Trees, their senses, their brains
Kamal Lodaya, The Institute of Mathematical Sciences, Chennai

All animals have senses. This sensory information flows to their brain, and the brain may make the animal's body respond in a particular way. For example, if your sense of touch says there is an insect on your arm, you will move your other arm to scare or swat the insect away.

Plant sense
An insectivorous plant like the Venus fly-trap has a soft pad which acts as a trap. When an insect settles on it, there is a sensory response from the hairs on the pad. The trap shuts and the insect is then "eaten" by the plant. To avoid "mistakes" (for example because of wind or dust) the trap shuts only when the hair is stimulated twice by the insect. The question is: how does the plant do this? Where does the sensory information go? Is there a brain which responds?

Defence mechanisms
Plants have other defense mechanisms. The bursera tree in Mexico is known as the source of incense. When an animal tries to eat its leaves, it squirts a sticky pungent spray onto the animal, scaring it away. This spray is a resin (a gummy substance) that is made in the trunk of the tree and sent to the leaves.

Acacia trees are present in many of the southern continents. When they are grazed upon by animals they produce a tannin, an astringent molecule used in the production of leather from animal skins which causes cell membranes to go dry. The smell of the tannin is picked up by nearby acacia trees which then start to produce tannin themselves, to protect themselves from nearby animals.

Survival mechanisms
A Chinese orchid called "dendrobium" fools a hornet wandering near it by producing a bee pheromone which honeybees use to send an alarm. Hornets use honeybees as food for their larvae. The hornets jump on the orchid (whose flowers do not have nectar). The result is that they carry the pollen from their flowers to other flowers and the orchid reproduces.
So plants have many kinds of senses just as animals do. Jagadish Chandra Bose identified some of these senses way back in the 19th century, arguing that they can feel "pain". Now we know that plants have more than 20 kinds of senses. (Compare this with the five senses of human beings!) For example, some plants can measure humidity, some can detect an electric or a magnetic field.

Rooted in the Brain?
It is still not completely known whether plants have a nervous system or a brain which uses these senses and devises a response. One of the earliest people to think about these questions was Charles Darwin. He found that a plant's root tip "acts like the brain of one of the lower animals".
This theory is now supported by Stefano Mancuso at the International Laboratory of Plant Neurobiology in Florence, Italy. Mancuso and his co-workers found that the signals given off from a plant's root tip are the same as those given off by the neural cells in an animal brain.
But of course a tree has not one but lakhs of root tips. Mancuso thinks it might be better to think of a single plant as a colony with a distributed neural structure, rather than an individual. Having a single brain would make plants much easier to kill. But having multiple such nodes forming a network, immobile plants can withstand large attacks on their "bodies" by predators or due to other natural calamities. Plants have no brain not because they are not intelligent, but because having a brain would make them vulnerable.
Switzerland is the only country in the world where, since 2008, plants have been given moral and legal rights, and Swiss citizens are required to protect these rights.
Adapted from the article by Dipanjan Ghosh in Dream 2047, Vigyan Prasar, July 2017
Reference: S Mancuso and A Viola, "Brilliant green: the surprising history and science of plant intelligence", Island Press, USA, 2015.