Plant Stem

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:

1. 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.

2. Support the leaves so that they may access sunlight and water for photosynthesis.

3. Storage of both water and nutrients.

4. Perform photosynthesis in some cases, as green (often called herbaceous) stems can photosynthesize.

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.

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.

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).

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.

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.

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.

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.

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”).

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.

Corms often have scaly leaves, that are not overlapping like those of bulbs.

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