Do plants have memory and consciousness?

The author is a leading researcher in the field of plant sensory communications and he reviews the literature by asking hypothetical questions such as; does a plant has the ability to see, hear, feel, think and react to external stimuli like animals do. He has a unique way of expressing himself, and in the process tries to answer each question so that a reader can understand and appreciate the subject. He describes simple plant experiments that demonstrate how a plant responds to external stimuli. The author argues that even though the plants doesn't have a central nervous system (with a brain and a system of branching nerves and nerve endings), but certain experimental evidences lead to the conclusion that plants do have cell to cell communication and they do respond to internal and external stimuli like animals. This is also supported by genetics of plants and animals where many animal genes are found in plants. With his arguments and clever plant experiments described in this book, he convinces the reader that the traditional definition of memory and thinking has to be revaluated, since the "neurobiological" mechanisms at the molecular and cellular level are very similar in plants and animals. A recent study published in the journal Plant Cell, biologists found that one leaf of a plant named Arabidopsis thaliana exposed to light caused the whole plant to respond to the light; and this response lasted even after the light source was taken away. This shows that plants send electrochemical signals at cellular levels in leaves that were not exposed to light; and they were responding to the electrical signals. In a similar experiment with tomato leaves, it was found that wounded leaves leads to responses in the unwounded leaves on the same plant. These responses include the transcription of a class of genes for proteinase inhibitors in intact leaves. This was accomplished by the transmission of electrical signals with the coordinated functions of ion channels during the cell to cell communication.

Most animals choose their environment to find food, mate, and migrate with changing seasons, but plants have to adapt to their environment and hence have evolved complex sensory and regulatory systems. A head lettuce has to know if there are ravenous aphids so that it can defend by secretions against the invading aphids. An Elm tree knows when and where to branch if its neighbor is shading it from sun; and a cherry tree knows when to flower.

Plant can feel the touch and they also differentiate between hot and cold temperatures. Vines start rapid growth upon contact with an object like a fence and carnivorous parts snap/trap when an insect lands on its surface. The electrochemical signals generated in Venus flytrap are very similar to the action of a nerve during muscular contraction of an animal. The plant responses are not subjective, and they do not experience pain like animal do, but they feel mechanical stimulations.

Plants do not have ears but apparently they hear music since certain types of music has positive influence in the growth of plants, but some biologists insist that it may be related to very small temperature differences or even being touched by the sound vibrations caused by the music, which imply mechanical response rather than music response.

Experiments in space with arabidopsis plants have demonstrated that gravity is essential in amplifying the movement/growth in plants. Humans and plants respond to gravity in similar ways.

Plants benefit from rich and varied sensory inputs, but plants do not have memories and they don't cower at the thought of drought or dream. They don't watch or remember the way plants grew under their shade or in their immediate neighborhood. They however retain past events and recall the information at a later period. Tobacco plants know the color of the last light they "saw" and Willow tree knows if their neighbors have been attacked by caterpillars. These examples illustrate a delayed response to a previous occurrence which is a key component to memory.

The mechanism of memory is not completely understood. We know that electric signaling between neurons is important for memory formation and storage, but the molecular and cellular mechanism are sketchy. The latest research hints that our memory is infinite but only very small number of proteins is involved in memory process. There are several types of memory; sensory memory, short-term memory, long-term memory, muscle-motor memory, and immune memory. All these processes follow three simple mechanisms; encoding information, information storage and its retrieval. In Venus flytrap, the plant has a short-term memory in a mechanism by which it traps an insect lurking on the surface.

There are three levels of memory associated with increasing levels of consciousness. Procedural memory is associated with anoetic consciousness; semantic memory is associated with noetic consciousness; and episodic memory associated with autonoetic consciousness. Plants appear to have the lowest level of consciousness, the anoetic consciousness that refers to the ability of the plant to sense and react to external and internal stimulation.

Reference: What a Plant Knows: A Field Guide to the Senses by Daniel Chamovitz