Single-celled organisms are biochemical multitaskers — they need to be if they are to survive, as every process required for their existence must be doable by that single cell.
Multicellular life probably began as colonies of independent cells forming mutually beneficial communities. One cell type possibly benefitted from the waste products of another cell type, and vice versa.
Over time these communities would have become more complex, as cells evolved to compartmentalise, specialise, and interact beneficially with other, differently specialised cells. These groups of differentiated, highly-specialised cells eventually became organs, or groups of uniquely identifiable tissue within a greater multicellular organism.
Organs are specialists rather than multitaskers, and have lost their ability to exist independently. This disadvantage is vastly outweighed by the advantage existing in a mutually beneficial way with other specialists brings. An organ performing one or two highly specialised roles, and doing these well, can then rely on other organs to do their unique roles well. Collectively all benefit, including the greater organism, whose role is to protect the organs and ensure their continuation by reproducing. (Ironically enough via the specialised sex organs.)
The Role of Organs
The role of organs is to ensure their multicellular organism lives — it really is that simple!
Where it becomes more involved is defining what it is they do to ensure that organism lives.
All life*, whether bacteria, protists, algae, fungi, plants or animals, have seven traits in common — conversely anything exhibiting all seven of these is by definition alive:
- the ability to detect changes in the environment
- the ability to move in response to stimuli
- the ability to obtain nutrients
- the ability to metabolise those nutrients
- the ability to excrete waste products
- the ability to grow
- the ability to reproduce
(* By these definitions, viruses are not alive. This is apparently a controversial subject, but I personally don’t believe they are alive for the simple reason that they aren’t even cells by any definition, making them incapable of cellular respiration and thus independent existence.)
Again, a single-celled organism must do all these for itself, but we can easily point to which of our own organs does each of the above tasks: brain, eyes, skin, limbs, stomach, gut, blood (an organ by definition), kidneys, muscles, bones (also an organ by definiton), and the sex organs.
But what are the equivalents in plants?
Plant organs include leaves, stems, cones, flowers, fruit, and roots. Each has at least one highly specialised role no other can do, all working for the greater good of the multicellular plant they make up.
The Role of Plant Organs
Detecting Changes in the Environment
Leaves, stems and flowers all respond positively to light, while roots respond negatively.
Leaves respond to seasons and termperature changes.
Moving in Response to Stimuli
Stems grow up towards light.
Roots grow downwards in response to gravity.
Flowers may open or close depending on time of day.
Roots respond to water and nutrient gradients in the soil.
Roots uptake water and nutrients.
Stems transport water and food through the plant.
Leaves convert light into sugars. Other metabolic processes also occur in the leaves.
Some metabolites such as hormones are produced in more localised areas throughout the plant.
Excretion of Waste Products
Leaves shed waste oxygen in photosynthesis.
Leaves and roots shed waste carbon dioxide during cellular respiration.
Some stems may excrete substances via gums or resins.
Roots shed wastes via exudates.
Stems enable growth via height and/or width.
Roots grow in width and depth through the soil.
Cones are the sex organs of gymnosperms (conifers, cycads, ginkgo).
Flowers are the sex organs of angiosperms (flowering plants) including the jujube tree.
Other groups such as ferns have different sex organs again.
This was a very general and brief introduction to plant organs. Future chapters shall cover specific organs in more detail wherever relevant to jujube tree growth and development.
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