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Freshwater aquatic plant anatomy and physiology, and how you can make easy botany work for your planted tank
Welcome back to another installment of the Aquarium Science Series! Got some epic aquarium plants growing, but wondering how they work and why that matters? This post will give you an overview of some botany basics, with more specific posts on roots, plant genetics, leaf physiology and photosynthesis/respiration, etc. to follow later when we really dig into it.
Aquarium Plant Classifications
Want to get to know someone? Start with a name. Aquarium plants are no different. A lot of people overlook the importance of naming and classification of plants, but knowing how a plant is classified and what its scientific name means can reveal a lot about it. Wish you could do that with people...imagine someone's saying his name is Dustin and you just immediately knew he was an aquarium nut with a big greenhouse and a finger-biting Oscar.
Considering there are over 100K people named Dustin, though, it doesn't work so well. This is another advantage to understanding classification and scientific names: they let folks know specifically what you're talking about. The common name "Baby Tears" gets thrown around to describe more than one aquarium plant species. Duckweed is the same way.
If you want to know about one particular plant, having a universal name everyone recognizes is important. Scientists use a system called binomial nomenclature (two names) as part of taxonomy (system for classifying organisms based on similar characteristics) to do this. Plant taxonomy is broken down by Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species. There are also clades, which refer to a group descended from a common ancestor.
Each category reveals more specific information about the plant. Water wisteria, for example, is broken down like this:
Kingdom - Plantae; Phylum: Tracheophyta; Clade: Angiosperms; Clade: Dicots; Order: Lamiales; Family: Acanthaceae; Genus: Hygrophila; Species: H. difformis
This is packed full of information even if you've never heard of water wisteria before. Kingdom tells you it's a plant (duh). Phylum and clades reveal it's a vascular plant, an angiosperm, and a dicot (more on those below!). Order reveals it's part of the same group of plants as flowering herbs like mint and lavender. Family tells you it's an acanthus, which means it's most likely native to tropical areas like others in the family. Hygrophila, one of only two aquatic genuses in the acanthus family, confirms this and also confirms it's aquatic. And finally, there's difformis, which tells you exactly which of those aquatic plants you're dealing with, named because of its rough, irregularly shaped leaves.
Scientific names are written in Latin and italicized, with the genus capitalized and the species lowercase. In botany, you'll sometimes see this name followed by the name of the person who discovered the plant. If you're talking about multiple or an unknown species within a genus, you would see the genus followed by sp. (Singular) or spp. (Plural).
If you're talking about a cultivar, you would see that written after the genus and species in single quotations, not italicized. A variety would follow the species with var. (Name of the variety)
Example: Windelov Java Fern is a gorgeous Java Fern cultivar. Here's how you'd write the name:
HOW TO MAKE THIS WORK FOR YOUR PLANTED TANK: The scientific name gives you 100% certainty you're buying or discussing the plant you want, and tells you if the plant is a subspecies, variety, or cultivar. Taxonomy gives some info on how you might be able to breed or propagate an aquarium plant you want more of. It also helps you understand how plants transport their nutrients, which gives you insight on using fertilizers. And finally, it gives you an idea of how to care for a species even if you can't find information on it specifically, because of similarities with more common plants of the same classification.
Subspecies vs. Cultivar vs. Variety
To spring off the last part of the previous section, people often use these terms synonymously, but technically, they're not the same even though they all refer to variation within the same species.
The difference comes down to whether we messed with the plant or not.
The word "cultivar" is short for "cultivated variety" and is a plant variety humans altered and/or created for a specific trait that passes that trait onto offspring. Cultivars are often patented. This is on the same level as a category called a grex (that doesn't usually apply to aquarium plants since it refers to orchid hybrids).
A variegated cultivar of Water Wisteria
On the flipside, subspecies and varieties are natural differences within a species. The difference between them is subtler. Subspecies usually means plants that live in different geographical areas and vary in some way as a result. Variety ranks below species or subspecies and above form in taxonomy and refers to minor, permanent differences between plants that can be passed down genetically/hybridize with other varieties.
Vascular vs. Non-Vascular; Angiosperms vs. Gymnosperms; Monocot vs. Dicot
We mentioned above that the phylum and clades revealed water wisteria was a vascular, angiosperm, dicot. But what the heck does that mean/tell you? You've probably been able to distinguish between vascular and non-vascular without even realizing, just based on looks. Most common aquarium plants are vascular, which means they have specific tissues that transport water and nutrients throughout the plant called the phloem and xylem. The xylem moves water and minerals, and the phloem moves products from photosynthesis (mainly sugars to "feed" the plant). Vascular plants include species like Anubias, ferns, Aponogetons, etc. and typically have roots, stems, and leaves.
Anubias afzelli is an example of a vascular plant
Vascular plants can further be broken down into Angiosperms (which can be broken down into monocots and dicots) Pteridophytes (ferns) and Gymnosperms. There are no truly aquatic gymnosperms that can (realistically) grow in a tank, so we'll focus on the angiosperms. Angiosperms bear flowers, fruits, and enclosed seeds and include many common aquarium species. Monocot is short for "monocotyledon," which means its seeds only have one cotyledon (embryonic leaf) and are usually grass-lile plants. Dicots (dicotyledons) have two. Pteridophytes include ferns and horsetails and do not reproduce via flowers or seeds, using spores instead.
Non-vascular plants do not have xylem or phloem and use simpler structures to do everything. They include Anthocerotophyta (hornworts), Marchantiophyta (liverworts), and Bryophytes (mosses). They do not have seeds or have as rigid a structure as vascular plants. They also reproduce via spores.
Christmas Moss (Versicularia daubenyana), a non-vascular plant
Classifications Based on How Aquatic Plants Grow
This one is a little less sciencey. You can also categorize aquatic plants based on whether they will grow emerged, submersed, or floating. These are exactly what they sound like. Emergent plants grow partially submerged in water, partially above the water. Submersed (or submergent) plants grow completely underwater. Floating plants...float. They're not rooted in substrate. There are also amphibious plants, which can grow both aquatically and on land (like the African Water Fern). Some species can grow more than one way.
HOW TO MAKE THIS WORK FOR YOUR PLANTED TANK: Taxonomy told you what the plant was; this tells you how you can grow it. Maybe you can try growing some plants out of the top of your tank for a unique look, or maybe you can float one instead of planting it or vice versa. Having this knowledge lets you give your plant its best shot at survival while also letting you get creative.
Aquatic Plant Anatomy/Physiology
Difference in Anatomy and Physiology
Both are important, but the difference is essentially "What?" Vs. "How and Why?" Anatomy is the study of structure - the specific parts your plants have and where they're located. Physiology is the study of why those parts are there: processes and function.
Basic Plant Parts/Functions
The three most basic parts of the plant are the roots, stem, and leaves. The roots are responsible for nutrient/water uptake and anchoring the plant in the substrate. The shoot is made up of stems and leaves. The stem is there mostly for transport of nutrients and support of the other parts of the plant. And finally, the leaves are the site of photosynthesis (process plants use to create food from sunlight) and gas exchange.
These all have more specific parts with more specific functions, listed and labeled below (Again, we'll have more specifics on each of these in future posts!).
- Root cap - the very tip of the root; main function is protection
- Two main root types. Taproots are root systems which grow downward with little branching. Fibrous root systems have a lot of secondary branching
- Root hairs - finer, fragile root parts that increase root surface area and absorption
- Nodes - locations of buds where leaf/stem/flower growth occurs
- Internode - the space between nodes. The length of the internode can sometimes give insight into whether your plant is growing healthily
- Petiole - the part that attaches a leaf to a stem. If a leaf is attached directly to a stem and doesn't have one of these, it's called sessile. Don't confuse it with a rachis, which is the main stem of a compound leaf. Alternanthera have sessile leaves.
- Compound vs simple leaves: simple leaves, like the ones in the picture, are single leaves (one leaf makes up the entire leaf structure). Compound leaves are comprised of more than one leaf (leaflets). Watercress is a good example of an aquatic plant with compound leaves
- Leaf blade - the main part of the leaf, site of chloroplasts that carry out photosynthesis
- Venation - the veins in a leaf. Can be formed in a variety of ways (pinnately, palmately). The midrib is a large/prominent vein down the center of the leaf. Veins contain xylem and pholem responsible for transport
- Chloroplasts - internal part of the leaf; small, disc-shaped parts of plant cells responsible for photosynthesis. Contain the pigment chlorophyll, which allows plants to "capture" light.
External structures of Hygrophila corymbosa 'Cherry Leaf'
HOW TO MAKE THIS WORK FOR YOUR PLANTED TANK: If you start noticing problems with a particular plant part, you can figure out how the plant will be affected based on that part's function and how to support it. Knowing each part also helps you diagnose problems since some only affect certain areas. Knowing what normal parts look like help you gauge your plant's health.
Modified/Specialized Plant Parts: How to Plant Plants with Rhizomes, Stolons, Bulbs, etc.
Some plants have modified stems, roots, etc. to help them out. Plants like Anubias, Java Fern, and African Water Fern have what's called a rhizome (AKA rootstalk/creeping rootstalk). A rhizome is a modified horizontal stem that sends out shoots/roots and can be used to start a new plant. A stolon (often called a runner) is similar, but grows aboveground. You'll see stolons a lot on carpeting plants. Don't confuse rhizomes with rhizoids. We mentioned a rhizome was a modified stem; a rhizoid is more of a simplified root/attachment system for primitive/non-vascular plants like mosses and liverworts.
A bulb is basically a round, underground storage unit for the plant's entire life cycle, and it can remain dormant for a while. Crinum, Aponogetons, and so on are good examples of aquatic bulb plants. It typically has a pointed tip and a rounded tip, and the pointed tip is where the rest of the plant will emerge. At the rounded end, it also has a basal plate, which distinguishes it from other root/stem-like structures, where roots will grow.
Bulbs are often confused with tubers, which which also act as mini plant storage units, but develop in a different way. Instead of having a basal plate, tubers develop nodes all over their surface where shoots/roots can grow. Potatoes are actually tuber plants, and the "eyes" are the nodes/axillary buds where new shoots and roots can emerge. Several Aponogeton species have tubers and rhizomes.
On that note, important note: the word bulb often gets used to describe all of these structures even though it's not exactly correct. So make sure if someone is selling you a "plant bulb" you know what they're referring to.
Basic Structures Labeled on an Anubias nana. The rhizome is the thicker, horizontal stem where all the leaves are growing from
HOW TO MAKE THIS WORK FOR YOUR PLANTED TANK: These specialized structures tell you how to plant and grow these species. Aquatic plants cannot have their rhizomes or stolons buried, or they may rot, since they're technically a stem. You want to grow bulbs with the pointed tip up (if you're not sure which end is which because they're both kinda pointy, you can grow the bulb sideways).
How Are Aquarium Plants Able to Grow Underwater?
Though aquarium plants share a lot of similarities with terrestrial plants, the underwater environment makes things more challenging: how do they get enough light and oxygen? How do they avoid rotting/overwatering? How do they reproduce surrounded by water? How do they absorb nutrients? You couldn't stick a rose bush underwater in your tank, so how can you stick aquarium plants in there?
The answer: their adaptations are cool as hell.
If you cut open some emergent plants, it'd look like a sponge. That's because of an adaptation found in a lot of aquatic plants called aerenchyma (Greek for "air infusion"). These open spaces between cells allow for gas exchange throughout the plant even in low-oxygen environments. There are air channels in sumberged plants called lacunae that function similarly and also provide structure. Some plants can respire anaerobically (without oxygen) via ethylene.
In low light environments, some species can use a different biochemical pathway of photosynthesis (C4 instead of C3) which is more efficient.
And finally, aquatic plants can reproduce via fragmentation, which allows pieces to break off and re-establish themselves somewhere else as a new plant (Imagine if someone lost an arm and a kid grew out of it...). The rhizomes and stolons we mentioned earlier also help with reproduction. Vallisneria can actually create structures called peduncles (female) and spathes (male; they look like little boats), which stretch up to the water surface to interact and reproduce.
HOW TO MAKE THIS WORK FOR YOUR PLANTED TANK: knowing these adaptations give you hints on the best ways to supply aquarium plants with what they need and which species may survive in your tank.
Sources/Further Reading for My Fellow Nerds:
Winterton, S.L., Scher, J.L., Burnett, J., and Redford, A.J. 2018. Aquarium and Pond Plants of the World, Edition 3. USDA APHIS Identification Technology Program (ITP) and California Department of Food and Agriculture. Sacramento, CA. - An Aquarium Plant ID Tool
Check out the rest of the Aquarium Science Series:
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