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Over thousand and thousand years plants developed a complex sensory and regulatory system [Chamovitz2013, page 4]. Their inability of locomotion forced them to become masters in an adaptive development. Therefore, it is obvious to research plant functions in more detail. This chapter adopts the content structure from Daniel Chamovitz’s book “What a plant knows” [Chamovitz2013]. He compared the plant sensory system with the human system. In relation to Human Plant Interfaces this structure is very useful to present the most obvious differences between the human and the plant sensory system. The starting sections will explore the senses see, smell, touch and hear. Afterwards, the following sections will investigate the plant ability of orientation and memory. This summary mentions only the basics of plant physiology related to this thesis based artistic research. For more details about plant physiology, the book “Physiology and Behaviours of Plants” by Peter Scott (2008) is a very good recommendation and for a regular basis the Journal Plant Signaling & Behavior is highly recommended.

The sense sight

A plant cannot see like a human does, but it can recognize certain kinds of light. Light is an electromagnetic wave and appears in different frequencies. These different frequencies are important for the plant’s lifecycle and health. The position of the plant sight sensor is located in the tip of the stem; to be more accurate for young plants, it is located in the tip of a seedling [Chamovitz2013, page 15, 16].

Moreover, plants do also notice the direction of light [Chamovitz2013, page 10]. The growing process directed to the light source is called phototropism [Chamovitz2013, page 18]. The Cryptochrome of the plant transforms blue light and causes the plant’s growth. Additionally, it manages the circadian clock of plants [Chamovitz2013, page 29], too.

Plant impact	Growing (and circadian clock)	Flowering
Scientific term	Phototropism	Photoperiodism
Location of the sensor	Tip of the stem	Leaves
Receptor	Cryptochrome	Phytochrome
Wavelength of light	Blue light	flowering on needs red light. This is normally the case during the morning hours.	flowering off needs far red light, which exist usually during evening.

Another sight sense is located in the plant leaves and manages the flowering process [Chamovitz2013, page 22]. The flowering process is influenced by the amount of red light or by the length of the night [Chamovitz2013, page 20]. This ability of sensing is determined as photoperiodism [Chamovitz2013, page 18]. The phytochrome of the plant leaves measure the red light and takes over the role of a light activated switch. Depending on the kind of red light the flowering process is turned on or off. The plant is able to remember the last color of red light [Chamovitz2013, page 21]. Far red light exists in the evening and turns off the flowering. During the morning the amount of red light is the biggest and that turns on the flowering process. When the plant really starts flowering depends on its kind. A plant can be a short or long day plant [Chamovitz2013, page 19]. A long day plant needs long sunny days to flower. They start flowering when the days changing from short to long. This is mostly the case during the time period of late spring and early summer. In contrast to that, short day plants flower when the days getting shorter. Usually this condition is complied after the end of June, when the nights are getting longer. Short day plants need a certain time of darkness for their flowering stimulus.

The mechanisms of phototropism and photoperiodism are important for plant displays that use light as an impact factor. The different light waves can influence the appearance of a plant display massively. Chapter 3.1 will explore plant displays in more detail.

The sense smell

The plant’s mechanism of smell is more complex than the mechanisms of sight [Chamovitz2013, page 34]. Plants emit odours for attracting humans and animals. Moreover, its odours are used for communication between different plant parts as well. Plants from the same species understand these signals and can react on it [Chamovitz2013, page 33]. How plants behave on this smell stimuli differs from species to species. Every plant distinguishes oneself in behaviour and signalling. Every plant communication through smell is a communication with its environment.

 

 

Almost every plant has the ability to perceive odour or scent through stimuli [Chamovitz2013, page 35]. The plant extracts a unique chemical (signalling) that can bind to a unique receptor (communication). Depending on the plant certain behaviour will be executed (see figure above).

For instance, rapid fruit ripening is caused by Ethylene, which exists in smoke [Chamovitz2013, page 37]. Even some plants use smell to find its best location. For example, the parasite plant Cascuta is very strong attracted by the tomato smell [Chamovitz2013, page 42], because tomatoes provide the best nutrients for the Cascuta compared to other plants. More complex is the chemical signalling between plants and insects, which is often related to realm of plant defence. In 1983, scientists revealed the plant ability to send warning signals of leaf-eating-insects [Chamovitz2013, page 43]. For instance, after a tree received the warning signal, it starts to enrich its leaves with phenolic and tannic chemicals. After this process, the leaves are unpalatable for the insects. Some plants can attract with their changed smell other insects that will kill the leaf-eating-insects. Entomologists investigate exactly these kinds of communication between plants and insects and they document its ecological impact [Chamovitz2013, page 45].

The production of chemical units is not only addressed to plants and animals. Humans benefit from this process, too. For example, many plants are used for medical approaches [Chamovitz2013, page 54] [Kastner2012, page 163]. Moreover, our emotional connection to plants is strongly based on smell. Humans can also smell for example good food, trouble, and stress. The smell of a plant can be perceived as positive or negative by a human [Chamovitz2013, page 58-59]. It depends on the human judgement if the experience is correlated with positive or negative emotions.

However, these chemical changes within a plant can provide us interesting information about our environment. Thus it is an additionally resource for monitoring environmental data with the help of biosensing. This data can be used for a better understanding of natural driven ecosystems.

The sense touch

The plant communication through smell is ambient to its environment. In contrast to this, the plant`s behaviour to touch is very limited and directed to certain parts of a plant. Plants detect tactile sensations through touch, wind or temperature [Chamovitz2013, page 62]. The touch sensations are distinguished in soft and hard interactions (see figure underneath). For instance, grass provides a soft touch interaction. It usually bends, when it is touched. In contradistinction to grass, thorns do not really bend, when they got touched. When the pressure on the thorns is too strong, then the chance is high that they break. Therefore thorns can be assigned to hard touch interactions with a plant.

However, tactile sensation has mostly a negative impact on the plant growth [Chamovitz2013, page 62] and health [Chamovitz2013, page 76]. Frequently touched plants or in a windy environment grow normally less well than in a more plant friendly environment. The tactile signal within a plant is caused by plant’s change of action potential on a cellular level. The rapid concentration change of additional ion and calcium increases the electrical activity of a cell [Chamovitz2013, page 65]. In human bodies this electrical signal travels through the nervous system to the brain, and then the brain stimulates the muscles. The plant does not have such a nervous system. The signal of the plant travels through the body and its behaviour depends on its species. The action potential (touch signal) of a plant can be detected with an electrode [Chamovitz2013, page 70]. Electrodes are very common tools in human medical purposes (e.g. EEG, ECG). Consequently, plants can provide a resource for touch interfaces in context of Human Computer Interaction.

 

 

As previously mentioned, most plants react negatively to touches, nevertheless some plants developed their own behaviour to deal with touches. For instance, the plant Mimosa pudica closes its leaves immediately after it got touched. Several minutes later it opens the leaves again [Chamovitz2013, page 72]. These open and close behaviours cause higher peaks of action potential within the plant. It is much easier to detect these peaks than the ones of other plants. Moreover, the open and closure states of the plant’s leaves can provide a visual feedback for human plant interfaces. The venus flytrap also open and close its trap, but its mechanisms differ from the Mimosa pudica. The venus flytrap has several hairs inside the trap. In order to spring the trap at least two of these hairs have to be touched within a time period of twenty seconds. John Burdon-Sanderson revealed that the touch detection of the flytrap is based on an electrical signal [Chamovitz2013, page 69]. Around 2007, Alex Volkov, T. Adesina, and E. Jovanov established an artificial electrical signal that causes the flytrap to close without any stimulus from the hairs [VoAdJo2007]. This experiment is particularly appealing in the context of plant displays. Their scientific finding could enable plant displays an immediate visual response based on the flytrap venus plant.

The biochemist Janet Braam explored the plant Arabidopsis thaliana and its cellular metabolism changed by touch. She revealed that the genes and hormones of the plant can get continuously altered by touch. Her result is a plant with modified cells that can light up when they are getting touched. Then an ultra-light sensitive camera can detect these touch areas. This might be an interesting approach for recognizing touch interactions for Human Plant Interfaces. Furthermore, the changes in the cell are stored in the TCH Genes of the plant [Chamovitz2013, page 78]. In that relation, it seems a plant is capable of memorizing touch interactions over several generations. This finding might be useful for plant based data sculptures.

When touch has such strong impact on plants, the question of plants do feel pain is very close. Based on current scientific knowledge we assume plants cannot experience pain like humans do. The human perception of pain is always related to emotions; therefore it is subjective [Chamovitz2013, page 82]. According to David Chamovitz plants have no capacities for an emotional experience [Chamovitz2013, page 172] and for this reason plants does not feel pain in Chamovitz opinion. On that account plants can be utilized for biosensing applications without any concerns.

The sense hearing

Hearing is a sense for rapid communication between individuals and animals. Living organisms with capabilities in locomotion and fast movements benefit strongly from the sense hearing [Chamovitz2013, page 109]. Plants cannot really change their locations nor do they perform fast movements. If plants do something close to moving it happens usually very slow. Therefore, it is obvious that plants are a counterpart to rapid movements. Hence, the sense hearing does not make much sense for plants. That might be also the reason why plants can be assigned as deaf living organisms [Chamovitz2013, page 107].

Even more, scientists are not focused strongly on this topic. Real scientific findings rarely exist, only some pseudo-scientific research. For instance, Dorothy Retallack investigated plant behaviour on different kind of music [Chamovitz2013, page 93-95]. She assumes that plants grow better with classical music than with rock music. Her results do not fulfil the scientific requirements and cannot be used for scientific assignment.

Nonetheless, some plants get influenced indirectly by an acoustic environment. Ultra-sonic vibrations caused by insects or deep bass sound can have an impact on plants [Chamovitz2013, page 107-108]. In addition to that, Janet Braam proved that vibrations have no effect on the plant’s touch genes and for this reason they are not inherited to the next generations [Chamovitz2013, page 95]. Moreover, speakers can radiate heat while they play music. This radiated heat can influence the plant health indirectly [Chamovitz2013, page 101]. All the mentioned indirect factors can affect the living system of a plant, but they cannot be connected with the sense hearing.

Orientation

It seems obvious for us that the stem of the plant grows up and the roots grow down. But how does a plant knows what is up and down? How does a plant know what is soil and what is air? The mechanisms of proprioception and gravitropism answer these questions.

The proprioception of humans work very differently compared to plants [Chamovitz2013, page 130]. For instance, the human sense of balance is located in the ears [Chamovitz2013, page 113]. In contrast to this, the plant sensors are distributed in two different areas of a plant. The root tips contain gravireceptors which detects the direction of gravitation [Chamovitz2013, page 119 + 125]. The plant ability of gravitropism enables the roots to grow towards the gravitation centre. The plant assumes that the gravitation force is the best orientation source for its roots growth [Chamovitz2013, page 119 + 125].

The growing process of the stem and branches is influenced by several parameters: Gravitation [Chamovitz2013, page 137], light, and sometimes smell (e.g. in case of the plant Cascuda). As mentioned during the exploration of the sense sight, the tip of the stem can detect the direction of the light source and bends its stem to this direction. Every plant has its own movement characteristic. Some plants grow in a circle or ellipse movement, some grow faster, others grow slower [Chamovitz2013, page 131]. Charles Darwin (1880) was fascinated about the plant movement and documented it in his book “The movement of plants” more accurate. Moreover, the current botanists Andreas Bresinsky, Christian Körner, Joachim W. Kadereit, G. Neuhaus, Uwe Sonnewald documented the plant movements and growing behaviours very systematic [BrKaNeSo2008, chapter 7].

The scientific findings about the orientation sense of the plant are important in relation to plant displays. The plant ability of gravitropism can enhance the application scope of plant displays massively (e.g. vertical plant displays). Moreover, the artists have to decide if the use of plant movement is appropriate or not.

Memory

The human reaction and behaviour is often based on memories and experiences from the past. A big amount of this information is stored in the human brain. Plants do not have a brain, but that does not mean they cannot remember anything. Every kind of plant developed their own unique memory system. Each plant that bloom has the ability of remembering the last light condition. Plants know how to react on pests and how to call for help through emitting their certain odours. A venus flytrap can store the last state of an action potential.

In context of touch genes, plants inherit their memory to their new generations (Epigenetic) [Chamovitz2013, page 161]. All these memories of a plant have one characteristic in common. They react on their conditions with a much more delayed response than human or animals do [Chamovitz2013, page 161].

If the memory system of a plant is useful for Human Plant Interfaces depends on its application. Surely, the number of various memory systems is as big as the number of different kinds of plants. A classification of the plant memory specifications related to Human Plant Interfaces does not exist yet and would be very desirable. For this reason it is not possible to give any outlook about utilizing plants memory capabilities.

Plant Physiology and Human Plant Interfaces

The domain of plant physiology is a very multidisciplinary field. Biochemists, physicians, entomologists and many others work together in this area. The connection to Human Plant Interfaces rarely exists in this field, although it provides many new possibilities in biosensing and interaction design within natural environments. Chapter 3 of this thesis is dedicated to this topic. It describes how some of these mentioned findings are applied for artistic applications.

References:

[Chamovitz2013] Chamovitz, Daniel (2013). What a plant knows. Scientific American / Farrar, Straus and Giro, 2013.

[Kastner2012] Kastner, Jeffrey (2012). Nature - Documents of Contemporary Art. MIT Press, 2012.

[BrKaNeSo2008] Andreas Bresinsky, Christian Körner, Joachim W. Kadereit, G. Neuhaus, Uwe Sonnewald (2008). Strasburger - Lehrbuch der Botanik. Springer Spektrum Akademischer, 2008.

[VolAdJo2007] Volkov, Alexander G.; Adesina, Tejumade; Jovanov, Emil (2007). Closing of Venus Flytrap by Electrical Stimulation of Motor Cells. In Plant Signal Behav. 2007 May-Jun; 2(3): 139–145.