Plants (Plantae Haeckel, 1866), also called vegetables, are single- or multi-cellular, photo-aerobic eukaryotic organisms with chloroplasts of primary endosymbiotic origin. There are more than 400.000 cataloged plant species. Plants include trees, shrubs or bushes, grasses, creepers, succulents, ferns, mosses, green algae, and many others.
Most plants are included in the group of Angiosperms, with about 250.000 species, which are distinguished from other groups by the production of flowers, followed, after pollination, by the formation of seeds enclosed and protected within a fruit.
The most important branches of biology that deal with the study of plants are botany for systematics and anatomy, plant physiology for their functioning, and plant ecology, which studies the distribution of plants and the effect of environmental factors that influence this distribution, as well as the interactions between plants and other organisms.
In biology, plants have some fundamental characteristics:
- they are formed by eukaryotic cells, that is particularly evolved cells, equipped with a nucleus protected by a membrane;
- they are autotrophic organisms, for the energy supply they carry out the chlorophyll photosynthesis, a set of biochemical reactions, which allows them to capture part of sunlight by transforming carbon dioxide into sugars and other substances;
- the cell walls are structured with a cellulose base and the cells themselves can store starch as a reserve energy source.
The precise limits of the kingdom of Plants, as far as lower and particularly unicellular organisms are concerned, have been the subject of partly discordant assessments. Initially, the kingdom of Plants (more exactly the kingdom Vegetable, see below) also included heterotrophic organisms (such as animals) such as Fungi, and all bacteria and archeobacteria. Later, Plants were restricted to only multicellular autotrophic organisms, referring all unicellular organisms also autotrophic to the kingdom of Protists.
Today prevails the tendency to bring back in the kingdom of Plants the autotrophic unicellular organisms, provided that eukaryotes. This applies in particular to green algae, traditionally included in the Protists; they would be part of the kingdom of Plants, because they have cells with cellulose walls, contain the same type of chlorophyll as terrestrial plants and produce starch by photosynthesis.
There are also other positions, such as that of researchers who still consider Plants a well circumscribed taxonomic group, from which they reiterate the exclusion of algae. Even more controversial is the placement of the red algae or Rhodophytes, which have a less close relationship than the green algae with the higher plants. Prokaryotes capable of photosynthesis, particularly the group of blue algae (more correctly called Cyanobacteria), remain unanimously excluded.
Because of their structural simplicity and close phylogenetic proximity, green algae are considered ancestors of land plants. According to this hypothesis, about 400 million years ago, some freshwater green algae (the Charophyceae or the Charophytes according to the different taxonomic frameworks), peeped out on the shores of lakes briefly exposed to the air. These thin green strips around the water areas were the only vegetation on land, then completely deserted.
Photosynthesis conducted by plants and algae is the main source of energy and organic matter (the phytomass) in almost all ecosystems. This process led to a radical change in the composition of the original atmosphere, caused by an increase in the amount of oxygen, which now occupies 21% of its volume. This allowed the development of aerobic organisms and later the arrival of life in the sub-aerial environment. Thanks to autotrophy, plants are the primary producers in terrestrial ecosystems, forming the basis of the food chain, on which the existence of animals and other heterotrophic organisms depends.
Plant species play an important function within the water cycle (evapotranspiration) and other biogeochemical cycles. Some plants have co-evolved with nitrogen-fixing bacteria, which are essential to the nitrogen cycle. In addition, root development has a well-defined role in soil evolution (pedogenesis) and, together with the canopies that form the plant cover, in preventing its erosion.
Plants are also the dominant organisms of the various terrestrial biomes, whose names derive from their characteristic vegetation types. Numerous animals have co-evolved with plants, both assuming specialized forms and behaviors to foster a mutualistic association that, at times, becomes so close as to literally bind the two species for “life,” because the disappearance of a particular plant causes the immediate extinction of the symbiotic animal species and vice versa. While the plants provide dens, sites for reproduction and food in quantity, some animals, called pollinators, promote the pollination of flowers, others the dispersal of seeds. Myrmecophytes are plants that have co-evolved with ants, which defend them from herbivores or competing plants and fertilize them with their organic waste, in exchange for a home and, not always, food.
Besides bacteria and animals, plants frequently establish symbiosis with fungal species through their roots, forming an association called mycorrhiza: fungi help the plant to absorb water and nutrients present in the soil; the plant offers in exchange the carbohydrates produced by photosynthesis. Other species host inside endophytic fungi which protect the plant from herbivores by producing toxins. In Orchidaceae, seeds are lacking or lacking in endosperm and germination cannot occur without the help of a specific fungus.
Like all living things, plants can be sensitive to molecules because their cells are equipped with receptors for such substances; they use these receptors, for example, to receive information from the environment. If root cells pick up on the presence of nutrients such as nitrogen and phosphorus, root growth will turn in the direction of the elements. Plants are also able to react in real time to a mechanical stimulus. Carnivorous plants have this characteristic.
For example, dionea has sensitive hairs on its trap leaves that detect the presence of insects and allow the traps to close immediately, taking less than a second, and Mimosa pudica retracts its leaves if touched. Plants detect light by molecules on their leaves (such as phytochromes) that act as receptors. Several plant species can sense soil moisture, gravity, CO2 or other chemical compounds. As a passive defense they use hundreds of molecules, such as salicylic acid, morphine, nicotine and caffeine. These molecules make the plant unpalatable or poisonous.
The emission of some molecules occurs when predation occurs; for example, mugwort, when wounded, emits chemical compounds that cause nearby plants to react. Tobacco, cotton, or Peruvian bean, when attacked by insects, produce molecules that attract other predatory insects that rid them of their attackers. Plants use the filaments (mycelia) of fungi that live in symbiosis with the roots, exchanging chemical signals, forming a network much larger than that of the roots alone.
Charles Darwin had already surmised, in his The power of movement in plants (1880), that roots could be considered the site of plant information processing phenomena. Each segment of the rootlets has a particular function when it enters the soil. It is able to sense environmental conditions and produces and propagates electrical signals. Mimosa is also able to locally store past events. After a few of these harmless strokes it stops closing its leaves, showing the phenomenon of habituation.
Evidence from the study of tomato, tobacco and other plants has shown that each plant emits high-pitched sounds when stressed, in the range of -65 db spl between 20 kHz and 100 kHz and therefore the sound emitted is quite loud and can be heard from several meters away in part by humans, other animals, insects and other plants. These sounds were recorded by a team led by Itzhak Khait at Tel Aviv University in Israel, and subjected to an artificial intelligence program that was able to predict from just listening to the sound emitted even amidst ambient noise, the state of the plant as dry, severed or intact. When the plant is intact and healthy, it emits very few sounds, practically negligible and often not even detectable, it practically remains silent, while when it is subjected to stress, the ultrasound is sharp, detectable and acute. The emission of sounds is produced by cavitation, the production of small bubbles inside stressed plants. Some plants are more “talkative” than others, for example the “whine” of the tomato is three times more frequent than tobacco, and also the “motivations” are different, for example some plants make more noise if they have little water than if they are damaged.
Animals such as humans can hear, only if they are in perfect health, a part of the sounds emitted by plants, whereas other mammals such as bats, for example, are able to clearly hear the whole spectrum of sounds emitted, and therefore it is assumed that for them a forest is too noisy to sleep in, or that insects such as moths that also perfectly hear the whole range of sound emissions of plants, prefer to lay their eggs on healthy and therefore quieter trees. Perhaps some of the legends on this topic, such as the heartbreaking scream that the Mandrake emits when it is pulled out, has a basis in truth as perhaps some people can actually hear the very loud and annoying sound, and probably many pets hear it well and clearly.
As an application, this discovery opens the door to the design of artificial intelligence systems and algorithms in crops that provide watering for plants when they ask for it.