Do you remember John Wyndham's book The Day of the Triffids? It was a hair-raising vision of a world terrorised by monster alien plants, which almost did for humanity.

The potential of genetically modified organisms (GMOs) to turn our world upside down is much less dramatic than that. But many people, including some scientists, believe that potential exists.

The first concern is what might happen if new genes introduced to plants from other organisms escaped into other plants.

The genes are inserted to do a specific job, by conferring new characteristics on the plant.

But they could spread in several ways. They could be carried as pollen by the wind into neighbouring fields.

And they might possibly jump the barriers between species, and show up in unpredictable places, with altogether unpredictable effects.

Eradicating food sources

The next worry is what the genes may do to biodiversity. English Nature, the government's wildlife adviser, has been warning for months now that this is a real concern.

Pesticides are devastating the skylark

It does not want an outright ban on GMOs. But it does want time for research on the effect of herbicide-tolerant and insect-resistant GM crops.

If a plant is modified to tolerate a particular herbicide, then you can use higher concentrations of the chemical to kill weeds - which were once famously described as "plants growing in the wrong place".

And if a GM plant resists insects, the insects which used to feed on it will go elsewhere.

In both cases, the result will be the same, less food for wildlife.

English Nature says there is "strong evidence that a major factor" in the steep decline over the last two decades of 10 farmland bird species - including the skylark - is the increasing use and effectiveness of pesticides.

GMOs might well accelerate the process of turning much British farmland into a largely sterile outdoor factory.

Monopolies in the making

A third concern is the power that GMOs will concentrate in the hands of the companies that produce the seeds.

One GM species being developed by Monsanto, for example, has been modified to be resistant to a pesticide called Roundup, also made by the company.

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A farmer who buys Roundup-resistant seed from Monsanto will also be expected to buy Roundup itself, to get rid of unwanted species in the same field.

So GMOs can be part of a drive to establish ever-stronger agricultural monopolies.

This is part of what bothers development campaigners who are working with third world farmers to help them to raise their yields.

A US company which Monsanto is planning to acquire is seeking to patent a technique which, in Monsanto's own words, would "prevent the germination of seeds from genetically improved plants".

Using what is being called "the terminator gene" would mean that seeds "harvested at the end of a growing season could not be saved and replanted the following year".

So poor farmers would have to abandon the age-old practice of using this year's seed for next year's harvest.

Instead, they would go back to Monsanto each year to buy new seed.

GMOs raise an awful lot of questions. If the sceptics are to be persuaded that they really are as good as their supporters say, we need time to find the answers.