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Plant Breathing & Ricin

Plant leaves appear to regulate their ‘breathing’ by conducting simple ‘computer-like’ computations; and the hort facts on Ricin!
by Art Drysdale
by Art Drysdale


Art Drysdale, a life-long resident of Toronto and a horticulturist well known all across Canada, is now a resident of Parksville, British Columbia on Vancouver Island, just north of Nanaimo. He has reno-vated an old home and has a new garden there. His radio gardening vignettes are heard in south-western Ontario over radio station Easy 101 FM out of Tillsonburg at 2 PM weekdays.

Art also has his own website at

February 8, 2004

Tiny holes in plant leaves open and close thanks to quick computations.
Image courtesy and © SPL.

According to Philip Ball, on January 21, online in the journal Nature, “Plants appear to 'think', according to US researchers, who say that green plants engage in a form of problem-solving computation.

“David Peak and co-workers at Utah State University in Logan say that plants may regulate their uptake and loss of gases by 'distributed computation' -- a kind of information processing that involves communication between many interacting units.

“It's the same form of maths that is widely thought to regulate how ants forage. The signals that each ant sends out to other ants, by laying down chemical trails for example, enable the ant community as a whole to find the most abundant food sources.

“This might not sound much like what a computer does, but it is. In distributed computation, signals exchanged between components of the system define the process for solving a problem. Researchers are now exploring the possibility of using distributed computing with swarms of simple robots to carry out tasks, such as searching a landscape, more efficiently than a single, more sophisticated robot could manage.

“David Peak and colleagues say that a cellular-automaton model can explain the way that plants regulate their uptake of carbon dioxide (CO2), which they use for photosynthesis, and their loss of water vapour.

“Leaves have openings called stomata that open wide to let CO2 in, but close up to prevent precious water vapour from escaping. Plants attempt to regulate their stomata to take in as much CO2 as possible while losing the least amount of water. But they are limited in how well they can do this: leaves are often divided into patches where the stomata are either open or closed, which reduces the efficiency of CO2 uptake.

“By studying the distributions of these patches of open and closed stomata in leaves of the cocklebur plant, David Peak and colleagues in Utah found specific patterns reminiscent of distributed computing. Patches of open or closed stomata sometimes move around a leaf at constant speed, for example.

“The statistics of the size of these patches, and of the waiting times between the appearance of successive patches, are the same as those for a model of cellular automata, the researchers say. The individual leaf stomata appear to act like simple computers, responding to what their neighbouring stomata are doing.

“The researchers think that transient patchiness may be the price the plant pays for a reasonably efficient and simple form of computation. It is a sign of the plant 'thinking' while it figures out the best solution to the problem of how much to open its stomata.”

* * *

Three shots of the castor bean plant including the poisonous seed heads.
Author photos

In the news again this week is the no-antidote poison Ricin. Though the general media have occasionally mentioned that it comes from a plant, and once on CNN I even noted a fleeting photo of the plant with its decorative fruit, little has been said about the plant, the castor bean (Ricinus communis) itself. Those of us old enough to remember when Castor Oil was “good for ails ya” probably know that the oil from which it is (was) made comes from the seeds produced by the castor bean plant.

Many gardeners like the castor bean for their gardens because it is an extremely fast growing annual (perennial or subshrub in the tropics) and valuable for screening or as a contrast against blank walls. While the plant is listed with several variations of the botanical name, generally, as far as gardeners are concerned, the two types available are Ricinus sanguineus which has red foliage and grows to about 2.4 m (8’), and R. zanzibariensis with green foliage and grows to at least 4.5 if not 5 metres (15-16’).

As I said in my book Gardening Off The Ground, “Castor oil bean plants are available from garden supply stores, or they may be started indoors from seed. But beware--the seeds are poisonous. If you have children around, you should probably forget about starting your castor oil beans from seed and buy plants instead.”

You should also remember that the seeds in the decorative pods produced in late summer too are poisonous. Many suggest cutting off the flowers as they develop so no seed pods are produced.

Scientists claim that as little as one milligram of Ricin can kill an adult. Children have been known to gather up castor bean seeds and put them on a string to make a chain of beads. This too is dangerous if the point at which the seeds are punctured comes in contact with skin, poisoning can occur. Even contact with the leaves has been known to cause sickness.

Keep in mind that if you were wondering about the possibility of being poisoned by castor oil, the Ricin within the seed pods cannot mix with the oil so there is no Ricin in castor oil!

They are attractive plants, but I do not recommend them around homes where there are children. Use them sensibly.

First part courtesy of Nature magazine online this month.


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