Theory

The plant stem is a crucial component to study plant anatomy. Primarily, the stem provides support, allowing the plant to stand upright and hold up its leaves, flowers, and fruits. Stems can be herbaceous, being soft and flexible, or woody, providing greater structural support. Overall, the plant stem is a versatile structure that supports, nourishes, and propagates the plant.

An example of a dicot stem is that of Amaranthus. Its stems are typically herbaceous i.e., they are soft and flexible rather than woody. These stems can be green or reddish, often featuring a ribbed or grooved surface.

Anthocyanins in Amaranthus are valuable not only for their vibrant colour but also for their significant health benefits. These pigments contribute to the plant's attractiveness and play a role in health. As natural antioxidants and anti-inflammatory agents, amaranth anthocyanins are of great interest in the food, nutraceutical, and pharmaceutical industries.

A transverse section of an Amaranthus stem, when observed under a microscope, reveals several distinct tissues arranged in a characteristic pattern:

Epidermis: The epidermis is the outermost cell layer of plant stems and leaves, typically composed of a single layer of tightly packed cells. Its primary functions include protecting against physical damage and pathogens, regulating gas exchange through stomata, and preventing water loss with a waxy cuticle. These roles are crucial for the plant's health and its ability to carry out essential processes like photosynthesis and respiration.

Cortex: The cortex, positioned beneath the epidermis, comprises multiple layers of parenchyma cells. These cells, characterized by their often irregular arrangement and presence of intercellular gaps, facilitate gas exchange within the plant. Additionally, the cortex receives structural reinforcement from collenchyma cells, which may also be found within this tissue layer. Collenchyma cells contribute to the plant's stability and flexibility by providing additional support against gravitational forces and mechanical stress. Consequently, the cortex, with its parenchyma and collenchyma cells, plays a vital role in maintaining the structural integrity and physiological functions of the plant.

Collenchyma is often found in the stems and petioles (leaf stalks) of plants. In celery, the "strings" you encounter while eating are actually strands of collenchyma cells, giving the stalk its crunch and flexibility.

Vascular Bundles: They are found scattered all through the stem in a characteristic design of dicotyledonous plants, each vascular bundle includes:

Phloem: The phloem is present on the outer side of the vascular bundle. It is in charge of moving the plant's photosynthetic by-products from the leaves to other parts of the plant.

Cambium: The cambium, positioned between the phloem and xylem, serves as a layer of meristematic tissue vital for the plant's secondary growth. Its primary role is to enable the expansion of the stem's diameter, thereby facilitating the plant's overall growth and development.

The vascular cambium is responsible for creating tree rings. Each year, the cambium produces a new layer of xylem (wood) and phloem, which can be seen as rings in a cross-section of a tree trunk. By counting these rings, you can determine the age of a tree.

Xylem: The xylem, located internally within the vascular bundle, plays a crucial role in conducting water and essential nutrients from the roots to various parts of the plant. It comprises vessel elements and tracheids, specialized structures facilitating the efficient transport of fluids.

The tallest plant in the world is Sequoia sempervirens (D. Don) Endl.,The xylem's role in water transport, structural support, resistance to cavitation, efficient water use, and secondary growth are all vital for Sequoia sempervirens to achieve and maintain its remarkable height.

The medullary ray, also known as a vascular ray, is a structural feature found in the anatomy of plants, particularly in the stems of vascular plants. These create beautiful patterns known as "ray flecks" in certain types of wood, like oak and maple. These patterns are highly prized in fine woodworking and furniture making for their aesthetic appeal.

Pith: The central region of the stem contains a tissue called the pith, composed of large, loosely arranged parenchyma cells. It serves as a storage tissue and plays a vital role in maintaining the structural integrity of the stem.
Pericycle: The pericycle, sometimes located just inside the endodermis, possesses the capability to initiate the formation of lateral roots or contribute to secondary growth processes.
Endodermis: In stems, the endodermis may not be as prominent as in roots, but certain regions may exhibit a layer of endodermis that regulates the movement of substances between the cortex and the vascular tissues.

Modified stems are specialized stems that have evolved to perform functions other than or in addition to their primary role of supporting leaves, flowers, and fruits, as well as transporting nutrients and water. Rhizome: Ginger, turmeric (Store nutrients and allow vegetative reproduction), Tuber: Potato, Yam (store nutrients, and allow vegetative reproduction), Bulb: Onion, Garlic, Tulips (Store nutrients and water, and support vegetative reproduction), Tendril: Pea, Cucumber (allowing the plant to climb and reach sunlight), Thorn: Rose (Provide protection).

Cauliflory is a fascinating phenomenon where flowers and fruits develop directly from the main stems or trunks of trees, rather than from new growth or branches. This adaptation is especially common in certain tropical plant species. e.g. Couroupita guianensis Aubl., Artocarpus heterophyllus Lam.

Dicot and monocot stems exhibit distinct differences primarily in their vascular bundle arrangement, growth patterns, and internal structure. In dicot stems, such as those of sunflowers (Helianthus annuus L.), the vascular bundles are arranged in a ring around the perimeter, which allows for secondary growth and the development of wood and bark. This is in contrast to monocot stems like those of corn (Zea mays L.), where the vascular bundles are scattered throughout the stem’s ground tissue, lacking the vascular cambium necessary for secondary growth, resulting in stems that grow in length but not in thickness. Additionally, dicot stems have distinct regions of pith and cortex, while monocot stems have a more uniform ground tissue without distinct pith and cortex. These structural differences influence how these plants grow and adapt to their environments.

Fig. Transverse section of a dicot stem