Recently, I wrote an article on the amazing level of
intelligence and advanced organization exhibited by other life forms. [1] In
thinking about this, I realized that the article concentrated on the animal
kingdom, which represents only a fraction of the life on earth. So what about plants? We assume that plants are insentient, yet, in
perusing the question more deeply, it turns out plants have their own amazing
abilities. In fact, these abilities
include sensing, communications, memory, and perhaps even thought. This article explores these abilities,
especially the ones typically thought only to be enjoyed by the more advanced
animal species.
By virtue of the fact that plants use light for
manufacturing food, they have a well-developed sense of light detection,
including the ability to sense color and quality. This ability, not dissimilar from sight, is used
for a variety of functions. There are
photoreceptors in plants sensitive toward blue light that are used to guide the
plant to bending its leaves, harvesting the maximum amount of light. They are also responsible for controlling
flowering and growth. The same receptors
happen to be found in animals and are responsible for the circadian
rhythm. Red light stimulates the
germination of seeds and the synthesis of chlorophyll. Darker red light, found predominantly under
dense plant cover, encourages stems to grow longer to find brighter light. [2, 3]
Plants are able to detect chemicals in the air and the
ground, analogous to our senses of smell and taste. Plants synchronize the ripening of their own
fruit as well as fruit on neighboring plants by having the ripe fruit emit
ethylene. The dodder, a parasitic plant,
finds food by detecting chemicals released into the air by nearby plants and
then selects the plants that it finds tastiest.
When plants sense nitrates in the soil, they develop lateral roots to
capture this nutrient. They also sense
chemicals from life forms attacking the plant, allowing them to defend against
the threat. For instance, when corn is
attacked by Spodoptera caterpillars, it detects a chemical in their saliva. In response, the corn releases chemicals that
attract the caterpillars’ predator wasps.
[4, 5]
Plants also have a sense of touch, the most obvious example
being the Venus Flytrap, where its sensing hairs trigger the quick closing of
the trap. There are about 1000 species
of other plants which have instant reaction to slight touch; however, all
species of plants have some reaction to touch.
For example, when certain types of cucumber plants are rubbed, they
reduce their stem length, widen the diameter, and stiffen within a two day
period. Plants in fact can be more
sensitive to touch than humans; a Sicyos tendril responds to a weight 1/8 that
of what a human can detect. [6, 7] Unlike animals, plants are rooted and
therefore can’t flee when they hear a threatening sound. So it does not seem necessary for plants to
hear. Yet, stories abound of plants
growing faster in response to music and Scientific American reports that recent
research indicates that plants may respond to vibration, which is the essence
of sound. [8]
Plants are also capable of communicating with one
another. Plants are able to signal to
each other via their root systems when there is a water shortage. A maple tree under attack by bugs releases a
pheromone which, when received by nearby trees, triggers them to produce
chemicals to fight off the invaders. As
previously mentioned, the corn attacked by caterpillars attracts the wasps that
eat these caterpillars. Finally, besides
coordinating the ripening of fruit, trees also are able to coordinate amongst
themselves the blooming of their flowers, possibly as a defense mechanism
against pests. Trees, according to Brian
Ford, space themselves to prevent competition for food, light, and water. [9, 10, 11]
These types of behaviors seem quite advanced for plants,
which do not have complex nervous systems as do animals. Yet, plants have other ways of communicating
within the organism. Plants have
“bundled sheath cells, which, like nerves, send electricity from cell to cell
and enable communication with other parts of the plant. Plants use hormones to transmit messages to
other parts of the plant. For instance,
leaves send a message to the tip of a shoot in order to initiate
flowering. Glutamate receptors, found in
the human brain and which are responsible for memory formation and learning,
are also found in plants and are believed to mediate cell-to-cell
communications. Stomata in plants,
besides regulating water evaporation, air, and carbon dioxide absorption in
leaves, sense and react to light and the chemical nature of the atmosphere. They also sense and respond to touch and
chemicals released by organisms on the leaf surface. They respond to chemical messages from the
root to assist the plant in conserving water when the roots experience dry
conditions. In the Parthenocissus, a creeper,
stimulating a single touch cell transmits to all other cells in the tendril to
initiate coiling. Plants, like animals,
can be immobilized by anesthesia. [12,
13, 14]
Despite not having an obvious brain, plants still exhibit
memory with the Venus Fly Trap providing a dramatic example. In order to close, the plant needs to have
two of the hairs on its leaves touched by a bug. When one is touched, it remembers that contact
for about 20 seconds. If a second hair
is touched in that time frame, it snaps shut.
There are certain plants that when stressed will pass stress resistance
to the next generation. This is not
accomplished by mutation but by changes in gene activity, a form of
transgenerational memory. Some flowers
that are unpollinated by insects will spring their own anthers, which infers
that the plant remembers this is necessary for pollination. Plants seem to remember trauma and will
compensate for it – a dandelion in a mown lawn will often flower close to the
ground, apparently to avoid being cut by the lawnmower. The tendrils of pea plants will curl when
physically stimulated while under the effects of light. This will not occur, however, when the plant
is in the dark, but it will remember the stimulation for up to 1 ½ hours. If within this time the plant senses light,
it will curl. [15, 16]
When looking at these capabilities of plants, is it possible
that they can think, too? Shoot growth
in plants partially depends on a hormone generated in the roots. Darwin proposed that the tip of the root
acted like the brain in plants; his proposal is the subject of current research
into plant thought. Daniel Chamovitz
writes: “…the entire plant is analogous to the brain.” [17] A recent study showed that plants
differentiate between types of light in order to make seasonal adjustments to
their disease immunity and against environmental factors such as drought and
cold. Quoting Professor Christine Foyer
of the University of Leeds in an interview with BBC News “Plants have to
survive stresses, such as drought or cold, and live through it and keep
growing…this requires an appraisal of the situation and an appropriate response
- that's a form of intelligence.”
[18] Given the complex set of
senses, communications, and memory that plants exhibit, despite their lack of
an obvious brain it is not hard to come to the realization that plants may well
think, albeit in a very different way from animals.
Plants have their own array of
complex talents that in many cases mimic more sophisticated animals. Daniel Chamovitz: “People have to realize
that plants are complex organisms that live rich, sensual lives.” [19]
Because plants have evolved in a very different fashion from animals,
their array of skills are not obvious to us, yet they contribute to the amazing
hardiness of plants in our ecosystem. In
many regards, plants are our close cousins, not distant relatives. Therefore, the mechanisms plants use for
their existence and survival can teach us a lot about the sustainability of our
ecosystem as well as what it takes to thrive and perpetuate our own species.
References:
2. http://news.softpedia.com/news/The-Sense-of-the-Plants-84523.shtml
4. http://news.yahoo.com/plants-think-110000318.html
5. See 2.
6. See 2.
7. See 3.
8. See 4.
9. See 4.
10. See 2.
11. See 3.
12. http://www.bbc.co.uk/news/10598926
13. See 4.
14. See 3.
15. See 4.
16. See 3.
17. See 4.
18. See 12.
19. See 4.
Photo credit: Cousin Dawn
