What would you think if I told you that one of the most versatile plant parts is the stem? I’m not kidding, you can fry them, mash them, or even base your state’s entire personality on them. And no, I am not talking about celery stalks, but potatoes. That’s right, the beloved potato is actually a plant’s stem! The stem of plants is an organ that helps support other plant parts, transport nutrients, store food, and sometimes even reproduce!
Definition of a plant stem
The plant stem is an organ part of the plant shoot system along with the leaves. The stem of plants supports many other plant parts including the leaves, flowers, fruits, buds, and branches.
So what does the stem do for the plant? Besides support, stems also provide the transport of water and nutrients throughout the plant. The stem of a plant is typically above ground and functions as the main body of the plant. Stems come in a variety of different forms, they may be branched or unbranched and may be underground as well (tubers, rhizomes, etc.).
So how exactly would you define a plant stem? They come in many different shapes and sizes, and sometimes are specialized to perform specific functions. Overall, we can say that the plant stem is a supportive plant organ, typically growing aboveground, and supporting the buds, leaves, and sexual reproductive structures.
Plant stems are organs that are part of the main plant body supporting other shoot system organs and growths (leaves, sexual structures, buds), and are involved in transport and storage as well.
The function of the stem in plants
The stem has a number of functions that earn it its role as the plant organ that functions as a central body. The functions of the stem in plants include:
Transport of water through the xylem tissue from roots to leaves. Additionally, transport through the phloem of products from photosynthesis to other parts of the plant.
Support the leaves so that they may access sunlight and water for photosynthesis.
Storage of both water and nutrients.
Perform photosynthesis in some cases, as green (often called herbaceous) stems can photosynthesize.
Stems versus their environment
Plants have adapted to many habitats, from the tundra to the desert. This means that plant stems vary greatly.
One example of a plant stem specially adapted to its environment is that of the cactus. Cacti thrive in dry environments (such as deserts) and therefore have extremely reduced leaves or even none at all! So what does the stem of a cactus plant do? The stems of cacti can photosynthesize to make up for the lack of leaves. They are also fleshy or succulent to help store water for when it is sparse!
Structure of the plant stem: diagram
Nodes are points on the stem where the leaves may grow from. Between them, are the internodes, which are the spaces on the stem between nodes. The axillary buds may develop in the area where the petiole of the leaf connects to the stem, known as the “axil”. These structures are shown in the following plant stem diagram.
The term bud refers to an undeveloped shoot that may become a flower, leaf, or perhaps a branch, which is an extension of the shoot system and will have its own node-internode pattern.
Cells and tissues of the plant stem
The cells of the plant stem and the parts it supports (shoot system) develop from the region of the embryo known as the shoot apical meristem. This region is made up of meristematic tissue, which is the undifferentiated tissue associated with cell growth and division. As the plant grows, the shoot apical meristem maintains apical dominance.
Fig. 1 - Comparison of the internal structure of monocot and dicot plants stems. The different tissues are shown.
Apical dominance inhibits the development of lateral buds as necessary so that the plant can grow vertically towards the direction of light.
Like other parts of the plant, the stem also contains the three types of permanent tissue: dermal, ground, and vascular (Fig. 2). These tissues have specific functions in the stem and their cells are specialized to perform certain functions, which means they are differentiated.
- Dermal tissue covers stems, just as it does the other plant organs, roots and leaves. The epidermal cells of stems may be specialized to be stomata or trichomes, like on leaves.
- Vascular tissue is an essential part of the stem as the main plant body, and thus, the main route of transport between the roots and leaves.
- Ground tissue also plays a big role in stems as a storage tissue (parenchyma) and support (colenchyma and sclerenchyma).
Thorns, prickles, and trichomes are all physical structures that make accessing the stem of plants less accessible and are part of the plants defense to hungry herbivores and omnivores because they poke and sting. But some plants go so far as to recruit their own personal security guards.
The acacia tree has specialized thorns, that grow large and provide fully stocked condos for the ants, and in return, the ants protect the tree from hungry animals by attacking and stinging them!1
Primary versus secondary growth in stems
Primary plant growth contributes to the initial growth of the stem and its lengthening. The apical meristem controls this stem elongation, encouraging the plant to grow taller. Some plants only ever experience primary plant growth- i.e. most herbaceous plants.
Secondary growth contributes to the thickening of the stem. Secondary growth is controlled by the lateral meristem. The lateral meristem consists of the vascular cambium and the cork cambium. These cambium tissues are meristematic and can produce new tissue through cell division.
The vascular cambium cells divide to produce secondary xylem inside and secondary phloem outside. As the newer tissues are produced the older xylem tissues are pushed to the center of the stem in plants experiencing secondary growth where they provide support for the plants.
Secondary plant growth helps provide extra support for plants that grow larger. Having the cork cells and the old xylem (fortified with lignin) provides extra layers of support as plants continue vertical growth. That is why the tree growth form is characterized by an increase in width or secondary plant growth. Some herbaceous plants may experience the production of the secondary xylem and phloem but not the production of cork cells- as that would make them woody. Plants that have bark formed from the cells of the cork cambium are considered woody.
Most secondary growth occurs in dicots and gymnosperms. Monocots do not often experience secondary growth.
Types of plant stems
There are diverse types of plant stems. Plant stems, like roots and leaves, come in many different shapes and sizes and serve a number of specialized functions for certain plants. In fact, a lot of plant parts you eat are stems. Especially parts that people mistake for roots a lot of the time such as potatoes, garlic, and even ginger!
Vines
Vines are considered a plant growth form in which the stems of these plants rely on the support of other plants or objects for support. You may be familiar with some types of plants that exhibit this growth form including grapes or poison ivy. Vines tend to send out tendrils that allow them to grow and climb other plants.
Some vines are considered parasitic and grow at the expense of other plants!
Rhizomes and stolons
Rhizomes are stems that grow horizontally under the soil. They may be adapted to provide extra food storage for the plant as well (such as in ginger plants). Stolons are also horizontally growing stems also known as runners. They usually grow just above or below the soil surface.
Both rhizomes and stolons have the ability to carry out vegetative reproduction. Vegetative reproduction is a method of reproduction where a plant produces new plant organs (roots and shoots) off of non-reproductive structures, and the new plant organs have the ability to exist as separate plants.
If a plant is in an environment with fewer resources, this method of reproduction may be more successful than investing a lot of energy into sexual reproduction.
Vegetative reproduction is the asexual reproduction of plants by producing vegetative plant parts (stems, leaves, roots) that can grow into new, independent plants.
Tubers, corms, and bulbs
Tubers are also modified stems that grow underground. Tubers are adapted to act as storage for a plant, often storing sugars in the form of starches. They are mainly composed of parenchyma tissue, which if you recall from learning about plant tissues, oftentimes functions as a storage tissue. This extra food storage and underground stem help some species survive winter conditions in seasonal climates (known as “overwintering”).
Potatoes are one example of tubers, and the eyes of potatoes are actually buds that can form new plants via vegetative reproduction!
Bulbs and corms are similar to tubers in that they are short vertical underground stems that also store starches and food for the plant. Bulbs often have fleshy leaves (i.e. tulips) that are overlapping and may emerge when conditions are favorable or act as storage if not.
Tulips, lilies, onions, and garlic are all common examples of plants with bulbous stems.
Corms often have scaly leaves, that are not overlapping like those of bulbs.
Examples of plants with corm stems include crocuses and taro root (which is actually an underground corm).
Fig. 3: examples of rhizomes (ginger - top left), bulbs (onion - top right), taro root (corms - bottom left), and potatoes (tuber - bootom right). Source: pixabay, edited
Plant Stem - Key takeaways
- Plant stems are organs that typically grow aboveground and are the main plant body.
- The plant stem functions to transport water and nutrients, as storage organs, and to support the vegetative and reproductive plant parts.
- Primary growth is observed when a stem is lengthening. Secondary growth contributes to the increase in girth in some plants which are typically called woody plants.
- Some plant stems grow underneath the soil and function as starch storage (i.e., tubers) or may also help the plant carry out vegetative reproduction (i.e., rhizomes and stolons).
- Tubers are also modified stems that grow underground. Tubers are adapted to act as storage for a plant, often storing sugars in the form of starches.
References
- Katherine Unger Baillie, "The mutualistic relationship between ants and acacias", Omnia Upenn, Oct. 31, 2019.
- Fig. 3: Ginger: (https://pixabay.com/photos/ginger-fresh-ginger-food-organic-5108742/) by WebTechExperts; onion bulb: (https://pixabay.com/photos/onion-bulb-garlic-crop-plant-5843310/) by NWimagesbySabrinaEickhoff; potatoes: (https://pixabay.com/photos/red-potatoes-potatoes-food-tuber-1353476/) by Brett_Hondow; corm: (https://pixabay.com/photos/taro-root-tuber-caladium-root-2825925/) by pisauikan. All images free to use under the Pixabay License (https://pixabay.com/service/license/).
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