The Differences Between Plants

Read this article in Spanish.

Featured image: A container filled with bryophytes and Venus Fly Traps (Dionaea muscipula) inside one of the greenhouses of UW-Madison Birge Hall’s Botany Greenhouse.

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What makes a plant a plant? This very general term may seem too simple, because when we refer to plants colloquially we often talk about trees, shrubs, cacti, vegetables, flowers, weeds, moss, and many more. What are the differences between all of these examples?

As humans, we notice patterns in our environment and learn the differences between most plants at a macroscopic level. Most people can identify plants like ferns, mosses, pines, oaks, cacti and all the plants that look superficially like them. Educated plant identifiers learn the small differences between these plants, and will need to use more precise language to refer to groups of coniferous, deciduous, vascular, and non-vascular plants.

When considering the nature of plants, it helps to start at the smallest and most fundamental differences: the cell. Cells were discovered when early observers used microscopes and discovered small capsule like structures inside cork. Cork, as in the cork in the top of a wine bottle, is traditionally sourced from the outer bark of the Cork Oak, Quercus suber. These cells were large enough to see with early microscope technology. Later, more and more types of cells were found across a mind boggling quantity of living organisms large and small. When comparing cells commonly found in plants to cells found in other organisms, their organelles (cellular or small ‘organs’) stand out.

Plants, algae and cyanobacteria cells have a unique organelle called the chloroplast. Fully functioning chloroplast organelles conduct chemical reactions in the sunlight that eventually produce glucose, a sugar used as fuel. This multi-step process is known as photosynthesis. Some people like to say that plants eat the sun.

Plants are some of the most important organisms on Earth, because they are at the start of most terrestrial and aquatic food chains. If an animal doesn’t need plants to survive, its prey probably does.

There are exceptions and quirks to photosynthesis in organisms. One exception is Indian pipe, Monotropa uniflora, which does not produce chlorophyll, one of the pigmented chemicals inside chloroplast organelles that play a role in photosynthesis. In contrast, some of the photosynthetic organisms that we consider algae, like the potentially toxic Blue Green Algae, are actually Cyanobacteria. Even though the mucky mats of bluish green goop on our lakes looks like algae, the cells of these bacteria lack certain features that all plants, algae, and fungi have: a cell wall.

The cell wall gives plant and fungi cells a rigid structure. Put all together, these box-like structures organize themselves to complete different tasks. These formations coalesce and create all the different tissues in plants like those in leaves, branches, bark, roots and more. They can have structural purpose or they can have an active purpose, such as transporting nutrients and water.

Plants can be divided into two groups: Vascular and non-vascular plants. Vascular plants have the capacity to transport water upwards and outwards, but non-vascular plants do not. Examples of non-vascular plants are mosses, liverworts, and hornworts. Whereas, vascular plants include ferns, and all other terrestrial plants like conifers and deciduous trees. In essence, any individual plant that has a large enough three-dimensional structure needs a system to absorb water and distribute it, where as other plants like mosses that are small do not require this.

Vascular systems are complex. For trees, the actively living part of the tree is near the surface of the trunk underneath the bark. A small ring around the portions of tissues underneath the bark called the cambial layer, dividing outwardly and inwardly to produce the phloem layers (nutrient transport), the xylem layers (water transport and structural). Xylem cells go dormant and eventually become inactive, these cells are the interior or ringed portion of the tree.

Tree rings indicate the rate of growth of these cells during the season, as the season cools the band of cells becomes denser with an aggregated darker color. Without seasons, tree rings would not be easy to see. Think about what happens during large-scale volcanic events!

Increase of metabolites or junk in the cells gives the heartwood (center of a tree trunk) a darker color in some species.

As the tree grows in diameter, the bark cell layer is split and grooves can be created throughout surface as it slowly rips and heals.

Vascular systems of plants are also different for monocots (monocotyledon) and dicots (dicotyledon). A cotyledon is the primordial leaf that emerges from a seed. These two groups (monocots vs. dicots) are differentiated by the number of primordial leaves formed at the initial stages of growth. Other patterns follow: vascular system arrangements, and leaf arrangements. Monocots like grasses, palms, and other plants have vascular bundles scattered across the structure of the stem. Dicots have a thin cambial layer, where vascular tissues are formed.

Deciduous trees lose their leaves for a season (winter), and coniferous or evergreen trees keep their leaves green the whole year or until they naturally decay. Tamaracks, Larix laricina, and Larches, Larix spp., are unique in that they are deciduous conifer trees unlike Pines, Pinus spp., and Spruces, Picea spp. Many plants in tropical regions can be considered evergreen, since they stay green year round.

Plants are also distinct in their reproductive strategies, which is the way an organism can duplicate itself or create offspring. Asexual strategies include plant tissues that extend the root or stem outwards so that new identical bodies can be created. Sexual reproductive strategies involve the production of reproductive organs that disperse and receive genetic information. In sexual reproduction, genetic information is combined to produce unique offspring.

Sexual reproduction of plants is complicated. Some plants have male and female organs on the same plant (monoecious), others have distinct male and female plant specimens (dioecious), some need other genetically distinct specimens to reproduce (self-incompatible) and some can fertilize themselves. (self-fertilization or self-pollination). Trends in sexual reproductive strategies are used to categorize plants.

Three of most noticeable groups of plants you’ll encounter: gymnosperms, angiosperms (magnoliophyta), and ferns (pteridophyta).

Gymnosperms, or naked seed plants, include Cycads (Cycadophyta), conifers (Pinophyta), Ginkgos (Gingkophyta) and Gnetophyta. Colloquially, many conifers are often referred to as pines. In reality, there are a variety of types of distinguishable conifers: pines, spruces, firs, hemlock, yews, larches, tamarack. Gymnosperms, like conifers, have reproductive organs that release pollen into the air and receptacle structure that are fertilized. When fertilized cones are produced, each seed is developed in the sheath of a cone scale. Some conifers like yews have fleshy cones. Instead of tough dry sheath, the seed of a yew is covered by partially by a red flesh.

Angiosperms, or fruiting plants, are the largest grouping of plants. These plants have flowers and store their seed in a specialized casing. Some flowers have female parts (pistils) and male parts (stamens) in one flower (perfect flowers), and female and male parts on separate flowers (imperfect flowers). Pistils receive pollen in an often flat sticky opening called the stigma.

At the bottom of a pistil is the ovary, which can be located above the main part of the flower receptacle as a bulbous mass (hypogynous or superior ovary) or hidden, imbedded or surrounded by the receptacle (epigynous or inferior ovary). Stamens store and release pollen from an oblong dusty masses called anthers.

Some flowers like sunflowers are composite flowers, and each small section within what we call a sunflower is a small flower capable of producing a dry fruit, which is what we call ‘sunflower seeds’. Flowers, their function and their structure are used to categorize them into more specific groups.

As an example, flowers of the Bean family or Fabaceae have unique pattern that has been classified as a papilionaceous flower. The flower has 5 parts, two of which are in symmetry. At the top is the banner, which is broad and faces out and up. The two parts in symmetry are the wings, which are below the banner and are sometimes fused together. The last part is the keel, which protrudes out from the bottom often between the wings.

We know the difference between many fruits like bananas, oranges, and apples. Many people can observe that bananas are most similar to plantains, oranges to grapefruit, and apples to pears. When studied closely, the anatomy of fruits shows that bananas and plantains are most similar to berries, oranges and grapefruit have many small berry like structures in them and are called ‘hesperidia’ (plural), and apples and pears belong to the group of fruit called ‘pomes’. Maples, box-elder, elm and ash trees have fruits called samaras, which are winged achenes. Achenes are a special designation for simple dry fruits, and are often mistaken for seeds like in strawberry “seeds” and caraway “seeds”. Strawberries are unique because the small seeds on the flesh are small dried fruit, and the tissue that is consumed is an accessory tissue or receptacle. Many fruit tissues are enlargements of different types of tissues located in the flower.

Pteridosperms, or “winged seed”, refers to the plants we know as ferns. Ferns are unique, because instead of relying on flowers for reproduction they release spores like fungi. This is a simple and archaic form of sexual reproduction. When a spore from a fern leaf becomes fertilized, it becomes a small plant-like structure called a gametophyte or gamete bearing plant. Gametes are the small cellular structures that fuse during fertilization. When the gametophyte is wet, material is shared between pore-like structures containing male and female gametes, which will fertilize to produce the fern or the sporophyte. The sporophyte is the spore bearing structure.

Curiously, what we know as a moss is a gametophyte. Whereas, what we know as ferns and most other plants are the sporophyte structures or spore/seed containing structure.

When studying reproduction, we are interested in genetic variability. Genes are stored in chromosomes, which are structures containing incredibly long and intensely coiled strings of DNA. Ploidy refers to the amount of chromosome pairs found in a cell. Plants are unique because certain species can exhibit polyploidy or more than 2 sets of chromosomes per cell. In many plants, the reproductive cells or gametic cells like those of the pollen and the embryo sac contain half of the genetic material (haploid) so that when they combine (fertilize) they create new combinations with the same amount of genetic information normally found in regular cells (somatic cells). When somatic cells divide, they are programmed to duplicate and maintain the same amount of information (mitosis). When reproductive cells or gametes are being formed, they are dividing but their genetic information is also divided (meiosis). This process allows for gametic cells to fuse together or fertilize without raising the amount of genetic information that is commonly found in somatic cells. After fertilizing, the first cell to begin the process of exact duplication is called the zygote. This entire process is not fool-proof, and problems in any stage of cell division and recombination can result in genetic disorders of varying degrees of impact on the function of the organism.

Some seedless varieties of plants like seedless watermelons are hybrid crosses between diploid (2 sets of chromosomes per cell or 2n) and tetraploid (4n) varieties. These hybrid crosses are triploid (3n), and cannot undergo successful meiosis or division of the genetic information in reproductive cells.

If we look at plants at a microscopic and atomic level, we can also differentiate plants by the chemicals they secrete. Plants because of their lack of movement, rely on chemicals or phytochemicals (plant chemicals) to poison their prey. Some can affect the nervous systems of insects, birds, and mammals in different ways. These compounds are referred to as secondary compounds, since they are not always essential to the primary functions of plant biology. Some of these are medicinal (anti-biotic, anti-fungal, and anthelmintic [anti-worm]), some can alter your mind (psychoactive), most will make you sick, many can damage organs over time, and some will kill you. Many of the plants we eat have been bred over thousands of years to have negligible quantities of these substances. Some plants that we eat like beans need to be soaked and cooked to destroy toxic secondary compounds. Less popular foods or foods eaten traditionally by indigenous people for survival can have toxic effects when eaten in the quantities that we are used to eating. So be careful when working with plants, and be very aware of the consequences of plant chemistry.

We’ve covered many different aspects of plants. There’s so much more to learn, but this should be enough to provide an idea of why there are so many different types of plants and what are the patterns we use to differentiate them from one another.

Photography: Gustavo M. Meneses Gingles
iPhone 5s, 6, SE; Olympus OMD E-M10 Mark II; Canon EOS Rebel T6i

Author: Gustavo Meneses
Published: 2021-04-11
Revised: 2024-03-26

References

“Pteridophyta.”. Council for the Indian School Certificate Examination. BIOLOGY4ISC. Accessed Mar 26, 2024. https://biology4isc.weebly.com/pteridophyta.html