Can plants see?
PUBLISHED: 16:31 20 July 2020 | UPDATED: 16:31 20 July 2020
Plant expert Martyn Baguley explores the age-old debate around whether plants have the ability to see
Anyone who likes to brighten up their domestic surroundings by having pot plants scattered around rooms will have noticed that they have an unrelenting habit of leaning towards sources of light – usually a window.
The habit has a name – phototropism, from the Greek ‘photo’ meaning light and ‘tropism’ from tropos, (turning).
It puzzled our ancestors for more than two thousand years. The earliest Greek philosophers, (500 to 300BC) couldn’t agree whether or not plants, like animals, were sensitive. Some, like Plato (427-347BCE) said they were, others, like Aristotle (384-322 BCE) said that they were totally insensitive and argued this is what distinguished plants from animals.
His contemporary, Theophrastus, argued the reason plants tracked light was because the sun removed fluid from the illuminated side. That simple explanation persisted until the 17th century, when botanists began to do experiments with plants, but it wasn’t until 1864 that the German botanist Julius von Sachs discovered phototropism in plants is influenced only by blue light, which explains why houseplants bend towards windows but not towards lamps.
He argued that changes in directional growth in plants were due to light rays ‘irritating’ the plant cells. And that’s where advances in the study of phototropism rested until Charles Darwin (no introduction necessary) and his son, Francis, decided to bend their minds to the problem.
Three years and numerous simple but exquisite experiments later, with grass seedlings, they came to the conclusion that light was perceived by the tips of plant shoots but the bending response was caused by a ‘transmissible substance’ produced at the tip and transported down the shoot inducing curvature.
The ‘transmissible substance’ stumped them. Their work was published in 1880 - twenty two years after Charles’ seminal book Origin of Species had been published in a book with the title Power of Movement in Plants (It’s still available – but be prepared for a big read; their ‘simple experiments’ stretch to 590 pages of text and figures).
In the context of this article I can’t resist quoting one of the authors’ closing comments, ‘It is impossible not to be struck with the resemblance between the foregoing movements of plants and many of the actions performed unconsciously by the lower animals’.
The Darwins’ conclusions, initially rejected by some plant physiologists, stimulated worldwide research on plant responses to the environment. The ‘transmissible substance’ was identified in 1931 as a plant hormone and named ‘auxin’, the name being derived from the Greek word ‘auxein’, meaning ‘to grow’.
‘Photoreceptors’, proteins specially adapted to perceive light and initiate biological changes, weren’t discovered until 1959. Whilst the intricacies of phototropism were still being debated, during the spring and early summer of 1920 two American scientists, Wightman Garner and Harry Allard, planted plots of soya beans at two-week intervals. To their surprise, all the beans flowered at the same time, regardless of their age.
Some environmental factor was influencing the flowering. This was confirmed by more research that they carried out on some tobacco plants that produced strange results. Most tobacco plants flower during the summer but in 1906 a mutant variety appeared in the State of Maryland that grew to a height of four and a half metres (15 feet) and had as many as a hundred large leaves, five times the number on the average tobacco plant. It obviously had huge commercial potential, but that could never be realised because it was always killed by frost before it flowered.
The mutant variety was called ‘Maryland Mammoth’. The scientists took cuttings of the variety and grew them in a greenhouse where, protected from frost, they flowered in December, even though they were only half the size of the field-grown plants.
Plants grown from seeds also flowered in the winter. The scientists concluded that flowering was related to the number of hours of light the plants received and called the phenomenon photoperiodism. Numerous other experiments over subsequent years have identified three types of plant, ‘Short-day’ plants that only flower when the day length drops below a certain threshold, ‘Long-day’ plants that only flower when the day length rises above a threshold and some plants that are ‘Day-neutral’.
It is thought that some plants actually measure the length of night rather than day, the length of continuous darkness determining when a plant flowers. How this happens puzzled botanists for decades but current thought is that plants, like us, have a ‘body clock’ and photoperiodism is an interaction between that and light cues from the environment, plants only flowering when their body clock is synchronised with the ambient light.
That stimulated botanists worldwide to work on the subject in the 1940s, and the science progressed rapidly over subsequent years. The Darwins had shown that phototropism was stimulated by the light-sensitive ‘eye’ of plants located at the tip of shoots and consequently it was thought that that was the location of the ‘eye’ for photoperiodism. But then it was discovered that shining light on any part of a plant – even a single leaf - in the dark was sufficient to influence flowering of the whole plant. But cut off all the leaves leaving only the stem and tip and light has no influence on the plant. Scientists and horticulturalists soon realised the considerable commercial potential of plant photoperiodism. Cut flowers, plentiful during summer months, fetch relatively low prices, but they would be much more valuable if they were available at other times of the year like New Year, Christmas, Mother’s and Valentine’s Days.
Today, light management is widely used by commercial horticulturalists to improve the productivity of flowers, crops of lettuces, strawberries, cucumbers, peppers, tomatoes and many more plants. Research over many years has shown that plants are able to detect from the quality of light they receive if they are shaded by neighbouring plants and get a better share of sunshine by increasing their upward growth.
Visit any old Derbyshire broadleaved wood and you will be sure to see a few trees with small diameter, twisting trunks that have struggled to grow skyward and occupy spaces between the crowns of dominant neighbouring trees. Plants can also detect the time of day and time of the year by sensing and using different wavelengths of light. It is this ability that determines the onset of bud dormancy in preparation for winter in hedges and trees growing in cold climates.
Does all this help us answer the question ‘can plants see’? Let’s go back to basics and look up a dictionary definition of sight (I’ll be doing more of that using the Concise Oxford Dictionary).
‘Sight: the faculty of seeing with the eyes’, I could have guessed. Move on...
‘Eyes: the organ of sight in humans and other animals; light detecting organ in some invertebrates’, Mmmm - ‘light detecting organ’?
‘Organ: part of an organism having a special vital function.’ There is ample evidence that different parts of plants do have the ‘vital function’ of detecting light and this controls their growth performance.
So, can we say that they see?