5.2 Anatomy of dicotyledonous plants

Differences between Monocotyledons and Dicotyledons (ESG7D)

All plants are classified as producing seeds or not producing seeds. Those that produce seeds are divided into flowering (angiosperms) and non-flowering (gymnosperms). Flowering plants are further divided into monocotyledonous and dicotyledonous (monocot and dicot) plants.

In angiosperms, the cotyledon is part of the seed of the plant. The number of cotyledons (mono- or di-) is used to classify flowering plants. Monocotyledonous plants have one cotyledon, dicotyledonous plants have two. Plants belonging to each group have a number of features in common, such as the leaf and root structure, the strength of the stem, the flower structure and flower parts. Some differences between monocots and dicots are summarised in Figure 5.3.

In addition to the differences listed above, monocots and dicots have important differences in their roots. Monocots have a network of fibrous roots and dicots have tap roots.

In the previous chapter you learnt about the key plant tissues involved in support and transport functions, namely the xylem, phloem, collenchyma and sclerenchyma. Recall that these tissues are involved in both transport and supporting roles in plants. In different parts of the plant, tissues are arranged differently. In this section, we will study the overall structure (or anatomy) of dicotyledonous plants.

Root anatomy (ESG7F)

Root systems are responsible for the following functions:

• absorption of water and organic compounds;
• anchoring of the plant body to the ground; and
• storage of food and nutrients.

When a seed germinates, the first structure to appear is the root or the radicle. This becomes the primary root. Other roots that branch out of the primary root are called secondary roots (Figure 5.4). The growing root tip is protected by the root cap as it moves through the coarse soil. The root cap is slimy in nature to facilitate easy movement. Above the root cap is the apical meristem. In this meristematic region, cells divide continuously by mitosis to produce new cells. In addition to mitosis, newly divided cells undergo elongation in the same direction of root lengthening.

Above the region of cell elongation, thousands of tiny root hairs are found in the root hair region. The function of the root hairs is to absorb water and dissolved mineral salts from the soil. As the root grows, it thickens and may produce lateral roots in the mature region as shown in Figure 5.5.

There are two major types of root system.

1. Taproot system: root system comprising one primary root and many secondary roots branching off the primary root. Examples of taproots include carrot and beetroots where the roots serve a storage function. Taproots are found in dicotyledonous plants.
2. Fibrous root system: system with no dominant primary root but many secondary roots of similar size. Fibrous (adventitious) root systems are common in monocotyledons. Examples include coconuts and grasses.

Tissue distribution in root

The different tissues in the root have a distribution which is common to all dicotyledonous plants and is shown in Figure 5.6.

The epidermis is a single layer of cells on the outside that protects the inner tissues. The epidermal layer of the root has no waterproof cuticle as this would prevent the absorption of water. Structurally, the cells of the root hair (shown in Figure 5.7) have large central vacuoles and cover a large surface area which allows water to enter these cells readily via osmosis.

The cortex consists of parenchyma cells. These large, thin-walled cells have leucoplasts to store starch and large vacuoles to store water and dissolved sugars. Intercellular spaces between parenchyma cells facilitate the movement of water from the root hair cells on the outside of the plant to the xylem on the inside of the plant.

The endodermis forms the innermost layer of the cortex. It is a layer of tightly-packed, modified parenchyma cells. The radial and transverse cell walls are thickened with a water-impermeable, waxy suberin layer, known as the Casparian strip. This layer helps to regulate the flow of water from the cortex into the stele, rather than allowing the water to spread to all the root cells. To aid in directing water, there are also thin-walled passage cells in the endodermis, directly opposite the xylem, allowing water to move into the xylem rapidly.

The stele, or vascular cylinder (responsible for transporting water and minerals), consists of the pericycle, phloem, cambium and xylem. The pericycle is the outermost layer of the stele, and consists of one or more rows of thin-walled meristematic parenchyma cells. It is in close contact with the xylem and phloem tissues of the root. It functions in the formation of lateral roots.

The phloem tissue is responsible for transporting food from the leaves of the plant to the cells of the root. The cambium separates the xylem and phloem tissues from each other. This is the area where secondary growth of xylem and phloem tissues occur. Xylem tissue is responsible for transporting water and dissolved mineral salts to the xylem tissue of the stem and leaves. These cells are strengthened with lignin for support. The pits in the cell walls allow for the lateral movement of water. Figure 5.8 shows stained root tissues visualised by confocal microscopy. It shows the internal structure of root cells, including the epidermis, cortex, endodermis and pericycle.

Stem anatomy (ESG7G)

Stems usually grow above the soil surface and towards the light from the sun. Depending on the hardness of the stem, we can distinguish between herbaceous stems, which are leafy non-woody structures, and woody stems. Woody stems are harder than herbaceous stems.

Stems have four main functions.

• Support for the plant as it holds leaves, flowers and fruits upright above the ground. Stems keep the leaves in the light and provide an attachment for flowers and fruits.
• Transport of water, mineral salts and sugars between roots and shoots in the xylem and phloem.
• Storage of nutrients.
• Production of new living tissue: stems contain meristematic tissue which generates new tissue.

The main stem develops from the plumule of the embryo and the lateral branches develop from the buds. Nodes and internodes are regions found on the stem. Nodes are the regions from which leaves and lateral branches develop, and the regions between nodes are known as internodes (shown in Figure 5.9). Stomata, or pores, can be found in stems of younger plants. We will subsequently discuss the tissues present in the dicot stem. The trunk of a tree (shown in Figure 5.10) is the stem.

The internal structure of the dicotyledonous stem

Figure 5.11 shows a schematic arrangement of tissues in a dicotyledonous stem. Details of each tissue type are described in this section.

Epidermis: A single layer of cells that covers the stem, and is in turn covered by a waxy cuticle. The waterproof cuticle helps prevent water loss and thus prevents the inner tissues drying out. Since the function of the epidermis is to protect underlying tissues, epidermal cells are tightly packed and have thickened walls. The epidermis may contain hair-like outgrowths known as trichomes, and stomata with guard cells. Stomata present in the epidermis allow for transpiration and gaseous exchange for respiration and photosynthesis.

Cortex: A region which comprises of collenchyma, parenchyma and the endodermis

• Collenchyma: A few layers of living cells that lie under the epidermis. These cells are not lignified but do have unevenly thickened cell walls. Collenchyma cells are thickened in the corners, but thin-walled elsewhere. The thickened corners strengthen the stem and provide support, while the thin-walled sections allow for flexibility in the wind. Collenchyma cells contain chloroplasts which produce food for the plant during photosynthesis.
• Parenchyma: Found beneath the collenchyma cells and makes up the bulk of the cortex. The cells are thin-walled, and there are intercellular spaces which are important in gaseous exchange. Parenchyma stores synthesised organic food (mostly starch) produced elsewhere in the plant.

Vascular cylinder or stele: Comprised of the pericycle, vascular bundles and pith

• Pericycle: Commonly found in roots and, in lower vascular plants, also in stems. In higher vascular plants, however, a distinct layer of cells may not be present. The pericycle, if present, may be composed of either thin walled parenchyma cells or sclerenchyma cells with relatively thin or heavily thickened walls. In plants undergoing secondary growth, the pericycle contributes to the vascular cambium often diverging into a cork cambium.
• Vascular bundles: Characteristically organised in a ring inside the pericycle of the dicot plant. Mature vascular bundles are made up of water-conducting xylem, cambium, and food-conducting phloem. The phloem is located on the outside of the bundle and the xylem towards the centre (see Figure 5.11). The phloem and xylem is separated by meristematic tissue known as cambium, which is responsible for secondary thickening. Xylem has lignified cell walls which helps it fulfil its two important roles, namely; strengthening and supporting the stem, and transporting water and minerals from the root system to the leaves. The function of phloem is to transport synthesised food from the leaves to other parts of the plant.
• Pith (or medulla): Occupies the large, central part of the stem. The pith is made up of thin-walled parenchyma cells containing intercellular spaces. Where the parenchyma extends between vascular bundles as thin bands it is known as medullary rays, and can be continuous with the pith and cortex of the parenchyma. The cells of the pith store water and starch, while the intercellular spaces allow for gaseous exchange. The medullary rays facilitate transport of substances from xylem and phloem to the inner and outer parts of the stem.

Secondary growth (ESG7H)

Secondary growth is seen clearly if you examine the stump of a tree. During every growing season the stem of a plant increases in width. This is known as secondary thickening. Towards the end of the first year of growth, the parenchyma cells between the vascular bundles become meristematic. This means that they actively start dividing by mitosis, and link up with the cambium that occurs in the vascular bundles to form a cambium ring in the stem (see Figure 5.12). The cells in the cambium ring start dividing to form secondary phloem (on the outside of the cambium ring) and secondary xylem (on the inside of the cambium ring). Each year another ring of secondary phloem and secondary xylem is formed, making the stem grow wider.

It is not possible to see the layers of secondary phloem but the secondary xylem layers are visible. These form rings called annual rings which can be used to work out the age of a plant.

As new rings are formed each year, the older rings are pushed inward and the xylem vessels collapse due to the pressure. The wood in the centre becomes denser and harder than the wood at the surface and is called heartwood. The youngest annual rings found on the outside transport water. This wood is not as dense and is called sapwood. The light-coloured rings are called spring wood. They are formed during spring and summer when the growing conditions are favourable. These rings are therefore relatively broad and light in colour as the xylem cell walls are thin. The dark-coloured rings are called autumn wood. They are formed during autumn and winter when the growing conditions are unfavourable. The rings are therefore relatively narrow and dark in colour as the xylem cell walls are thick. Rings may be thicker in some years than in others, depending on the amount of rain and nutrients received.

There is another type of cambium in plants called cork cambium. Cork cambium forms when an outer layer of parenchyma in the cortex becomes meristematic and produces cortex cells to the inside and cork cells to the outside. This forms the dry outer cells of the bark on a stem.