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The meristems are the stem cell niches in plants, regions of the plant that contain undifferentiated stem cells. These cells are classified as stem cells because they fit the criteria of being self-renewing, undifferentiated, totipotent or pluripotent, and are found in a specialised area called a niche. Both plants and animals have stem cells that can generate new cells. However, plants not only have the ability to regrow entire organs from their stem cells, but can also regenerate themselves from any one of their cells. That is, if the organism were to be dealt cataclysmic damage - such as a tree being cut down to its base - it would be able to regenerate completely. This is due to the fact that all of a plant’s cells, unlike an animal’s cells, are totipotent. When the plant is wounded, the differentiated cells around the wound will dedifferentiate into a group of cells - called a callus - that can grow into any new organ the plant needs. Depending on the concentrations of hormones the callus is exposed to, it will differentiate into different organs. Higher concentrations of auxin lead to more root formation and higher concentrations of cytokinin lead to more shoot formation. From this callus can be grown new organs or even an entirely new plant. Although it is true that all plant cells can be totipotent and self-renewing, they do not fulfill the two other criteria that would make them stem cells, being undifferentiated and being in a niche. Thus, not all plant cells are stem cells, only the undifferentiated cells in the meristems count as stem cells.

The meristem is the region of the plant that contains undifferentiated stem cells, the stem cell niche in plants. These are classified as stem cells because they fit the criteria of being self-renewing, undifferentiated, totipotent or pluripotent, and are found in a specialised area called a niche. Both plants and animals have stem cells found in niches, however, a unique feature of plant cells is that they are all totipotent. That is, if the organism were to be dealt cataclysmic damage - such as a tree being cut down to its base - it would be able to regenerate completely. This process takes place through the formation of a callus. When the plant is wounded, the differentiated cells around the wound will dedifferentiate into a group of cells, the callus, that can grow into any new organ the plant needs. Depending on the concentrations of hormones the callus is exposed to, it will differentiate into different organs. Higher concentrations of auxin lead to more root formation and higher concentrations of cytokinin lead to more shoot formation. Though it is true that an entire plant can be regenerated from a single one of these dedifferentiated cells, that does not mean all plant cells are stem cells. Stem cells must be undifferentiated and found in a niche, as such, although all plant cells can regenerate an entire organism, most of them are already differentiated and are not found in a niche.

There are many different plant cell types, but there are a few unique types that distinguish plants from animals. Meristem cells are found in the root apical, shoot apical, leaf bud, and vascular cambium meristems of plant, and are the regions of new plant cell synthesis. Undifferentiated cells in the meristems will turn into leaves, flowers, roots, xylem, phloem, and other plant organs depending on internal and external stimuli. Xylem and phloem are the vessels through which water and sugar flow, respectively. Xylem are dead cells, consisting of thin tracheids and large vessels, transporting water from root to shoot. Phloem are live cells, comporting two cell types joined together via branched plasmodesmata. The sieve element is largely devoid of organelles and even lacks a nucleus, leaving it mostly hollow, which allows for the transfer of nutrient laden liquid throughout the plant. The companion cell is linked to the sieve element and provides it with the cellular products it cannot make itself. Root hairs are epidermal cells found on roots that extend via tip growth in order to increase root surface area, allowing for more water and nutrient absorbance. Pollen is the plant equivalent of animal sperm and serves to fertilize female plant gametes. It is made up of two, sometimes three, nuclei of two types, vegetative and generative. The vegetative nuclei is responsible for the growth of the pollen tube, which extends into the stigma of the plant in order to allow for the male gametes produced by the generative nuclei to fertilize the egg and the endosperm. Stomata and trichomes are specialized structures found on any aerial plant surface. Stomata are openings that allow for gas exchange and are regulated by guard cells who open and close in response to blue light. Trichomes come in two forms, glandular and non-glandular, and more than one type of trichome can be found on a plant. The glandular form releases chemicals when burst that can deter predators or attract pollinators. The non-glandular form can be used as physical defence, trapping the predator on the plant, where it will die of thirst. Additionally, the non-glandular form provides an increased boundary layer that minimizes airflow around the aerial parts of the plant, thus decreasing water evaporation.

Microtubules in plant cells are responsible for a series of structures and processes absent from animal cells. In plant cells, microtubules stack at the cell cortex as parallel loops. These loops act as the tracks on which cellulose synthase moves while it synthesizes cellulose microfibrils. As such, the cellulose microfibrils are produced parallel to the cortical microtubules. Disrupting either of these processes results in isotropic growth. Next, during cell division, cortical microtubules will accumulate into a band around the nucleus known as the preprophase band (PPB). The PPB dissolves before spindle assembly leaving behind division markers that label where the cell plate will join the cell wall. Lastly, microtubules play another important role in plant cell division during the formation of the phragmoplast. The phragmoplast is a complex of microtubules, actin, and vesicles that is generated after nuclear separation and serves as the cytokinetic mechanism of plant cell division. To generate the cell plate precursor to the cell wall that will eventually divide the new plant cells, microtubules are arranged parallel to one another pointing towards the region demarcated by the division markers. Along the microtubules are transported vesicles containing callose that will fuse to form the cell plate, growing it until it reaches the pre-existing cell wall. This mechanism generates a precursor cell wall made of callose that will eventually be replaced with cellulose. Additionally, the formation of the cell plate involves production of primary plasmodesmata due to the presence of the endoplasmic reticulum (ER). The ER blocks the cell plate in certain areas generating holes in the cell wall that will become the primary plasmodesmata between the newly divided cells.

Signals transmitted between animals usually have to travel some distance, and are often affected by their environmental. The first kind of environmental effect is attenuation, which is simply defined as an increase in the faintness of a sound with increasing distance. Sounds propagate spherically and base attenuation predicts a 6 dB decrease in sound intensity for each doubling of distance. However, sounds rarely conform to this model and experience excess attenuation due to the environment. Excess attenuation is affected by foliage density, temperature, humidity, and many other environmental conditions. Generally speaking, high frequency sounds will attenuate faster because they tend be more absorbed by the atmosphere than low frequency sounds. However, low frequency sounds close to the ground tend to suffer from interference due to sound waves reflecting off of the ground. The second kind of environmental effect on signals is degradation. Sound reflecting off of the environment will cause successive elements to become difficult to distinguish and will blur element form, effects collectively known as reverberation. This effect is especially noticeable in high frequency, rapidly modulated sounds, which suffer from greater scattering and element blurring. Scattering occurs more in high frequency sounds as they tend to bounce off of objects instead of wrapping around them like low frequency sounds. In forests, there are many objects that will cause reverberation, causing element form to break down, especially at high frequency. Rapidly modulated sounds, frequency or amplitude modulated, will degrade much more quickly in high object density environments, due to scattering and interference. Songs with elements in quick succession, such as rapid trills, will become more blurred the higher the object density in the environment. Whistles however will retain their frequency and won’t be blurred when they reflect off of the environment. As such, it is often the case that birds will communicate using lower frequencies and simpler songs in forests as opposed to open environments.

Neurons communicate with one another in two ways, electrical synapses and chemical synapses. Electrical synapses are less common than chemical synapses and allow neurons to communicate with one another purely via ion exchange. The gap of an electrical synapse is approximately 3 nm wide and is bridged by gap junctions, themselves made up of connexons. These synapses facilitate communication in the nanosecond range - known as ultrafast transmission - and are especially found in areas of the body where response synchrony is required. Chemical synapses are the most common synapse type in the body and are an order of magnitude wider than electrical synapses, spanning ranges of 20 to 50 nm. A variety of chemicals are passed across this synapse type. First, amino acids are rapid transmission small molecules that directly create an excitatory or inhibitory response in the postsynaptic cell. Glutamate is an excitor while glycine, and GABA are inhibitors. Second, amines are slightly larger organic molecules that have an amine group, and include substances such as acetylcholine, dopamine, and epinephrine, that generally act as neuromodulators. Of these, acetylcholine is the only fast transmitter, and also serves as a direct excitor like glutamate. Third, neuropeptides are short polypeptides that act as neuromodulators, affecting how receptors will respond to different neurotransmitters. Neurotransmitter reception can yield a  wide array of effects, ranging from simple inhibitory or excitatory postsynaptic potentials to modulatory effects with many downstream responses. The variety of neurotransmitters and their associated responses is often pinned as playing a strong role in our neural complexity.

Although plants are sessile organisms with unmoving cells, plant cell organelles are in constant motion. Cytoplasmic streaming is a phenomenon exclusive to plant cells wherein organelles will rapidly migrate around the cell. Streaming is controlled by myosin motors anchored to an actin filament network. This phenomenon necessitates only the actin cytoskeleton as a series of experiments showed that streaming doesn’t require the cooperation of microtubules. Additionally, these experiments demonstrated that actin filaments can maintain cytoplasmic streaming by simply disassembling and reassembling recycled actin monomers. In light of these discoveries, there are three non-mutually exclusive theories regarding the mechanism of cytoplasmic streaming. Active streaming theory asserts that myosin motors riding along actin filaments walk organelles around the plant cell. Passive streaming theory posits that the bulk of organelle movement around the plant cell is driven by a cytosolic stream produced by the active transport of only a few organelles. Endoplasmic reticulum (ER) anchor theory states that the majority of organelles are bound to the ER and that myosin motors move the ER, thus dragging the organelles anchored to it. Research has provided evidence for all three of these theories, and it has yet to be shown if they contribute to streaming in equal part or if one model predominates.

The electrochemical gradient is one of the most important locomotive forces in animal cells. The first part of the electrochemical gradient is, counter-intuitively, the concentration gradient. This refers to the difference in concentrations of a small molecule across a membrane. For example, in axons, there is a downward potassium gradient out of the axon, that is, there is a higher concentration of potassium in the axon than outside of it. Which means, all other conditions being equal, potassium would flow out of the axon if given the chance. Another example of this phenomenon is osmosis. Plant cells employ a proton pump to push protons into the extracellular matrix against their concentration gradient. They then use the energy generated by the proton flowing down its concentration gradient and back into the cell to import metabolites. This generates a higher solute concentration inside the cell than outside the cell. Water flows from low solute potential to high solute potential, or from high water concentration to low water concentration. The plant cell takes advantage of this phenomenon and increases rate of water diffusion into its cytosol in order to increase turgor pressure. The electrical gradient refers to the difference in charges across a membrane. Charges can interact across membranes as they are, generally, only a few nanometres across. This produces a countering force to the concentration gradient. For example, if the intracellular side of a membrane has a high concentration of both positive and negative ions and is only permeable to the positive ions, then the positive ions should flow down its concentration gradient to the extracellular side of the membrane. However, as it does, it will generate a greater positive charge on the extracellular side of the membrane, which will attract the high concentration of negative ions to the intracellular side of the membrane, generating a negative charge. This charge will pull the positive ions from the extracellular side of the membrane back to the intracellular side despite the concentration gradient. The charge at which the electric force counters the diffusion force is known as the equilibrium potential of the ion. This mechanism is employed across animal cells to passively maintain asymmetrical charge and ion concentrations.

Action potentials are the units of transmission across nerve cells. Action potentials were first observed by Hodgkin & Huxley in a squid’s giant axon. Through experimentation and mathematics, they hypothesized a namesake model that suggested that there were gated channels controlling the rise and fall in membrane potential. As technology developed, this model was vindicated. Through the application of suction to a microscopic region of the axon, ion channels could be isolated and their current measured. When current was run through the axon, it was shown that the channels produced a short-term inward current that stops until the membrane returns to below resting membrane potential. Later studies using tetrodotoxin and other poisons yielded information about the structure of these channels. Today, the canonical information taught about action potentials is that there is a resting membrane potential around -65 mV, an action potential threshold of -40 mV, a rising phase, an overshoot above 0 mV, a falling phase, an undershoot under -65 mV, and a return to resting potential. The action potential is often referred to as an all-or-nothing response, as a neuron will fire once the membrane potential reaches threshold. Whether or not this occurs within a neuron is a complicated process that often involves thousands of computations. However, once the action potential reaches threshold, voltage-gated sodium channels open, sodium floods into the axon due to the concentration gradient and the electrical gradient, causing membrane depolarisation. Then, voltage-gated potassium channels open approximately 1 ms later - a system known as a delayed rectifier - allowing potassium to go down its electrochemical gradient to rush out of the axon, causing hyperpolarisation. The sodium channels lock preventing re-depolarisation, forcing the depolarisation to travel down the axon in a chain reaction of opening and closing voltage-gated channels. Then the sodium/potassium pump returns the membrane to resting potential, allowing for the next action potential to fire.

Though plants are sessile organisms with unmoving cells, the inside of plant cells is in constant motion. Cytoplasmic streaming is a phenomenon exclusive to plant cells wherein organelles will migrate around the cell at speeds that exceed most intracellular motion. This motion is controlled by myosin motors that travel along an actin filament network. Though microtubules are the main scaffold for protein motors in animal cells, plant cells favour their actin network. Additionally, through a series of experiments, it was shown that streaming didn’t require the cooperation of the microtubule cytoskeleton nor did it need continuous production of actin monomers. Thus, actin filaments can maintain cytoplasmic streaming simply through disassembly and reassembly of recycled monomers. There are three non-mutually exclusive theories regarding the mechanism of cytoplasmic streaming. Active streaming theory suggests that myosin motors on actin filaments move the organelles around the plant cell. Passive streaming theory posits that the bulk of organelle movement around the plant cell is driven by a cytosol stream produced by the movement of only a few organelles. Endoplasmic reticulum anchor theory states that organelles are bound to the endoplasmic reticulum and that myosin motors move the endoplasmic reticulum, thus dragging the organelles anchored to it. Research has provided evidence for all three of these theories, and it has yet to be shown if they contribute to streaming in equal part or if there one model predominates.