Chapter 6

Anatomy of flowering plants

Both plants and animals have structural similarities and differences in their outward appearance that can be easily viewed. Furthermore, the interior structure of living creatures shares certain characteristics. The internal structure and organization of higher plants are discussed in this chapter. Anatomy is the study of a plant's interior structure. Cells are the basic unit of plants, and cells are organized into tissues, which are then organized into organs. Different organs of a plant have different internal structures. Monocots and dicots are considered to be physically distinct within angiosperms. Internal structures also exhibit environmental adaptations.

Tissues

A tissue is a collection of cells that share the same origin and typically execute the same function. Different types of tissues make up a plant. Tissues are divided into two categories: meristematic and permanent tissues, depending on whether or not the cells forming them are capable of dividing.

A. Meristematic Tissues

Meristematic tissue is a type of plant tissue that can divide continuously throughout its existence. Plant growth is generally restricted to meristems, which are specialized regions of active cell division. Meristems come in several forms in plants. Primary meristems include apical and intercalary meristems, which arise early in a plant's life and contribute to the production of the primary plant body.

Apical meristems are meristems that form at the terminals of roots and shoots and create primary tissues. The root apical meristem is found at the root's tip, while the shoot apical meristem is found at the far end of the stem axis. The axillary bud is formed by cells left over from the shoot apical meristem after the production of leaves and stem elongation. These buds can develop a branch or a flower and can be found in the axils of leaves.

Intercalary meristem is a type of meristem that arises between mature tissues. They are present in grasses and restore sections of the plant that have been eaten by grazing herbivores. They are found in the leaves and internodes. These contribute to the internode's lengthening. It is also present in monocots, and pine trees. It contributes to the plant's height.

The secondary or lateral meristem is found in the mature portions of roots and shoots of many plants, especially those that generate woody axes and develop later than the primary meristem. They are meristems that are cylindrical. Lateral meristems include the fascicular vascular cambium, interfascicular cambium, and cork-cambium. The secondary tissues are produced by these cells. Following cell division in both primary and secondary meristems, newly produced cells become architecturally and functionally specialized, and their ability to divide is lost. Permanent or mature cells are the cells that make up the permanent tissues. Specific parts of the apical meristem produce dermal tissues, ground tissues, and vascular tissues during the creation of the main plant body.

Figure 1  a): Shoot Meristem  
b) Root Meristem

 

B. Permanent Tissues:

Permanent tissues are made up of mature cells that have lost their ability to divide and have taken on a permanent shape, size, and function as a result of meristematic tissue development and differentiation. These tissues' cells can be alive or dead, thin-walled or thick-walled. Permanent tissue cells do not usually divide any further. They are divided into simple tissues and complex tissues.

Simple tissues

are permanent tissues with all cells having the same structure and function.Only one sort of cell makes up a basic tissue. Plants have parenchyma, collenchyma, and sclerenchyma as simple tissues. The parenchyma cells form a majority of the living cells in the plant. The parenchyma's cells are usually isodiametric. They come in a variety of shapes, including spherical, oval, round, polygonal, and elongated. Their cellulose-based walls are quite thin. They might be tightly packed or with minor intercellular gaps. Photosynthesis, storage, and secretion are all processes performed by the parenchyma. In most dicotyledonous plants, the collenchyma is found in layers beneath the epidermis. It can be found as a single layer or in patches. It is made up of cells that have thickened at the corners due to cellulose, hemicellulose, and pectin deposition.Collenchymatous cells can be oval, round, or polygonal, and chloroplasts are common. When chloroplasts are present, these cells digest food. There are no intercellular spaces. They offer mechanical support to the plant's growth portions, such as the young stem and leaf petiole. Long, narrow cells with thick, lignified cell walls and a few or many pits make up sclerenchyma. They are frequently devoid of protoplasts and lifeless. Sclerenchyma can be classified as fibers or sclereids based on differentforms, structures, origins, and development. Fibers are thick-walled, elongated, cells that can be found in groups throughout the plant. Sclereids are dead cells that are spherical, oval, or cylindrical in shape and have very thin cavities (lumen).These are often found in nut fruit walls, fruit pulp such as guava, pear, and sapota, legume seed coats, and tea leaves. Organs are supported mechanically by sclerenchyma.

Figure 2: Simple Permanent Tissue

Complex tissues

 are permanent tissues that contain a variety of cell types.They are made up of multiple cell types that work together as a unit. Plants have complex tissues called xylem and phloem. From the roots to the stem and leaves, the xylem serves as a conduit for water and minerals. It also gives the plant parts mechanical strength. Tracheids, vessels, xylem fibres, and xylem parenchyma are the four types of elements that make up the xylem. The xylem of gymnosperms is devoid of vessels. Tracheids are long, tube-like cells with lignified walls and tapering ends. These have no protoplasm and are dead. The inner layers of the cell walls feature different types of thickenings. Tracheids and vessels are the principal water transportation elements in blooming plants.A vessel is a long cylindrical tube-like structure composed of multiple vessel parts, each with lignified walls and a big central chamber. The vascular cells are also protoplasm-free. Members of the vessel are linked through perforations in their common walls. Angiosperms are characterized by the presence of vessels. The walls of xylem fibers are thicker and the central lumens are destroyed. These can be septate or aseptate in nature. Xylem parenchyma cells are live, thin-walled cells with cellulose-based cell walls. They store food items such as starch or fat, as well as other compounds such as tannins. The ray parenchymatous cells are responsible for water radial conduction.

Protoxylem and metaxylem are the two forms of primary xylem. Protoxylem refers to the first created primary xylem elements, while metaxylem refers to the later formed primary xylem components. The protoxylem is located at the organ's center (pith), while the metaxylem is located near the organ's perimeter. Endarch is the name for this sort of primary xylem. Protoxylem is found on the periphery of roots, while metaxylem is found in the centre. Exarch is the name for this type of primary xylem configuration. Food supplies are transported via phloem from leaves to other sections of the plant. Sieve tube elements, companion cells, phloem parenchyma, and phloem fibres make up phloem in angiosperms. Albuminous cells and sieve cells are found in Gymnosperms. Sieve tubes and companion cells aren't present.

Sieve tube elements are similar to companion cells in that they are long, tube-like structures that are aligned longitudinally. The sieve plates are formed by perforating their end walls in a sieve-like pattern. A developed sieve element has a big vacuole and peripheral cytoplasm but no nucleus. The nucleus of companion cells controls the actions of sieve tubes. Companion cells are specialized parenchymatous cells that are intimately linked to sieve tube elements. Between their common longitudinal walls, pit fields connect the sieve tube elements and companion cells. The partner cells aid in maintaining the sieve tubes' pressure gradient. Elongated, tapered cylindrical cells with rich cytoplasm and nucleus make up phloem parenchyma.The cellulose cell wall has pores through which plasmodesmata connections between cells can be made. Food and other compounds like as resins, latex, and mucilage are stored in the phloem parenchyma. The majority of monocotyledons lack phloem parenchyma. Sclerenchymatous cells make up phloem fibers. The primary phloem is devoid of them, whereas the secondary phloem has them. These are long and unbranched, with sharp, needle-like apices. Phloem fibers have a very thick cell wall. These fibers lose their protoplasm and die as they mature. Protophloem is the first created primary phloem, which has tiny sieve tubes, and metaphloem is the second formed primary phloem, which has larger sieve tubes.

Figure 3: Xylem and Phloem
Figure 4: Classification of plant tissues