Botany-Anatomy of a plant- stems and leaves

(Unedited version)
Originally appeared in the January 2007 issue of
Tropical Fish Hobbyist Magazine

Last month I gave a very brief review of the historical prospective of botany. Now that we have a bit of an idea of how this field of study developed, let’s look at how we describe plants.

Evolution of plant structure

The generally believed evolutionary picture of plants goes something like this. Some bacteria developed the ability to photosynthesis. These are the cyanobacteria, or blue-green algae that can sometimes invade the aquarium. The algae were the next to evolve, they eventually colonize freshwater environments. It’s believed that the first land plants evolved from freshwater algae. Fossils of the spores from these early plants have been used to date this development, believed to have occurred 480 million years ago or more.

Mosses and liverworts, club mosses and horsetails, and ferns, all seedless vascular plants were the next to evolve. Seed ferns (plants known by their fossils that may have been the link between the ferns and the seeded plants) and cycads, and the gymnosperms (cone bearing plants) developed next. About 140 million years ago the first angiosperms appeared. These are the vascular, flowering seeded plants. Their methods of reproduction were far superior to those of the plants that had evolved earlier and they eventually became the dominant plants on the planet as they are now.

Until very recently there was a great deal of debate about the order of evolution, and some of the groupings of the angiosperms. The flowering plants have two basic groups, the monocots and dicots. The terms monocot and dicot refer to the number of first leaves present, one or two, though there are other features that distinguish the two. But there are also some plants that display features of both, including water lilies which confuse the issue. Recent evidence from new scientific techniques has helped to clarify the relationships of these plants. It’s no believed that the dicots were in fact the first to develop and the monocots evolved later from a dicotyledon ancestor.


Myriophyllum heterophyllum (A) has 4-6 whorled pinnate leaves. Hygrophila difformis (B) has varied opposite leaves. In desirable conditions in the aquarium the leaves will be pinnatifid. Annubius barteri (C) leaves grow along a rhizome. They are ovate with an acute tip and rounded base. These Cryptocorynes (D) have lanceolate leaves and an acute tip. Bases on many cryptocorynes can be variable.

Terms for plant parts

The different types of plants that have evolved have some similarities and differences in the structure of their growth. The structure of a typical moss, fern or angiosperm is different and so the different parts of these plants, even though they may look similar, can have different names.

It's much easier when you're researching information on aquatic plants if you're already familiar with the terminology. It's absolutely a necessity if you're trying to identify a plant using a field guide or dichotomous key. A dichotomous key is a written reference to help identify a particular organism. In a dichotomous key you are given a list of numbered paired choices. You chose which statement best describes the organism you're trying to identify. At the end of that statement will be another number. You go to that number and then are given 2 more choices. This process repeats until the correct identification is reached.

Mosses and Liverworts

We see mosses and liverworts in the aquarium quite a bit and they have both increased in popularity in recent years with many new mosses and a few new liverworts coming in to the hobby. Mosses and liverworts have spores but not the protection of seeds. They’re also non-vascular, which means they don’t have an internal system of vessels to transport water and nutrients. These plants need to be in moist environments to live and reproduce.

Mosses don’t have true roots, though some have very root looking rhizoids. Mosses also don’t have true leaves, though most people, including botanists, call them leaves. The technical term for the leaf like structures on moss is phyllid. Some liverworts have leaf like structures, but the liverworts we grow in our aquariums, generally Riccia and more recently Pellia, are simpler. These plants don’t really have much in the way of distinctive parts and stem and leafy like structures are called the thallus.

Ferns

Club mosses, horsetails and ferns are more developed than the mosses and liverworts and have a vascular system. This allows them to grow larger and farther away from their water source. They still have a less developed reproduction system than the seeded plants though and still need water for reproduction.

There are several ferns seen regularly in the aquarium. Water ferns, Java ferns, Bolbitis, Salvinia, and 4-leaf clover are some of the common names of the aquatic ferns we grow in our tanks. Ferns have a rhizome and a root. The leaf like part of a fern is called a frond. Fronds can grow very differently to serve different functions, for example what looks like roots hanging down from Salvinia are actually a very modified frond. The parts of a more typical vegetative frond are; the stipe which is the lower part of the stem, the rachis which is the upper part of the stem where the more leafy parts of the fern grow out from, and finally those leafy parts which are called the pinna.

Gymnosperms

I don’t believe there are any gymnosperms used regularly in the aquarium, though I know some folks have used cypress trees growing out the top of tanks. Gymnosperms develop seeds in cones. Those most common to us are the ones that fill many of the forests as pine and fur trees.

Angiosperms

When looking at a plant the parts we see first are the stems and leaves. The places on the stem where leaves and more stems grow is called a node. When cutting stemmed plants to root this is also the place where new root growth will appear. The part of the stem in between the nodes is called the internode. When a stem grows along the substrate to form a new plant, then the stem is called a stolon. Several aquarium plants do this. Vallisneria, Sagittaria and pygmy chain sword are some examples.

The leaf itself includes both the leaf stem which is called a petiole and the main part of the leaf, the blade or lamina. Some leaves don't have a petiole, and they're called sessile, while those with a petiole are called petiolate. At the base of the petiole where it attaches to the stem are also one or more small axillary buds which can grow to form a new branch.


The Limnophila heterophylla (A) seen in the background can have as many as 14 pinnate leaves in whorls. The large Ceratopteris thalictroides (B) is a fern with pinnate fronds growing along a short, usually erect rhizome. Rotala rotundifolia (C) is to the left and has varied leaf arrangement going from opposite to small whorls of 3 or 4 even on the same stem. The Echinodorus tennallus (D) plant in the front is stoloniferous and has lanceolate leaves.

Leaf shapes

Leaves can be quite different in appearance and are a good first step in recognizing a plant. Though some plants and particularly aquarium plants can have a very different looking leaf depending on the conditions the plant is grown in.

An important part of the identification of a plant through the leaf is the pattern of its veins. There are three major venation patterns. The first is called pinnate, in this pattern a main vein runs down the middle length of the lamina and smaller veins form on each side. Pinnate means that it resembles a feather, but I've always thought it looks like the body of a fish skeleton, or basic Christmas tree form. The second type of venation is called palmate. This refers to the hand with several large veins radiating from the base of the lamina. Think of a maple leaf and how the veins look like the spread fingers of a hand. The third type of venation is parallel. This is when many veins run parallel down the length of the lamina. Parallel venation is a major feature of the monocots.

When looking at the shape of the leaf the main shape, the shape of the tips and bases and the shape of the edge of the leaf are considered. The edge of the leaf is called the margin.

Leaf shapes you’ll see often in aquatic plant literature include; lanceolate- long and tapered at the end like a lance, ovate- egg shaped, oblong- long oval shape with long sides parallel, elliptic- long oval shape with rounded long sides, cordate- heart shaped, linear- very long and narrow.

Leaf shapes and margins are often mentioned together in literature or sometimes only one is mentioned if it’s the distinguishing feature of a plant. Entire margins are smooth; serrate has a jagged teeth like edge and undulate is wavy. Oak trees, or in the case of aquarium plants the Mexican oak plant is a good example of a lobbed leaf margin. Leaves described as pinnate, pinnatifid, bipinnate, are just as when describing plant veins, feathery in appearance. There are many aquarium plants that have developed these kinds of leaves.

Compound leaves and leaf arrangement

Some plants have multiple leaflets that grow on the petiole instead of single leaves. Plants with a single leaf per petiole are called simple leaves. When they have multiple leaflets they're called compound leaves. You can tell the difference between a compound leafs leaflet and a simple leaf but checking where the leaf attaches to the stem. If there is no axillary bud then it's a leaflet and not a leaf. Compound leaves are pinnate when multiple leaflets grow opposite each other along a modified midvein that looks like a stem. This modified vein is called a rachis. When the end of the pinnate leaf ends in two leaflets it's called even pinnate and when it ends in a single leaflet it's called odd pinnate. Bipinnate leaves are further divided with leaflets on branches coming out the sides of the rachis. If 4 or more leaflets grow attached to the end of a petiole it has a palmate compound leaf. If there are 3 leaflets, like in clover, it's called trifoliate.

Leaf arrangement is determined by how many leaves are at each node and how they grow. If there is a single leaf at each node the leaf arrangement is alternate. If there are 2 leaves at each node, they will grow across from each other and form an opposite leaf arrangement. Whorled leaf arrangements consist of 3 or more leaves at each node. Rosette plants grow leaves in a circular pattern. It looks like all the leaves are coming from the same place, but they actually have very, very close nodes, with almost no internode between them.

Adding a few more botany terms to your vocabulary can help you in making identification faster and easier. Hopefully it will also help you to have a deeper understanding of how complicated the underappreciated world of plants really is.

Questions or Comments?

If you have questions or Comments about this column, join the Natural Aquariums Forum and post them here.


Welcome to the Jungle | Into The Forest | The Creepy Crawlies | A Clearing in the Thicket | Algae Eaters for the Planted Aquarium
North American Natives | Why things go wrong Pt 1 | Why things go wrong Pt 2 (Algae) | Algae Eating Shrimp | Lo-Tech Tanks
Welcome to the Fish Room | The Stemmed Plants | Mosses | A Livebearer Biotope | Planted Tank Social | The Genus Hygrophila | Cyanobacteria
Easy Plants | What I Did Last Summer | Decorations in the Planted Tank | Botany-An Introduction to Plant Biology | Botany-Anatomy of a plant
Botany-How Plants Work | Easy Rosettes | Going High-Tech | Floating Plants | Dealing with Success | Bringing the Outside In | Vallisneria
Hair Algae | Flowering Aquarium Plants Part 1 | Flowering Aquarium Plants Part 2 | Liverworts in the Aquarium | Elements of Design
Planted Aquarium Maintenance | More Mosses | Invaders | Ferns in the Aquarium | Setting up a Planted Aquarium
Seaweeds of the Pacific Northwest | Proserpinaca | Hardware for the Planted Aquarium | Rotala | Neocaridina Shrimp | Lo-tech Tank Tips


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