Anatomy of Flowering Plants
Internal Structure and Function of Flowering Plants
The study of the internal structure and functional organization of flowering plants, or angiosperms, involves an analysis of tissue systems and their specific roles within various plant organs. This field explores the anatomy of monocotyledonous and dicotyledonous plants, focusing on how cells are grouped into tissues and how these tissues are further organized into systems. The three primary tissue systems include the epidermal tissue system, the ground tissue system, and the vascular tissue system. Each system carries out specialized functions ranging from protection and secretion to transport and mechanical support.
Epidermal Tissue System
The epidermal tissue system forms the outermost protective covering of the entire plant body, including leaves, stems, and roots. It consists of epidermal cells, stomata, and epidermal appendages such as trichomes and hairs. The epidermis is typically the outermost layer, characterized by a continuous arrangement of cells with no intercellular spaces (ICS). These cells are living and possess a thick outer wall in certain environments. In stems and leaves, the epidermis is covered by a waxy, thick layer called the cuticle, which is instrumental in preventing water loss ( loss). Notably, the cuticle is absent in roots to facilitate the absorption of water and minerals.
Stomata are specialized structures present primarily in the epidermis of leaves and sometimes in stems. Each stoma is composed of a stomatal pore, two guard cells, and surrounding subsidiary cells. The guard cells are living cells containing a nucleus and chloroplasts, which are essential for photosynthesis and the regulation of the opening and closing of the stomatal pore. In dicots, the guard cells are kidney-shaped, whereas in monocots, they are dumbbell-shaped. The primary functions of stomata include the exchange of gases ( and ) and the regulation of transpiration. Subsidiary cells are specialized epidermal cells located in the vicinity of the guard cells that assist in stomatal movement.
Epidermal appendages include root hairs and trichomes. Root hairs are unicellular elongations of the epidermal cells (often called epiblema in roots) that significantly increase the surface area for the absorption of water and minerals from the soil. Trichomes are multicellular outgrowths found on the stem. They are often secretory in nature, producing chemical compounds that protect the plant from pathogens. Furthermore, trichomes help prevent water loss by providing an additional layer of protection against transpiration.
Ground Tissue System
The ground tissue system encompasses all tissues except for the epidermal and vascular tissues. It forms the bulk of the plant body and is differentiated into various regions such as the cortex, pericycle, pith, and medullary rays in the primary stem and root. This system is composed of simple tissues, including parenchyma, collenchyma, and sclerenchyma. Parenchymatous cells are typically thin-walled and living, often serving in storage or photosynthesis. Collenchyma provides mechanical support to young growing parts, while sclerenchyma consists of thick-walled, lignified cells that provide structural rigidity.
In leaves, the ground tissue is specifically referred to as mesophyll. The mesophyll consists of thin-walled, chloroplast-containing cells (mesophyll cells) which house the chlorophyll pigment necessary for photosynthesis. In dicot leaves, the mesophyll is further differentiated into palisade parenchyma and spongy parenchyma, whereas in monocot leaves, it remains undifferentiated.
Vascular Tissue System
The vascular tissue system is responsible for the long-distance transport of water, minerals, and nutrients throughout the plant. It consists of complex tissues: xylem and phloem, which are organized into vascular bundles. The xylem is responsible for the conduction of water and minerals, while the phloem translocates organic nutrients. The arrangement and presence of specific tissues within these bundles determine the type of vascular bundle.
Based on the presence or absence of cambium, vascular bundles are classified as open or closed. Open vascular bundles, characteristic of dicotyledonous stems, contain a layer of cambium between the xylem and phloem. This cambium is a meristematic tissue located beneath the bark region that helps in rapid cell division, leading to secondary growth (the formation of secondary xylem and secondary phloem). Closed vascular bundles, found in monocotyledonous stems and roots, lack cambium. Consequently, these plants do not show secondary growth, meaning no secondary xylem or phloem is produced.
Based on the arrangement of xylem and phloem, vascular bundles are categorized as radial or conjoint. In the radial arrangement, xylem and phloem are located on different radii in an alternating manner; this is the characteristic arrangement in roots. In the conjoint arrangement, xylem and phloem are situated at the same radius within the vascular bundle. Conjoint bundles are typically found in stems and leaves. Conjoint bundles can be further classified as collateral (where phloem is on the outer side and xylem on the inner side) or bicollateral.
Anatomy of Dicot and Monocot Roots
The dicot root is characterized by an outermost layer called the epiblema, which bears unicellular root hairs. The cortex is comparatively narrow. The endodermis is the innermost layer of the cortex and contains Casparian strips, though these are more prominent and less thickened than in monocots. The pericycle is responsible for the initiation of lateral roots and vascular cambium during secondary growth. In dicot roots, the number of xylem and phloem bundles varies from to . The pith is either absent or very small. Secondary growth occurs with the help of vascular cambium and cork cambium.
The monocot root features a very wide cortex. The endodermal cells have highly thickened Casparian strips, which are primarily visible in young roots. Unlike the dicot root, the monocot root possesses more than vascular bundles (). The pith is well-developed and large. A significant distinction is that secondary growth is entirely absent in monocot roots.
Anatomy of Dicot and Monocot Stems
In a dicot stem, the ground tissue is clearly differentiated into the cortex, endodermis, pericycle, and pith. The vascular bundles are arranged in a ring, which is a diagnostic feature. These bundles are open, wedge-shaped in outline, and lack a bundle sheath. Because of the presence of cambium between the xylem and phloem, the dicot stem undergoes secondary growth, eventually forming wood.
In a monocot stem, the ground tissue is not differentiated into distinct layers and is made up of similar cells throughout. The vascular bundles are scattered throughout the ground tissue rather than being arranged in a ring. Each vascular bundle is closed, oval or rounded in shape, and is surrounded by a sclerenchymatous bundle sheath. Secondary growth is absent due to the lack of cambium.
Anatomy of Dicot and Monocot Leaves
The dicot leaf, also known as a dorsiventral leaf, typically has stomata that are either absent or less abundant on the upper (adaxial) epidermis compared to the lower (abaxial) epidermis. The guard cells are kidney-shaped. The mesophyll is differentiated into two distinct parts: the upper palisade parenchyma (consisting of elongated, vertically arranged cells) and the lower spongy parenchyma (consisting of loosely arranged, nearly spherical cells). The hypodermis in the midrib region is collenchymatous to provide flexible support.
The monocot leaf, or isobilateral leaf, has stomata equally distributed on both the upper and lower surfaces. The guard cells are dumbbell-shaped. A key characteristic is that the mesophyll is undifferentiated, meaning there is no distinction between palisade and spongy layers. The hypodermis in the midrib region is sclerenchymatous, providing more rigid mechanical support compared to the collenchyma found in dicots.