Vascular Anatomy of the Floral Parts

Vascular Anatomy of the Floral Parts

Flower composition

     The flower consists of an axis (receptacle) and lateral appendages (floral parts or floral organs). The sepals and petals which constitute the calyx and corolla respectively are the sterile parts. The stamens and the carpels are the reproductive parts. The stamens compose the androecium, whereas the carpels compose the gynoecium.

Importance of floral anatomy

      The study of the vascular anatomy has helped in solving many problems of floral morphology. It has shown that many structures are not what they appear to be or what they are commonly taken to be.

      Morphologically the flower is a determined shoot with appendages, these appendages are homologous with leaves. The floral vascular skeletons, differ in no essential way from leaf stems. In the present text the flower is treated on the basis of the  homology between the flower and the shoot in their phylogeny and ontogeny.

Pedicel

The Pedicel and the receptacle have typical structure, with a normal vascular cylinder. The cylinder may be unbroken or it may contain a ring of vascular bundles. In the region where floral organs are borne, the pedicel expand into the receptacle. The vascular cylinder also expands and the vascular bundles increase somewhat in number, and finally traces begin to diverge.

The appendage traces are derived from the receptacular stele exactly as leaf traces are derived in typical stems.

Sepals

      The sepals are anatomically like the leaves of the plant. A sepal usually receives three traces derived from the same or different sources. As regards the morphological nature of the sepals, they have often been considered as equivalent to bracts and foliage leaves.

Petals

      In their vascular supply the petals are sometimes leaf like, but much more often they are like stamens. The petals may have one (very common), three or several traces.

Stamens

      A stamen generally receives a single trace which remians almost unbranched throughout its course in the filament. In the anther region it may undergo some branching.

Carpels

      The carpel is commonly looked as a leaf-like organ folded upward, i.e., ventrally with its margins more or less completely fused and bearing the ovules. This conception has been supported by the anatomy. The carpel has one, three, five or several traces. The three trace carpel is most common. The median trace which leaves the stele below the other carpel traces, is known as the dorsal trace because it becomes the dorsal (midrib) bundle of the folded organ. The outermost traces are known as ventral or marginal traces because they become the bundles that run along the ventral edge of the carpel. The ovule traces are derived from the ventral bundles.

The Plant Organs

The Root

Position: The lower part of the plant axis.

Functions: Anchorage and absorption of water and minerals.

The epidermis: Thin walled, without cuticle, having root hairs (piliferous layer), uniseriate or multiseriate (velamen) in air roots that act as absorptive tissue.

The cortex: Parenchyma, no chlorophyll except in some hydrophytes and aerial roots of epiphytes, act as storage tissue.

The exodermis: The outer subepidermal layer of the cortex, suberized or lignified, single layer or many layers, act as protective tissue.

The endodermis: The inner boundary of the root cortex, with casperian strip and passage cells, regulates the water passage to xylem.

The vascular cylinder: Surrounded by the pericycle that is the origin of the lateral roots, the vascular bundle of radial type, the protoxylem exarch.

Monocot roots                                                 

The xylem is polyarch; more than 17 arches.  

Dicot roots

The xylem is limited; from 2-12 arches.

The Stem

The epidermis: The outer layer of the stem, fiberous epidermis may be present in monoct stems, trichomes may be present.

The cortex: Between the epidermis and the vascular cylinder, of parenchyma, chlorenchyma, collenchyma or fibers, in monocot stems the ground tissue is undifferentiated into cortex nor pith.

The starch sheath: The innermost layer of the cortex, containing starch grains.

The primary vascular system: External phloem and internal xylem (conjoint vascular bundles), the protoxylem endarch.

The pith: Parenchyma with leucoplastids, may contain crystals, tannins or sclereids.

Monocot stems                                                                  

The vascular bundles are:

numerous, scattered, collateral, closed, xylem vessels in the form of V or Y, presence of xylem cavity, presence of fiber sheath around the bundle.

Dicot stems                                                                  

The vascular bundles are:

few, arranged, collateral or bicollateral, opened, xylem vessels in the form of rows, absence of fiber sheath .

Root-Stem Transition

       The root and the stem make a continuous structure called the axis of the plant. The vascular bundles are continuous from the root to the stem, but the arrangement of vascular bundles is quite different in the two organs; the stems possess collateral bundles with endarch xylem, whereas the roots possess radial bundles with exarch xylem. The change of position involving inversion and twisting of xylem strands from exarch to endarch type is referred to as vascular transition, and the part of the axis where these changes occur is called transition region.

Type A: In Fumaria, Mirabilis and Dipsacus, and others, each xylem strand of the root divides by radial division forming branches, they swing in their lateral direction; one towards right and the other goes to the left. These branches join the phloem strands on the inside. The phloem strands, do not change their position and also remain unchanged in their orientation. They remain in the form of straight strands continuously from the root into the stem. In this type as many primary bundles are formed in the stem as many phloem strands are formed in the root.

Type B: In Cucurbita, Phaseolus, Acer and Trapaeolum and others, the xylem and phloem strands fork, the branches of the strands of both swing in lateral direction and join in pairs. After joining in the pairs they remain in the alternate position of the strands in the root. The xylem strands become inverted in their position and the phloem strands do not change their orientation. This way, in the stem, the number of bundles becomes double of the phloem strands found in the root. This type of transition is more commonly found.

Type C: In Lathyrus, Medicago and Phoenix, the xylem strands do not fork and continue their direct course into the stem. These strands twist through 180 degrees. The phloem strands divide soon and the resulting halves swing in the lateral direction to the xylem positions. The phloem strands join the xylem strands on the outside. In this type as many bundles are formed as there are phloem strands in the root (as in type A).

Type D: This type is rarely found and is known in only a few monocotyledons (e.g., Anemarrhena). In this type half of the xylem strands fork and the branches swing in their lateral direction to join the other undivided strands of xylem which become inverted. The phloem strands do not divide but they become united in pairs. These united phloem strands unite with the triple strands of the xylem. Thus, a single bundle of the stem consists of five united strands. In this type half as many bundles are formed in the stem as there are phloem strands in the root.

The Leaf

The epidermis: contains stomata and trichomes, upper (adaxial) and lower (abaxial).

The ground tissue: the ground tissue of the leaf is called mesophyll.

Types of leaves:

Dorsiventral:

Palisade below the upper epidermis, the spongy at abaxial surface.

Isolateral:

Palisade is found on both ad- and abaxial surfaces, one row at each side.

Isobilateral:

Palisade found on both sides, two rows at each.

The vascular system:

The vascular bundles are called veins, the arrangement of veins is called venation, the phloem is directed towards the lower epidermis.

Monocot leaves

The blade is undifferentiated into midrib nor wings.

The ground tissue is undifferentiated into palisade nor spongy.

Vascular bundles are numerous.

Mechanical tissue of sclerenchyma beneath epidermis.

Dicot leaves

The blade is differentiated into midrib and wings.

The ground tissue is differentiated into palisade and spongy.

Vascular bundles are one median and numerous laterals.

Mechanical tissue of collenchyma beneath epidermis in midrib region.

Lecture 06

The Vascular Tissue System

The vascular tissue system consists of vascular bundles that distributed in the stele.

The stele is the central cylinder of stem & root delimited externally by cortex & enclosed pith or no at the center.

Functions:  •    Xylem functions in water conduction with mineral from roots to shoots.

                    •   Phloem functions in carbohydrates translocation.

Origin: Originate from procambium meristem.

Constituents:  •           Phloem (bast elements).

                         •          Vascular cambium.

                         •          Xylem (wood elements).

Vascular Cambium

Locality: In between xylem & phloem (in stems).

Characters: The cambial cells are living, thin-walled, able to divide, elongated with tapered ends, uniseriate or multiseriate.

Position:

•           Fascicular ( inside vascular bundle ).

•           Interfascicular( in between vascular bundles ).

Behavior:

• In monocot stems:

the cambium is completely differentiated into pimary  xylem & phloem so, the vascular bundle is closed

& secondary growth will not form inside the bundle.

• In dicot stems:

the cambium remains meristematic to give secondary

growth so, the vascular bundle is opened.

N.B. In roots:

the primary phloem & xylem originate from

pericycle side by side forming radial vascular bundle.

Phloem:  Complex tissue consisting of:

In Angiosperms (Dicots & Monocots)

•           Sieve tube cells.

•           Companion cells.

•           Phloem parenchyma.

In Gymnosperm (Conifers)

•           Sieve cells.

•           Albuminous cells.

•           Phloem parenchyma.

Portions of phloem:

•           Protophloem consists of (sieve tube cells & parenchyma) in angiosperms,

(sieve cells & parenchyma) in gymnosperms.

•           Metaphloem (sieve tube cells, companion cells & parenchyma) in angiosperms, (sieve cells, albuminous cells & parenchyma) in gymnosperm.

Xylem:  Complex tissue consisting of:. 

In Angiosperms (Dicots & Monocots):

•           Xylem vessels (function in water conduction).

•           Xylem (wood) fibers (xylary fibers) (give mechanical support).

•           Xylem (wood) parenchyma (aid in storage of food & water).

In Gymnosperms (Conifers):

•           Tracheids with bordered pits (water conduction).

•           Xylem (wood) fibers (mechanical support).

•           Xylem (wood) parenchyma (storage of food & water).

Portions of Xylem:

•           Protoxylem consists of (xylem vessels & xylem parenchyma) in angiosperms, (tracheides with bordered pits & xylem parenchyma) in gymnosperms.

•           Metaxylem consists of (xylem vessels, xylem fibers & xylem parenchyma) in angiosperms, (tracheides with bordered pits, xylem fibers & xylem parenchyma) in gymnosperms.

Development:

•           In Roots

The development of protoxylem is toward the pericycle & endodermis, the xylem strand is called centripetal or exarch.

•           In Stems

The development of protoxylem is toward the pith, the xylem is cetrifugal or endarch.

•           In hypocotyl of angiospermic seedlings

The development is both centripetal and centrifugal, the xylem is mesarch.

Xylem Vessel Lignification:

Lignification or perforation occurs on one end walls of vessel or on the lateral walls.

The perforated part is called perforated plate which may be simple or multiple.

Types:

    •       Annular            •           Spiral            •  Reticulate      •  Pitted           •Scalariform

Types of Vascular Bundles

A. Radial: Xylem & phyloem lie radially side by side (in roots).

• In dicot roots:                                         • In monocot roots:

2-12 xylem arches.                                     more than 13 arches.

B. Conjoint: Xylem & phloem lie on the same radius (in stems).

       •    Collateral: Phloem lies outwards only.                             

1.         In dicots:                                                     2.  In monocots: 

contain cambium                                                no cambium,

    (Opened)                                                           (Closed).

       •    Bicollateral:  Phloem present on

                                both sides of xylem.

C. Concentric: One type of vascular tissue surrounds the other. (Closed v. bundle).

•           Amphicribral:                                             •   Amphivasal:

Phloem surrounds                                          Xylem surrounds 

     the xylem .                                                     the phloem .

Secretory and Excretory Tissue System

Secretion: is the separation of certain substances from the protoplast.

           •            Secretory cell: the secretion formed is stored within the secretory cell.

           •            Excretory cell: the secretion formed is exuded from the secreting cell.

Nature of secreted substances:

           •            Products utilized by plants (resins, tannins& crystals).

           •            Products with special physiological function (enzymes&hormones).

Types of secretory tissues:

    A. laticeferous tissue: In which latex is found.

           •            Non-articulate latex ducts (latex cells): not fuse together.

           •            Articulate latex ducts (latex vessels): forming a complex network.

    B. Glandular tissue:

        1. External glands: occur on the epidermis.

           •            Glandular epidermal hairs: Glandular trichomes as in dermal system.

           •            Nectarines: in entomophilous plants (insect-pollinated) to attract insects.

        2. Internal glands: embedded in the interior tissues.

      •     Oil glands: contain essential oils (volatile and odoriferous), lysigenous in nature (formed by lyses of the cells forming large cavities of glands).

         •  Resin glands: in gymnosperms forming ducts of schizogenous nature (formed by separation of cells forming cavities with epithelial lining).

        •   Digestive glands: in insectivorous plants secreting protein-digesting enzymes.

         •  Hydathodes: called water stomata or water pores that exudates water under conditions of low transpiration and abundant soil moisture.

Vscular tissue system and secretory & excretory system

Ground tissue system

Ground  or Fundamental tissue System

Consists of the main bulk of plant body and extends from below the epidermis to the center, leaving vascular tissues apart.

Type of cells & tissues of ground systems

1.         Parenchymatous tissue

 Origin:   •       That of ground system develops from ground meristems.

                 •      That of the vascular elements derived from procambium.

Cell wall composition: with primary cell wall of cellulose, hemicellulose and pectic 

                                       compounds.

Functions: •     Play a role in healing of wounds where it can retain the ability to divide.

                   •    Storage.

                   •    Secretion.

Shapes & arrangement of parenchyma:

     •      Polyhedral:- with equal diamerers.            •      Lobed (Plicate) :- with folds.

     •      Armed:- with arms and air spaces.            •      Lacunate:- forming air spaces. 

2.         Chlorenchymatous Tissue

Nature: Parenchyma with chloroplasts.

Location: Characterizes the wings of leaves & herbaceous stems.

Functions:  •    Photosynthesis.

                    •   Storage.

Shapes and types:     •            Palisade & Spongy tissue                     •       Isodiametric.

3.         Collenchymatous Tissues

Origin: Develops from elongated cells rsemble procambium or from isodiametric cells of the ground meristem.

Cell wall composition: with secondary cell wall thickened with Pectin.

Locations:  •    below the epidermis in a cylinder or strips in stems

                     •  on one or both sides of mid-vein or wings in leaves.

Function:    • mechanical support:

Shapes:          1.          Angular                                                        2.            Annular

               Thickening lies on corners.                             Thickening around the cell .

                    3. Lamellar                                                       4.           Lacunar (tubular)

               Thickening in lamellae                                   Thickening in parts facing the

                (on tangential walls).                                                intercellular space.

4. Sclerenchymatous Tissue

Origin: Originate from procambium, cambium, ground meristem or protoderm .

Cell wall composition: With secondary cell wall thickened with lignin.

Functions: Mechanical supporting tissue

Classification of sclerenchyma:

1.         Fibers:- Elongated cells with pointed ends, occur in patches, bands or singly

     a.    Xylary (Wood) fibers: Derived from procmbium,  in between xylem elements.

     b. Extraxylary (Bast) fibers: Occur out side the xylem elements.

    •       Phloem fibers          •    Cortical fibers        •     Pericyclic fibers  •         Bundle sheath fibers

  In secondary phloem. May form cylinder.    Close to phloem.    Around V. bundle

2.         Sclereids:

       Development:  from parenchyma or from  sclereids primordia.

       Locations: Occur in epidermis, ground tissue & vascular tissue.

Shapes: 

1.         Brachysclereids (grit cells, stone cells): Unbranched, with  branched canal pits, in the gritty parts of the  pears.

2.         Macrosclereids (rod cells, palisade–like): Forming the palisade layer of many seeds.

3.         Asterosclereids (stellate cells):

Star shaped, with arms, in some xerophytes.

 4.        Osteosclereids (bone cells):

In hypodermal layers in xerophytic leaves.