

When an apple is compressed vertically, however, the core is parallel
to the direction of the force. It is not put under tension at all, say the
researchers, so the apple is much more elastic and can expand laterally.
It can withstand greater deformation without permanent damage.

The bruising of an apple is proportional to the energy absorbed. Khan
and Vincent find that the Rock Pippin variety, for example, can absorb 0.5
joules of energy before it is damaged. They conclude: ‘In normal storage
conditions individual apples seldom experience energies greater than 0.5
joules. Therefore, vertical orientation of apples during storage would considerably
reduce the risk of mechanical damage to the fruit.’

The skin of an apple also plays an important role in resisting damage.
Khan and Vincent found that apples split much more easily if the skin is
weak or damaged. ‘Any damage to the skin may considerably lower its role
as a crack preventer, and greatly weaken the structure of the entire apple,’
they say.

The researchers will be publishing their results very soon in the Journal
of Texture Studies.