Anatomy of Flowering Plants

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Anatomy definition: is the branch of biology concerned with the study of the structure of organisms and their parts.

 Difference between Morphology and Anatomy:

o Plant morphology or phytomorphology is the study of the physical form and

external structure of plants, whereas plant anatomy is the study of the

internal plant structure, mostly at the cellular/microscopic level.

 Cell – is the smallest structural and functional unit of an organism, which is

typically microscopic and consist of cytoplasm and nucleus enclosed in a

membrane.

 Difference between Protoplasm and cytoplasm:

o Protoplasm is the colourless material consisting of the living part of a cell,

including the cytoplasm, nucleus, and other organelles. Cytoplasm is the fluid

that consist of all of the content outside the nucleus and enclosed within the

cell membrane of a cell. The tissue

A group of cells having a common origin and usually performing common function are called tissues.

Plant Tissue

Meristematic Tissue Permanent Tissue

1. Apical meristem Simple permanent tissue Complex permanent tissue

2. Intercalary 1. Parenchyma 1. Xylem

3. Lateral 2. Collenchyma 2. Phloem

3. Sclerenchyma Meristematic Tissues or Meristems

(1) They contain immature and young cells and are capable of repeated divisions.

(2) Intercellular spaces are not present in meristematic tissue.

(3) They contain a homogeneous thin wall.

(4) They contain large nuclei associated with abundant cytoplasm.

(5) They are metabolically very active but they do not store food material.

(6) Only proto-plastids are present instead of plastids, chloroplast absent.

(7) Dense cytoplasm is present which contains several premature mitochondria.

(8) Vacuoles are absent.

(9) Meristematic cells are isodiametric (roughly spherical in shape, not a proper sphere but almost spherical) in shape.

Isodiametric shape of parenchymatic cell

Types of meristems

The meristems may be classified on the basis of their mode of origin, position or function:

(i) According to origin and development: On the basis of origin, meristematic tissues are of three types : (a) Promeristem or Primordial meristem: The promeristem originates from

embryo and, therefore, called primordial or embryonic meristem. It is present in

the regions where an organ or a part of plant body is initiated.

(b) Primary meristem: A primary meristem originates from promeristem and

retains its meristematic activity. It is located in the apices of roots, stems and the

leaf primordia.

typical promeristem. Some living permanent cells may regain the meristematic nature.

(ii) According to position: On the basis of their position in the plant body meristems are classified into three categories:

(a) Apical meristem: This meristem is located at the growing apices of main and

lateral shoots and roots. These cells are responsible for linear growth of an organ.

(b) Intercalary meristem: These are the portions of apical meristems which are

separated from the apex during the growth of axis and formation of permanent

tissues. It is present mostly at the base of node (e.g., Mentha viridisMint), base of

internode (e.g., stem of many monocots viz., Wheat, Grasses, Pteridophyts like

Equisetum) or at the base of the leaf (e.g., Pinus).

sides of stems and roots. This meristem consists of initials which divide mainly in

one plane (periclinal) and result increase in the diameter of an organ.

(iii) According to function: Haberlandt in 1890 classified the primary meristem at the apex of stem under the following three types: (a) Protoderm: It is the outermost layer of the apical meristem which develops into

the epidermis or epidermal tissue system.

(b) Procambium: It occurs inside the protoderm. Some of the cells of young

growing region which by their elongation and differentiation give rise to primary

vascular tissue constitute the procambium.

ground tissues like hypodermis, cortex, endodermis, pericycle, pith and medullary

rays.

(iv) According to plane of cell division: On the basis of their plane of cell division meristem are classified into three categories:

(a) Mass meristem: The cells divide anticlinally in all planes, so mass of cells is

formed. e.g., formation of spores, cortex, pith, endosperm.

(b) Plate meristem: The cells divide anticlinally in two planes, so plate like area

increased. e.g., formation of epidermis and lamina of leaves. (c) Rib or File meristem: The cells divide anticlinally in one plane, so row or

column of cells is formed. e.g., formation of lateral root.

Permanent Tissues

Permanent tissues are made up of mature cells which have lost the capacity to divide and have attained a permanent shape, size and function due to division and differentiation in meristematic tissues. The cells of these tissues are either living or dead, thin-walled or thick-walled. Permanent tissues are of three types:

(1) Simple tissues: Simple tissues are a group of cells which are all alike in origin, form and function. They are further grouped under three categories:

(i) Parenchyma: Parenchyma is most simple and unspecialized tissue which is

concerned mainly with the vegetative activities of the plant.

(ii) Collenchyma: The term collenchyma was coined by Schleiden (1839). It is the

tissue of primary body.

o The cells of this tissue contain protoplasm and are living.

o The cell walls are thickened at the corners and are made up of cellulose,

hemicellulose and pectin.

(iii) Sclerenchyma: It was discovered and coined by Mettenius (1805).The main

feature of sclerenchyma are:

o It consist of thick-walled dead cells.

o The cells vary in shape, size and origin. (2) Complex Permanent Tissue

Unlike simple permanent cells which look the same and are made up of one type of cells, complex permanent tissues are made up of more than one type of cells. These different types of cells coordinate to perform a function. Xylem and Phloem are complex permanent tissues and are found in the vascular bundles in the plants.

 Xylem- It consists of tracheids, vessels, xylem parenchyma and xylem fibres.

Tracheids and vessels are hollow tube-like structures that help in conducting water

and minerals. The xylem conducts only in one direction i.e vertically. The xylem

parenchyma is responsible for storing the prepared food and assists in the

conduction of water. Xylem fibres are supportive in function.

 Phloem- It consists of four of elements: sieve tubes, companion cells, phloem

fibres and the phloem parenchyma. Unlike the xylem, phloem conducts in both

directions. It is responsible for transporting food from the leaves to the other

parts of the plant. Phloem contains living tissues except for fibres that are dead

tissues.  Primary xylem is of two types- protoxylem and metaxylem. In stem,

protoxylem lies in centre and metaxylem towards periphery. This type of

primary xylem is called endarch.

 In roots, protoxylem lies in periphery and metaxylem lies towards the centre.

This type of primary xylem is called exarch.

 In gymnosperms, albuminous cells and sieve cells lack sieve tube and

companion cells.

Secondary growth

Unlike most animals, who grow to a specific body size and shape and then stop growing

(determinate growth), plants exhibit indeterminate growth where the plant will continue adding new organs (leaves, stems, roots) as long as it has access to the necessary resources.

Plants are able to continue growing indefinitely like this due to specialized tissues called meristems, which are regions of continuous cell division and growth. Meristematic tissue cells are either undifferentiated or incompletely differentiated, and they continue to produce cells that quickly differentiate, or specialize, and become permanent tissues

(dermal, ground, and vascular).

Meristems contribute to both primary (taller/longer) and secondary (wider) growth.

Primary growth is controlled by root apical meristems or shoot apical meristems, while secondary growth is controlled by the two lateral meristems, called the vascular cambium and the cork cambium. Not all plants exhibit secondary growth.

Key Points

 Indeterminate growth continues throughout a plant’s life, while determinate growth

stops when a plant element (such as a leaf) reaches a particular size.  Primary growth of stems is a result of rapidly-dividing cells in the apical meristems

at the shoot tips.

 Apical dominance reduces the growth along the sides of branches and stems, giving

the tree a conical shape.

 The growth of the lateral meristems, which includes the vascular cambium and the

cork cambium (in woody plants), increases the thickness of the stem during

secondary growth.

 Cork cells (bark) protect the plant against physical damage and water loss; they

contain a waxy substance known as suberin that prevents water from penetrating the

tissue.

 The secondary xylem develops dense wood during the fall and thin wood during the

spring, which produces a characteristic ring for each year of growth.

Key Terms

 Lenticel: small, oval, rounded spots upon the stem or branch of a plant that allow the

exchange of gases with the surrounding atmosphere

 Periderm: the outer layer of plant tissue; the outer bark

 Suberin: a waxy material found in bark that can repel water

Growth in Stems

Growth in plants occurs as the stems and roots lengthen. Some plants, especially those that are woody, also increase in thickness during their life span. The increase in length of the shoot and the root is referred to as primary growth. It is the result of cell division in the shoot apical meristem. Secondary growth is characterized by an increase in thickness or girth of the plant. It is caused by cell division in the lateral meristem. Herbaceous plants mostly undergo primary growth, with little secondary growth or increase in thickness. Secondary growth, or “wood”, is noticeable in woody plants; it occurs in some dicots, but occurs very rarely in monocots.

Figure: Primary and secondary growth: In woody plants, primary growth is followed by secondary growth, which allows the plant stem to increase in thickness or girth. Secondary vascular tissue is added as the plant grows, as well as a cork layer. The bark of a tree extends from the vascular cambium to the epidermis.

Some plant parts, such as stems and roots, continue to grow throughout a plant’s life: a phenomenon called indeterminate growth. Other plant parts, such as leaves and flowers, exhibit determinate growth, which ceases when a plant part reaches a particular size.

Primary Growth

Most primary growth occurs at the apices, or tips, of stems and roots. Primary growth is a result of rapidly-dividing cells in the apical meristems at the shoot tip and root tip.

Subsequent cell elongation also contributes to primary growth. The growth of shoots and roots during primary growth enables plants to continuously seek water (roots) or sunlight

(shoots).

The influence of the apical bud on overall plant growth is known as apical dominance, which diminishes the growth of axillary buds that form along the sides of branches and stems. Most coniferous trees exhibit strong apical dominance, thus producing the typical conical Christmas tree shape. If the apical bud is removed, then the axillary buds will start forming lateral branches. Gardeners make use of this fact when they prune plants by cutting off the tops of branches, thus encouraging the axillary buds to grow out, giving the plant a bushy shape.

Conifers Angiosperm Tree

Secondary Growth

The increase in stem thickness that results from secondary growth is due to the activity of the lateral meristems, which are lacking in herbaceous plants. Lateral meristems include the vascular cambium and, in woody plants, the cork cambium. The vascular cambium is located just outside the primary xylem and to the interior of the primary phloem. The cells of the vascular cambium divide and form secondary xylem (tracheids and vessel elements) to the inside and secondary phloem (sieve elements and companion cells) to the outside. The thickening of the stem that occurs in secondary growth is due to the formation of secondary phloem and secondary xylem by the vascular cambium, plus the action of cork cambium, which forms the tough outermost layer of the stem. The cells of the secondary xylem contain lignin, which provides hardiness and strength.

In woody plants, cork cambium is the outermost lateral meristem. It produces cork cells

(bark) containing a waxy substance known as suberin that can repel water. The bark protects the plant against physical damage and helps reduce water loss. The cork cambium also produces a layer of cells known as phelloderm, which grows inward from the cambium. The cork cambium, cork cells, and phelloderm are collectively termed the periderm. The periderm substitutes for the epidermis in mature plants. In some plants, the periderm has many openings, known as lenticels, which allow the interior cells to exchange gases with the outside atmosphere. This supplies oxygen to the living- and metabolically-active cells of the cortex, xylem, and phloem. Annual Rings

The activity of the vascular cambium gives rise to annual growth rings. During the spring growing season, cells of the secondary xylem have a large internal diameter; their primary cell walls are not extensively thickened. This is known as early wood, or spring wood.

During the fall season, the secondary xylem develops thickened cell walls, forming late wood, or autumn wood, which is denser than early wood. This alternation of early and late wood is due largely to a seasonal decrease in the number of vessel elements and a seasonal increase in the number of tracheids. It results in the formation of an annual ring, which can be seen as a circular ring in the cross section of the stem. An examination of the number of annual rings and their nature (such as their size and cell wall thickness) can reveal the age of the tree and the prevailing climatic conditions during each season.

Figure: Annual growth rings: The rate of wood growth increases in summer and decreases in winter, producing a characteristic ring for each year of growth. Seasonal changes in weather patterns can also affect the growth rate. Note how the rings vary in thickness.

Secondary Meristem in Monocot

Secondary growth is increase in the circumference / girth of the plant organs due to the formation of secondary tissues in stelar and extra stelar regions. Normally secondary growth takes place in roots and stem of dicotyledons and gymnosperms. Due to lack of cambium in monocotyledons, secondary growth is absent. But exceptionally, secondary growth takes place in some monocotyledons, such as palm, Yucca, Dracaena etc.

Difference between Dicot and Monocot Root

Dicot Root Monocot Root

Pericycle

Gives rise to cork cambium, parts of the vascular cambium, and Gives rise to lateral roots only lateral roots

Vascular Tissues

Has a limited number of Xylem and Phloem Has a higher number of Xylem and Phloem

Shape of Xylem

Angular or Polygonal Round or Oval

Number of Xylem and Phloem

2 to 8 8 to many

Pith

Absent or very small and undeveloped Larger and well developed

Conjunctive tissue

Parenchymatous Sclerenchymatous

Secondary growth Secondary growth occurs Secondary growth does not occur

Cambium

Present and formed by the Conjunctive parenchyma Absent

Xylem

Usually tetrarch Polyarch

Cortex

Comparatively Narrow Very wide

Covering

Older roots are covered by a Cork Older roots are covered by an Exodermis

Examples

Pea, beans, peanuts, etc. Maize, banana, palm, etc.

Secondary growth in Monocot

Most monocotyledons consist entirely of primary tissues. The usual vascular cambium is

absent from this group and so there is no normal secondary growth. However, in some

monocots, the thickening and elongation of stem occurs through primary thickening

meristem, diffuse secondary thickening and secondary thickening meristem.

Primary thickening meristem: This meristem is observed in palms, in the rhizomes of Musa (Banana) and in the bulbs of

Allium cepa (Onion) etc. In these plants, the shoot apex is not large and produces only a small part of the primary body. A considerable thickening occurs below the shoot apical meristem. This is due to the intensive cell division of primary thickening meristem.

This meristem lies below the young leaf bases and originates by periclinal division of the cells situated below the region of attachment of young leaf primordia. The meristem appears as a flat zone in longitudinal section of the developing embryo. The zone gradually assumes a concave form in the mature young plant.

The meristem consists of several layer of cells which are rectangular, elongated and oriented parallel to the surface of shoot apex. The derivatives of meristem are the ground parenchyma cells. Sometimes localized mitotic activity within this meristem forms procambial strands, which run horizontal to the surface. These procambial strands gradually mature into vascular bundles.

The term meristematic cap has been coined by Zimmermann and Tomlinson (1976) to designate the zone of procambium formation. Primary thickening meristem contributes mainly to the increase in width of stem and later it causes the elongation of young stem. In palms, Musa and a number of other monocotyledons with a similar growth habit the procambium originates from two sources —the shoot apical meristem and meristematic cap.

Diffuse secondary thickening:

In palm stem the ground parenchyma cells, close to and distant from the shoot apex (ex.

Roystonea, Actinophloeus etc.), expand along with proportional increase of the intercellular spaces, thus causing diffuse secondary growth. Sometimes these ground parenchyma and the procambium cells, destined to produce the outer fibres of bundle sheath, divide, expand and thus contribute to the diameter of stem. The diffuse secondary thickening occurs after the completion of expansion and elongation caused by primary thickening meristem. Secondary thickening meristem:

Secondary thickening with this meristem occurs in a number of monocotyledonous species such as, Xanthorrhoea, Dracaena, (Figs. 21.1 & 31.23) Cordyline, Aloe, Yucca, Kingia,

Dioscorea etc. This meristem is a type of vascular cambium, which originates in the parenchyma cells present on the peripheral sides of the entire mass of primary vascular bundles.

This region may be the inner layer of cortex or pericycle. Though this lateral meristem is distant from the stem apex, in seedling stages it develops in continuity to the primary thickening meristem. In mature plants these two meristems are found to be discontinuous. The secondary thickening meristem consists of several tiers of cells that may be of different shapes, such as fusiform, rectangular, polygonal or only with one end tapered. This cambium divides tangentially, some of the peripheral derivatives divide repeatedly and the others form secondary cortex. These parenchyma cells usually remain thin walled and sometimes may contain crystals.

In Xanthorrhoea these cells secrete resin, which form a sheath around the stem. A large number of tissues are formed to the inner side. The inner tissues form the ground parenchyma and vascular bundles. The ground parenchymatous tissues are termed as conjunctive tissues, which may sometimes become lignified. The secondary vascular bundles remain embedded in the conjunctive tissue.

The bundles are either collateral (e.g. Kingia) or amphivasal (e.g. Dracaena, Aloe etc.). In

Kingia, the phloem is surrounded by xylem only on three sides and so in cross section the vascular bundles appear as U-shaped. The xylem consists of small amount xylem parenchyma that may be lignified and tracheids only. Some localized region of the tiers of cambium is involved in the formation of a single bundle. The conjunctive tissues with embedded secondary vascular bundles show radial arrangement in contrast to primary strands, which are irregularly arranged.

Protective tissues are developed in the perennial monocotyledons by periderm and storied cork formation.

Periderm is formed with the origin of cork cambium or phellogen and the subsequent formation of phellem and phelloderm from it like dicotyledons. The periderm formation of

Dracaena, Aloe and Yucca has no difference than those of dicotyledons.

Storied cork:

It is a special type of protective tissue composed of cells, which are radially arranged in files and have suberized walls. The storied corks are observed in Curcuma, Cordyline and in many palms. These corks are formed by the activity of storied meristem. This meristem originates at the outer cortex. In contrast to phellogen, the storied meristem does not form a regular uninterrupted cylinder.

The cortical parenchyma cells, which are destined to be the initials of this meristem, become three to eight layered by periclinal division. The derivatives of this meristem become suberized and are termed as storied cork. The cork cells are arranged in irregular tangential bands and radial files.

Sometimes these bands may coalesce tangentially and radially when they enclose some undividing cortical cells, which also become suberized. Thus the storied cork along with enclosed suberized cells form a hard protective tissue (Fig. 21.2).