The naming of parts
Summary
Plants have two main systems: the root system below ground, and the shoot system (consisting of stems, leaves and flowers) above it. Roots anchor the plant in the ground and absorb water and nutrients. Stems transport water and nutrients throughout the plant, and position leaves for photosynthesis and flowers for pollination. Leaves are where the majority of photosynthesis takes place, where the majority of gaseous exchange takes place, and from where water can evaporate to create a suction force for transpiration. Flowers (in angiosperms only) are the reproductive organs, where pollination takes place, and contain the ovaries which develop into the fruit after pollination.
We are all familiar with most of the parts of the plants around us. We know what a flower is, what a leaf is, what a root is…but do we know why plants have flowers, leaves and roots, and how they help the plant to survive, grow and reproduce? Do we know how these different plant parts actually work? And can we apply that knowledge to help us in creating and maintaining our gardens?
This is the first of a series of articles looking into the morphology and anatomy of plants. Morphology? Anatomy? Probably best to start with some definitions:
Plant morphology is the study of the physical form and external structure of plants
Plant anatomy is the study of the internal structure of plants
Before being separated out in the mid-20th century, plant anatomy included plant morphology as a sub-discipline. To keep things simple, this is the approach that we’re going to use at Getting Dirty.
The high level anatomy of a flowering plant
Flowering plants (angiosperms - see this post for more details) have two main systems: the shoot system, which sits above the surface of the soil; and the root system, which lies beneath the surface. The shoot system consists of the plant’s stem, buds, leaves and flowers; and the root system is the plant’s roots. Gymnosperms and pteridophytes are similar at this level of detail, except that they have no flowers. Bryophytes are very different; as non-vascular plants, they don’t have roots or stems in the same sense.
The main parts of a flowering plant
Roots
A plant’s roots have three main functions: they anchor and support the plant in the soil, allowing it to grow taller without falling over; they absorb water and minerals from the soil and transport this to the base of the stem; and they store carbohydrates produced by the plant, often allowing the plant to regenerate from damage to the shoot system.
There are two main types of root system. “Taproots” (as shown above) are thick primary roots that can extend deep into the soil (a wild fig tree in South Africa was found to have a taproot that extended 120m deep!). Lateral roots branch outwards from the taproot, strengthening its role as a soil anchor, and increasing the surface area available to absorb water and nutrients. Taproots can sometimes store significant amounts of energy in the form of carbohydrates, allowing a plant to regenerate from a tiny piece of root. Anyone who has ever tried to remove an infestation of broad-leaved dock (Rumex obtusifolius) from their lawn can testify to this…
The other type of root system, “fibrous roots” lack a single primary root, and instead have large numbers of small branching roots emerging directly from the base of the stem. Fibrous root systems have a significantly larger surface area available for absorption, but provide less secure anchorage. An example is the grass we grow in our lawns (actually multiple different species, with the most common in the UK being dwarf ryegrass, Lolium perenne). The fibrous root system allows each individual plant to compete effectively for water and nutrients in a crowded environment; but it is quite easy to dig out turf (ask your neighbourhood badgers, if you have them…).
Roots can also be “adventitious”, growing from less common parts of the plant. Tomato plants (Solanum lycopersicum) can grow adventitious plants from their stems, allowing gardeners to replant them deeper and improve anchorage for the growing season. Some climbing plants, such as ivy (Hedera helix) use adventitious roots to cling to surfaces, supporting their weight as they reach ever higher towards the light.
Stems
The stems of a plant also have three main purposes: they transport water, minerals and carbohydrates from where they are sourced (either from the soil, or manufactured by the plant during photosynthesis) to where they are needed, whether for growth, repair, defence or reproduction; they physically support and position leaves in the optimal position for photosynthesis (which may differ in different plants); and they physically support and position flowers in the optimal position for pollination.
Stems are divided into sections called “internodes”, so named because they are the pieces of stem between the “nodes”, the places where leaves, flowers or branches of the stem attach. Where a branch or leaf attached to a stem, it forms an angle with the stem. This joint, above the attached branch or leaf is called an “axil”, and is one of two main growth areas on the stem, the other being the very tip of a stem, called the “apex”.
“Buds”, which may grow to become branches, leaves or flowers, form at either the axils or the apices (plural of apex). Buds are actually condensed stems (see the picture of a Brussel sprout below), complete with nodes, internodes, axils, and even their own axillary buds. These are wrapped inside small, thickened leaves (“bud scales”), which enclose the bud and protect it from the outside environment. Foliage buds contain multiple folded leaves, and flower buds contain the immature flower; if we carefully pull apart a bud, we can often determine which it is from the structures that you find.
Cross-section of a Brussel sprout
If we examine a stem on a deciduous woody plant that is more than a year old, we can see the scars where the prior year’s leaves were attached to the stem (the “leaf scars”). We can also see a scar where the previous year’s apical bud (the bud that forms at the apex) formed (the “bud scale scar”). By counting bud scale scars, we can tell the current year’s growth from that of previous years; this is very useful, since some plants flower on new growth and some on older growth - meaning that we can determine where to make pruning cuts without losing flowers in the current season.
Leaves
Leaves again play three critical roles in supporting a plant: perhaps most importantly, leaves are where the majority of photosynthesis takes place, using sunlight to produce carbohydrates to sustain the plant; unlike most other tissues in the plant, leaves are covered in tiny holes (called “stomata”) that allow gases and vapours (primarily carbon dioxide, oxygen and water vapour) to pass in and out of the plant, something that is essential for the key processes that sustain the plant; and the evaporation of water through the plant’s leaves creates a suction force that draw water up through the plant from the soil via the process of transpiration.
Individual leaves consist of a flat surface (the “blade” or “lamina”) and usually, a short stem-like “petiole” (where the leaf blade is attached directly to the stem, the leaf is said to be “sessile”). Leaves can be “simple” (just a single piece) or “compound” (with multiple “leaflets” joined together and attached to a single petiole). Within these two categories, there are many different shapes of leaf; and leaves have many different textures along their edges (or “margins”). Leaf morphology is worth an article in its own right!
Flowers and fruits
Finally, we come to the flower, perhaps the main reason that many gardeners grow many of their plants. Flowers are the sexual organs of the plants. They produce the basic units of reproduction, the “gametes”: the male pollen grains, and the female eggs. They are structured so as to optimise the likelihood of pollen from one plant being able to fertilise the eggs of another (the process of “pollination”). Different flowers have different structures, depending on the way that particular plant has evolved to achieve pollination: some by wind, some by water, and some by insects.
The eggs of a plant are contained within structures called “ovules”, and these themselves are contained within the “ovary” of the flower. Once pollination occurs, the eggs start to develop into seeds, and the ovary begins to swell, forming the “fruit” of the plant. All angiosperms bear fruit, albeit what we mean by a fruit in this context is very different from the fruit we might buy from a grocer…for example, the seed head on an opium poppy (Papaver somniferum) is the fruit of the poppy.
Detailed anatomy
Understanding how the roots, stems, leaves, flowers and fruits of a plant work together to help the plant survive, thrive and grow requires a more detailed picture of the anatomy of each of these structures. To get there, we will first need to take a detour into some of the internal structures of the plant: the ground tissue (collenchyma, sclerenchyma, and parenchyma), dermal tissue, and vascular tissue. These will be the subject of the next article.
Image credits
The diagram of angiosperm anatomy in this post has been shared under a Creative Commons Licence as follows:
Credit: Kelvinsong. Image source. Creative Commons Attribution-Share Alike 3.0 Unported Licence.
The diagram of a Brussel sprout in this post has been shared under the FreeImages Content Licence as follows:
Credit: IAmThatStrange. Image source. FreeImages Content Licence.