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What a Plant Knows:
A Field Guide to the Senses

A fascinating journey through Chamowitz's work as he explores how plants see, smell, feel, remember and orient themselves

Citation Information

Bender, Robert 2016. What a Plant Knows: A Field Guide to the Senses, URL = <http://www.rationalrealm.com/science/reviews/what-a-plant-knows.html>.

Publication Information

Chamowitz, Daniel, What a Plant Knows: A Field Guide to the Senses, New York: Scientific American/Farrar, Straus and Giroux, 2013, pp. 177, (paperback).

Meal worm in venus fly trap

Daniel Chamowitz, PhD, is Director of Plant Bioscience at Tel Aviv University in Israel. His book, What a Plant Knows: A Field Guide to the Senses, arose from the discovery that the genes in plants responsible for distinguishing light from darkness are also present in humans. Chamowitz recollects some amazing discoveries on the sensory abilities of plants. These abilities include sensitivity to light, to pheromones, to touch, to gravity, but, surprisingly, not to sound. We are accustomed to believing that these sensory abilities apply to animals with nervous systems and directing brains, not to plants, which lack nerves. The chemical and electrical processes may differ, but animals and plants do have a surprising amount in common in their ability to react to their environment's signals.

Chamowitz includes chapters on plant sensitivity to light, to touch, to smells, to sound and to gravity and concludes with a brief discussion of The Aware Plant. This kind of plant does not have the same awareness as an animal searching for its food and avoiding predators, but it does definitely respond to its environment in surprising ways. He discusses many little-known experiments and research projects that are the basis of what we now know.

What a Plant Sees

Plants are sensitive to light and bend towards it. Darwin showed that the growing tip is the sensitive part. Plant flowering is stimulated by night length, so if the length divided in two by a brief light period in the middle, flowering time is controlled (wonderful for Mother's Day). Plant sensitivity is to red and far-red light, so sunrise and sunset colours switch flowering on and off. Plants lack the rods and cones that enable our eyes to detect light and colour. However, they have other chemical means of responding. Sensitivity to flowering switches is in the leaves. So, if you strip all of a plant's leaves, the plant won't flower. Light receptors in plants tell it when to germinate, bend towards light, flower and when it's night-time. Annual growth and dormancy cycles, flowering and seed-set start in spring due to sensitivity to lengthening days. And plants grow faster when shaded by overhanging plants by forcing them to grow out into the light, which for them is food.

Humans, along with all other animals, have a circadian rhythm regulated by a biological clock that affects our waking/sleeping patterns, hunger and energy/lethargy patterns. Plants, too, have circadian rhythms of flower-opening and closing, leaf movement and photosynthesis. These cycles are regulated by light and dark, just as ours are. We have different chemistries, but all of these chemistries derived from the same, original single-celled organisms that lived billions of years ago, before animals and plants diverged into different evolutionary pathways. Plants suffer jetlag, just as we do, when moved to a different time zone, taking a few days to adjust to the new timing of light and dark. Plants moved from the northern to the southern hemisphere eventually flower in the southern spring, not the northern one, as they are responding to light signals.

What a Plant Smells

Book cover: What a Plant Knows: A Field Guide to the Senses by Daniel Chamowitz

Plants can detect odours emitted by other plants. Our noses have receptors sensitive to even a single molecule of fumes coming from the external world. Smells vary far more than colours, so we have many specific receptors sensitive to particular volatile chemical molecules.

Fruits ripen in response to the presence of ethylene gas, which is why a kerosene heater does the trick, as does incense and putting a ripe fruit in a bag with an unripe one. On the other hand, an electric heater fails to ripen fruit. Because ripe fruit emits ethylene fumes, this allows unripe fruit on a tree to detect other fruit ripening.  In this way, all fruits respond together to ripen about the same time, earlier ones setting off unripe neighbours. It is the same process that also sets off the autumn colours in dying leaves. Germinating parasitic plants first rotate their growing stems to detect the fumes of nearby hosts, then grow towards them to latch on. A 1983 study showed that a plant subject to insect attack gives off pheromones that warn neighbouring plants and other leaves on the same plant to produce defensive chemicals.

What a Plant Feels

Some plants are very sensitive to touch. For example, Venus flytraps have sensitive hairs inside their trap-lobes. So, if a large insect or small frog touches two within 20 seconds, it snaps shut. This action sets off digestive juices to dissolve the victim and absorb its proteins. The Venus flytrap can discriminate, as it is not set off by rain.

The South American Mimosa pudica (shy) has doubly divided fronds, like a wattle, which collapse on being touched. Chemical research has shown that in response to electrical signals set off by touch, the balance of phosphorus and sodium inside and outside the cell wall enter and exit through the cell membrane to signal water to flood in or out, to stiffen or collapse the cells.

What a Plant Hears

In 1974, Dorothy Retallack published her book, The Sound of Music and Plants, followed in the same year by Peter Tompkins and Christopher Bird's, The Secret Life of Plants. Through these books, the authors spread the idea that plants respond to music and to being talked to. So, millions of people started doing that. The research was all based on anecdotal evidence by amateurs, with no scientific rigour, and none of the conclusions have been confirmed by sound experiment. Plants don't respond to sound, as they have no need to.

How a Plant Knows Where It Is

Book cover: The Sound of Music and Plants by Dorothy Retallack

How do plants know which way is up and which is down? Roots grow down, shoots grow up. A growing seed up-turned will slowly reorient its growing tips, which Darwin showed is the site of the gravity sensor. Experiments in which seedlings were doused with mutagenic chemicals and then centrifuged at great speed show some lost the gravity sensor and subsequently grew down or horizontally.

A recent study with microscopes has isolated dense balls of matter in plant cells (statoliths) that fall to the bottom of cell fluid and tell the plant 'this is the down side' so root cells know to grow down and shoots know to grow up. Experiments conducted in the Space Station's zero gravity confirmed that if statoliths cannot fall to the bottom of cells, plants have no sense of 'up'. Other experiments showed that plants slowly wave in spiral movements as they try to correct their stance after being bent or blown, even in zero gravity. This waving happens much more strongly on Earth, under the full influence of its gravity.

What a Plant Remembers

Book cover: The Secret Life of Plants: A Fascinating Account of the Physical, Emotional, and Spiritual Relations between Plants and Man by Peter Tompkins and Christopher Bird

The Venus flytrap is set off when an insect touches two sensitive hairs within 20 seconds. So, it has a 20-second memory. This memory, which is electrical in nature, sets off calcium floods into and out of cells that eventually recede if not reinforced. If a flood is reinforced within 20 seconds, it passes a threshold that sets off snapping shut of the lobes and trapping of the insect.

Each year, there are two equinoxes; one in spring and the other in autumn. Flowering and fruiting in many plants is set off in spring, when the plant has a memory of a recent cold period, rather than in autumn when the recent period was very hot. In years of warm winters, this temperature-memory signal is absent, with the result that fruiting does not occur.

The Aware Plant

Plants show awareness of surrounding environments of light and dark, pheromones, injury by insect attack or fire and touch by passing animals. They possess an ability to respond in some way to these sensory experiences. To that extent, we have a shared biology, although we parted company from plants in evolutionary terms billions of years ago and are chemically and neurologically very different from them.

In this book, Chamowitz has brought together many fascinating experimental reports delving into the sensory capacities of plants, exploring their ability to communicate, to defend against attack and to modify their growth patterns. As well as a truly interesting read, the book contains a good reading list and an index at the end.

Copyright © 2016

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