question: How do plants and trees "know what to do"
comment: They don't have a central nervous system - their cells are largely autonomous and just act according to simple instructions. For instance, when the cells in a grass-like plant receive sufficient amounts of nutrients, water and sugar energy (from sunlight) they will initiate seed production. Much like the human body, a tree's genome will instruct it to build leaves at the end of branches (like how your genome tells your body to grow hair on the surface of your skin - it's not something you consciously chose to do). Leaf cells just automatically photosynthesize.
passages: Plants respond to environmental stimuli by movement and changes in morphology. They communicate while actively competing for resources. In addition, plants accurately compute their circumstances, use sophisticated cost–benefit analysis, and take tightly controlled actions to mitigate and control diverse environmental stressors. Plants are also capable of discriminating between positive and negative experiences and of learning by registering memories from their past experiences. Plants use this information to adapt their behaviour in order to survive present and future challenges of their environments.
   Section::::Defensive communication.:Communication.
  
  Many studies report that mycorrhizal networks facilitate the coordination of defenses between connected plants using volatile organic compounds and other plant defensive enzymes acting as infochemicals.
   Section::::Processes.:Pathway signals.
  
  Wounded tomatoes are known to produce the volatile odour methyl jasmonate as an alarm signal. Neighbouring plants can then detect the chemical and prepare for attack by producing chemicals which defend against insects or attract insect predators. Plants systematically use hormonal signalling pathways to coordinate their own development and morphology.
  
  Section::::Processes.:Pathway signals.:Neurochemicals.
  
  Plants produce several proteins also found in animal neuron systems, such as acetylcholine esterase, glutamate receptors, GABA receptors, and endocannabinoid signaling components. They may also use ATP, NO, and ROS for signaling in the same ways that animals do.
  
  Section::::Processes.:Pathway signals.:Electrophysiology.
   The concepts of plant perception, communication, and intelligence have parallels in other biological organisms for which such phenomena appear foreign to or incompatible with traditional understandings of biology, or have otherwise proven difficult to study or interpret. Similar mechanisms exist in bacterial cells, choanoflagellates, fungal hyphae, and sponges, among many other examples. All of these organisms, despite being devoid of a brain or nervous system, are capable of sensing their immediate and momentary environment and responding accordingly. In the case of unicellular life, the sensory pathways are even more primitive in the sense that they take place on the surface of a single cell, as opposed to within a network of many related cells.
   The ion flux across cells also influence the movement of other molecules and solutes. This changes the osmotic gradient of the cell, resulting in changes to turgor pressure in plant cells by water and solute flux across cell membranes. These variations are vital for nutrient uptake, growth, many types of movements (tropisms and nastic movements) among other basic plant physiology and behavior. (Higinbotham 1973; Scott 2008; Segal 2016).
  
  Thus, plants achieve behavioural responses in environmental, communicative, and ecological contexts.
  
  Section::::Processes.:Signal response.
   Section::::Types of plant cognition.:Plant memory.
   A plant's concomitant reactive behavior is mediated by phytochromes, kinins, hormones, antibiotic or other chemical release, changes of water and chemical transport, and other means. These responses are generally slow, taking at minimum a number of hours to accomplish, and can best be observed with time-lapse cinematography, but rapid movements can occur as well. Plants respond to volatile signals produced by other plants. Jasmonate levels also increase rapidly in response to mechanical perturbations such as tendril coiling.
   Section::::Plant physiology.:Plant hormones.
  
  Plants are not passive, but respond to external signals such as light, touch, and injury by moving or growing towards or away from the stimulus, as appropriate. Tangible evidence of touch sensitivity is the almost instantaneous collapse of leaflets of "Mimosa pudica", the insect traps of Venus flytrap and bladderworts, and the pollinia of orchids.
   Many plant organs contain photo-sensitive compounds (phototropins, cryptochromes, and phytochromes), each of which reacts very specifically to certain wavelengths of light. These light sensors tell the plant if it is day or night, how long the day is, how much light is available, and from which direction the light is coming. Shoots grow towards light and roots usually grow away from light, responses known as phototropism and skototropism, respectively. They are brought about by light-sensitive pigments like phototropins and phytochromes and the plant hormone auxin.
   Plant cognition
  
  Plant cognition or Plant Gnosophysiology is the study of the mental capacities of plants. It explores the idea that plants are capable of responding to and learning from stimuli in their surroundings in order to choose and make decisions that are most appropriate to ensure survival. Over recent years, experimental evidence for the cognitive nature of plants has grown rapidly and has revealed the extent to which plants can use senses and cognition to respond to their environments. Some research claims that plants have physical structures functioning in the same way as the nervous systems of animals.
  
  Section::::History.
   Environmental cues play a direct role in mediating the behavior of both plants and animals. For example, environmental cues, such as temperature change or food availability affect the spawning behavior of fish. In addition to cues generated by the environment itself, cues generated by other agents, such as ant pheromone trails, can influence behavior to indirectly coordinate actions between those agents.
   Many plants exhibit certain phenomena at specific times of the day; for example, certain flowers open only in the mornings. Plants keep track of the time of day with a circadian clock. This internal clock is synchronized with solar time every day using sunlight, temperature, and other cues, similar to the biological clocks present in other organisms. The internal clock coupled with the ability to perceive light also allows plants to measure the time of the day and so determine the season of the year. This is how many plants know when to flower (see photoperiodism). The seeds of many plants sprout only after they are exposed to light. This response is carried out by phytochrome signalling. Plants are also able to sense the quality of light and respond appropriately. For example, in low light conditions, plants produce more photosynthetic pigments. If the light is very bright or if the levels of harmful ultraviolet radiation increase, plants produce more of their protective pigments that act as sunscreens.
   Plants produce hormones and other growth regulators which act to signal a physiological response in their tissues. They also produce compounds such as phytochrome that are sensitive to light and which serve to trigger growth or development in response to environmental signals.
  
  Section::::Signals and regulators.:Plant hormones.
   In 2014, Anthony Trewavas released a book called "Plant Behavior and Intelligence" that highlighted a plant's cognition through its colonial-organization skills reflecting insect swarm behaviors. This organizational skill reflects the plant's ability to interact with its surroundings to improve its survivability, and a plant's ability to identify exterior factors. Evidence of the plant's minimal cognition of spatial awareness can be seen in their root allocation relative to neighboring plants. The organization of these roots have been found to originate from the root tip of plants.
   Section::::Defensive communication.
  
  Section::::Defensive communication.:Overview.
  
  Mycorrhizal networks can connect many different plants and provide shared pathways by which plants can transfer infochemicals related to attacks by pathogens or herbivores, allowing receiving plants to react in the same way as the infected or infested plants. A variety of plant derived substances act as these infochemicals.
  
  Section::::Defensive communication.:Defensive reactions.
   Plants can also interact with one another in their environment through their root systems. Studies have demonstrated that plant-plant interaction occurs among root systems via the soil as a medium. For instance, Novoplanksy and his students at Ben-Gurion University in Israel tested whether plants growing in ambient conditions would change their behavior if a nearby plant was exposed to drought conditions. His team wondered if plants can communicate to their neighbors of nearby stressful environmental conditions.
   Fourthly, plant physiologists study the ways that plants control or regulate internal functions. Like animals, plants produce chemicals called hormones which are produced in one part of the plant to signal cells in another part of the plant to respond. Many flowering plants bloom at the appropriate time because of light-sensitive compounds that respond to the length of the night, a phenomenon known as photoperiodism. The ripening of fruit and loss of leaves in the winter are controlled in part by the production of the gas ethylene by the plant.
   The tendrils of plants are especially sensitive to airborne volatile organic compounds. Parasites such as dodder make use of this in locating their preferred hosts and locking on to them. The emission of volatile compounds is detected when foliage is browsed by animals. Threatened plants are then able to take defensive chemical measures, such as moving tannin compounds to their foliage. (See Plant perception).
   Section::::Further research.
  
  In 2003, Anthony Trewavas led a study to see how the roots interact with one another and study their signal transduction methods. He was able to draw similarities between water stress signals in plants affecting developmental changes and signal transductions in neural networks causing responses in muscle. Particularly, when plants are under water stress, there are abscisic acid dependent and independent effects on development. This brings to light further possibilities of plant decision-making based on its environmental stresses. The integration of multiple chemical interactions show evidence of the complexity in these root systems.
   Some plants are capable of rapid movement: the so-called "sensitive plant" ("Mimosa pudica") responds to even the slightest physical touch by quickly folding its thin pinnate leaves such that they point downwards, and carnivorous plants such as the Venus flytrap ("Dionaea muscipula") produce specialized leaf structures that instantaneously snap shut when touched or landed upon by insects. In the Venus flytrap, touch is detected by cilia lining the inside of the specialized leaves, which generate an action potential that stimulates motor cells and causes movement to occur.
  
  In plants, the mechanism responsible for adaptation is signal transduction. Adaptive responses include:
   Plants also communicate via electrical signals, which is explored in Calvo et al. 2017. These electrical signals are mediated by cytosolic Ca2+ ions. Cytosolic calcium signals are mediated by hundreds of protein and protein kinases, and many of the signals also induce action potentials in plants. The phloem of the plant serves as the pathway for electrical communication, and as the plant grows and learns from its past, the phloem becomes increasingly cross linked. Plants respond to various environmental cues and elicit electrical responses internally to alter the function of the plant body. This can range from avoiding predation, releasing defense mechanisms, responding to changing temperature, changing growth direction, and sharing nutrients in the soil. This form of memory stored in the plant’s phloem allows it to better respond to similar stimuli in the future and shows how electrical signaling allows a plant to communicate with itself and alter its’ own physiology to better suit certain environmental cues (Calvo et al. 2017).
   Plants do not have brains or neuronal networks like animals do, at least in the traditional sense; however, reactions within signalling pathways may provide a biochemical basis for learning and memory in addition to computation and basic problem solving. Controversially, the brain is used as a metaphor in plant intelligence to provide an integrated view of signalling.
   Section::::Evolutionary and adaptational perspectives.:Adaptive mechanisms of mycorrhizal fungi and plants.
  
  Although they remain to be vigorously demonstrated, workers have suggested mechanisms which might explain how transfer of infochemicals via mycorrhizal networks may influence the fitness of the connected plants and fungi.
   Section::::References.
  
  Calvo et al. 2017. “Plant, Cell, & Environment. Are Plants Sentient?”
  
  Gorzelak et al. 2015. “Inter-plant communication through mycorrhizal networks mediates complex adaptive behaviour in plant communities.”
  
  Martin Heli and Richard Karban. 2010. “Explaining evolution of plant communication by airborne signals.”
  
  Phillip et al. 2011 “Pathways for below ground carbon transfer between paper birch and Douglas-fir seedlings”
  
  Runyon et al. 2006 “Volatile Chemical Cues Guide Host Location and Host Selection by Parasitic Plants.”
  
  Zebelo et al. 2012. “Plasma membrane potential depolarization and cytosolic calcium flux are early events involved in tomato (Solanum lycopersicon) plant-to-plant communication”.
   The idea of cognition in plants was first explored by Charles Darwin in the late 1800s. In the book "The Power of Movement in Plants" written together with his son Francis, he used a neurological metaphor to acknowledge the sensitivity of plant roots when he proposed that the tip of roots acts like the brain of some animals, even though plants neither possess actual brains nor nerves. 
  