Dicot Root and Stem — Definition

Definition

Imagine a plant as a complex building, and its roots and stems are like the foundation and the main support beams. In 'dicot' plants (like beans, sunflowers, or mango trees), the internal structure of their roots and stems is quite specific and different from each other, even though they are both part of the same plant.

Let's start with the dicot root. If you were to cut a dicot root cross-section and look at it under a microscope, you'd see several layers. The outermost layer is the 'epidermis', which has tiny root hairs that help absorb water and minerals from the soil.

Just inside this is a thick region called the 'cortex', mostly made of storage cells (parenchyma). Deeper still, you'd find the 'endodermis', a special layer with a waxy band called the 'Casparian strip' that controls what enters the vascular tissue.

Inside the endodermis is the 'stele', which contains the 'vascular bundles' – these are like the plant's pipelines. In a dicot root, these pipelines (xylem for water, phloem for food) are arranged radially, meaning they alternate with each other around a central point, often forming a star-like shape with 2 to 6 arms of xylem.

The xylem matures from the outside inwards (exarch), and there's usually a very small or no central pith.

Now, let's look at the dicot stem. Again, starting from the outside, there's the 'epidermis', which has a protective waxy layer called the 'cuticle' and sometimes small hairs (trichomes). Beneath the epidermis is the 'cortex', but it's structured differently from the root.

It often starts with a 'hypodermis' (made of collenchyma cells for support), followed by general parenchyma, and then an 'endodermis' (sometimes called a starch sheath because it stores starch). The most striking feature of a dicot stem is the arrangement of its 'vascular bundles'.

Unlike the root, they are not radial; instead, they are 'conjoint' (xylem and phloem together), 'collateral' (xylem towards the inside, phloem towards the outside), and 'open' (meaning there's a cambium layer between xylem and phloem, allowing for secondary growth).

These bundles are typically arranged in a ring around a central region called the 'pith', which is large and stores food. The xylem in the stem matures from the inside outwards (endarch). Understanding these distinct arrangements helps us identify whether we're looking at a root or a stem, and whether it belongs to a dicot plant.