Theme № 4

Anatomical structure of vegetative plants organs.

 

There are two plant groups, the Monocots and the Dicots.  The distinction between these two groups is not always clear, but some general trends are outlined below:

 

Primary and second secondary anatomic structure of root.

In vascular plants, the root is the organ of a plant body that typically lies below the surface of the soil. But, this is not always the case, since a root can also be aerial (that is, growing above the ground) or aerating (that is, growing up above the ground or especially above water). On the other hand, a stem normally occurring below ground (see rhizome). So, it is better to define root as a part of a plant body that bears no leaves, and therefore also lacks nodes. There are also important internal structural differences between stems and roots. The two major functions of roots are 1.) absorption of water and inorganic nutrients and 2.) anchoring the plant body to the ground. They often function in storage of food. The roots of most vascular plant species enter into symbiosis with certain fungi to form mycorrhizas, and a large range of other organisms including bacteria.

Root is descending axis of plant, normally below ground. Subapical region of roots (along section) has traditionally been divided into 3 regions: zones of the cellular division, cellular elongation, and cellular maturation. Primary tissues differentiate in or distally to zone of cellular maturation. An epidermis (in root call epiblema) surrounds root, which is usually one cell thick. Epiblema covers the entire root except the root cap and usually lacks stomata. In cross section of a root we can see zones: epiblema, primary bark (cortex) and stele. Primary bark has three layers: 1)exoderma with polyangles thick-walled cells; 2) mezoderma with oval cells and air spaces; 3) endodermis with casparian strip and conductive cells. Stele consists of pericycle – one layer of thick cells and radial vascular bundle.

In monocot roots vascular bundle is polyarch, closed, and radial. In dicot roots with primary structure vascular bundle is closed, radial, and tetraarch.

Dicot roots with secondary vascular bundle structure have opened collateral vascular bundles in stele, which are arranged in circle. This anatomic structure you can see in transition region (placing between root and shoot). Between them there are wide medullary rays which are start in primary xylem and are situated in the center of a root. Dicot roots with secondary nonbundle structure have solid  ring of vascular cambium, solid zone of phloem above it and solid zone of xylem below cambium. In the center there is primary xylem. Primary medullary rays stretch from it, Secondary rays are formed by cambium.

Monocot roots, interestingly, have their vascular bundles arranged in a ring. Dicot roots have their xylem in the center of the root and phloem outside the xylem. A carrot is an example of a dicot root.

Root Anatomy – Monocot Roots

Epidermis

• Dermal tissue

• Protection of the root

Cortex

• Ground tissue

• Storage of photosynthetic products

• Active in the uptake of water and minerals

Endodermis

• cylinder once cell thick that forms a boundary between the cortex and the stele

• even more distinct than dicot counterpart

• contains the casparian strip and conductive cells

Vascular Tissue

• Xylem and Phloem

• Forms a ring near center of plant

Pith

• Root hasn’t pith, in center of root is sclerenchim

Root Anatomy – Dicot Roots

Epidermis

• Dermal tissue

• Protection of the root

Cortex

• Ground tissue

• Storage of photosynthetic products

• Active in the uptake of water and minerals

Endodermis

• cylinder once cell thick that forms a boundary between the cortex and the stele

• contains the casparian strip, and conductive cells

Pericycle

• found just inside of the endodermis

• may become meristematic

• responsible for the formation of lateral roots

Vascular Tissue

• Xylem and Phloem

• Forms an X-shaped pattern in very center of root

In dicot roots, the xylem tissue appears like a 3-pronged or 4-pronged star. The tissue between the prongs of the star is phloem. The central xylem and phloem is surrounded by an endodermis, and the entire central structure is called a stele.

Anatomic structure of stems and rhizomes monocotyledones and dicotyledones.

A stem is one of two main structural axes of a vascular plant. The stem is normally divided into nodes and internodes, the nodes hold buds which grow into one or more leaves, inflorescence (flowers), cones or other stems etc. The internodes act as spaces that distance one node from another. In most plants stems are located above the soil surface but some plants have underground stems.

Stems have four main functions which are:

• Support for and the elevation of leaves, flowers and fruits. The stems keep the leaves in the light and provide a place for the plant to keep its flowers and fruits.

• Transport of fluids between the roots and the shoots in the xylem and phloem.

• Storage of nutrients.

• The production of new living tissue. The normal life span of plant cells is one to three years. Stems have cells called meristems that annually generate new living tissue.

Stem structure

Stem usually consist of three tissues, dermal tissue, ground tissue and vascular tissue. The dermal tissue covers the outer surface of the stem and usually functions to waterproof, protect and control gas exchange. The ground tissue usually consists mainly of parenchyma cells and fills in around the vascular tissue. It sometimes functions in photosynthesis. Vascular tissue provides long distance transport and structural support. Most or all ground tissue may be lost in woody stems.

Monocot stems

Vascular bundles are present throughout the monocot stem, although concentrated towards the outside. This differs from the dicot stem that has a ring of vascular bundles and ofteone in the center. Monocots rarely produce secondary growth and are therefore seldom woody.

Most monocots are herbaceous plants that do no attain great size. Stems have neither a vascular cambium not a cork cambium, and thus produce no secondary vascular tissues or cork. As in herbaceous dicot, surfaces of stem arc covered by an epidermis, but xylem and phloem tissues produced by procambium appear in cross-section as discrete vascular bundles, scattered throughout stem. Type of vascular bundle is closed collateral.

Rhizome of monocot has three zones: dermal, primary bark and stele in cross section. Dermal tissue is epidermis. Primary bark consists of oval crumbly cells of ground parenchyma, cells of endodermis is horseshoe-like and endodermis is doubled (it is a diagnostic feature of lily of the valley). There are two types of  vascular bundles in stele: closed collateral (near pericycle) and concentric centerphloem, which are chaotically arranged in parenchyma. Pith is in the centre.

 

Monocot stems differ from dicot stems in that they lack secondary growth

• No vascular cambium nor cork cambium

• Stems usually uniform in diameter

• Scattered vascular bundles (not in a ring like dicot stems)

Monocot stems, such as corn, palms and bamboos, do not have a vascular cambium and do not exhibit secondary growth by the production of concentric annual rings.

The following illustrations and photos show scattered vascular bundles in the stem cross sections of corn (Zea mays):

 

DICOT STEMS

Dicot stems with primary growth have pith in the center, with vascular bundles forming a distinct ring visible when the stem is viewed in cross section. The outside of the stem is covered with an epidermis, which is covered by a waterproof cuticle. The epidermis also may contain stomata for gas exchange and hairs. A cortex of parenchyma cells lies between the epidermis and vascular bundles. Cortex includes: collenchyma, chlorenchyma and endodermis. Vascular cylinder  includes: opened collateral bundles, arranged in a ring in stele. Between xylem and phloem is cambium in bundles of stem. Pay attention to a large amount of sclerenchyma and on large rays between bundles.

Woody dicots and many nonwoody dicots have secondary growth originating from their lateral or secondary meristems: the vascular cambium and the cork cambium or phellogen. The vascular cambium forms between the xylem and phloem in the vascular bundles and connects to form a continuous cylinder. The vascular cambium cells divide to produce secondary xylem to the inside and secondary phloem to the outside. As the stem increases in diameter due to production of secondary xylem and secondary phloem, the cortex and epidermis are eventually destroyed. Before the cortex is destroyed, a cork cambium develops there. The cork cambium divides to produce waterproof cork cells externally and sometimes phelloderm cells internally. Those three tissues form the periderm, which replaces the epidermis in function. Areas of loosely-packed cells in the periderm that function in gas exchange are called lenticels.

Secondary xylem is commercially important as wood. The seasonal variation in growth from the vascular cambium is what creates yearly tree rings in temperate climates. Tree rings are the basis of dendrochronology, which dates wooden objects and associated artifacts. Dendroclimatology is the use of tree rings as a record of past climates. The aerial stem of an adult tree is called a trunk. The dead, usually darker inner wood of a large diameter trunk is termed the heartwood. The outer, living wood is termed the sapwood.

In general, plants that complete their life cycles within one year (annuals) have green herbaceous stems. Their tissues arelargely primary, although cambium may develop some secondary tissues. Herbaceous dicot stems have discrete patches of xylem and phloem called vascular bundles, which occur in a ring that separates cortex from the pith, although in a few plants xylem and phloem are produced as continuous rings. As previously noted, procambium produced only primary xylem and primary phloem.

Arrangement of primary tissues in woody dicot stems is very similar to that found in herbaceous dicot stems during the early stages of  growth. As soon as the vascular cambium and the cork cambium start functioning, however, obvious differences begin to appear, the most conspicuous of which involve the secondary xylem, or wood.

Rhizome of dicot occur vascular and nonvascular bundle structure

 

 

 

Herbacous Dicot Stem Cross Sections

 

WOODY DICOTS

The “bark” of the tree consists of the periderm + the phloem

 

 

Gymnosperm stems

All gymnosperms are woody plants. Their stems are similar in structure to woody dicots except that most gymnosperms produce only tracheids in their xylem, not the vessels found in dicots. Gymnosperm wood also often contains resin ducts. Woody dicots are called hardwoods, e.g. oak, maple and walnut. In contrast, softwoods are gymnosperms, such as pine, spruce and fir.

 

 

A RHIZOME is underground modification of stem, which is formed  at perennially  herbacous  plants for an accumulation  nutritives and vegetative reproduction. Has the well formed stocking parenchime; mechanical and vascular tissues are expressed poorly. In medicine use rhizome of onion, fern, valerian and other.

 

Anatomic structure of leaves

 

Leaf – lateral vegetative organ of plants, the general function its are photosynthesis, transpiration

A generalized plant cell type, parenchyma cells are alive at maturity. They function in storage, photosynthesis, and as the bulk of ground and vascular tissues. Palisade parenchyma cells are elogated cells located in many leaves just below the epidermal tissue. Spongy mesophyll cells occur below the one or two layers of palisade cells. Parenchyma cells also occur withinthe xylem and phloem of vascular bundles.

If a typical leaf is cut transversely and examined with the aid of microscope, three regions stand out: epidermis, mesophyll, and veins. Epidermis is a single layer of cells covering the entire surface of leaf. Epidermis on lower surface of blade can sometimes be distinguished from upper epidermis by presence of stomata. The epidermis also secretes a waxy substance called the cuticle. These layers protect the leaf from insects, bacteria, and other pests.

The mesophyll (“middle leaf’) includes the major photosynthetic tissues: palisade parenchyma (is found only in dicot beneath the upper epidermis, may be one or more cell layers thick, depending on plant, packed with chloroplasts; is a primary site of PSN in dicot); spongy parenchyma (is found in both monocot and dicot, in dicot, will be found below the palisade layer; in both, forms large air spaces for “storage” for carbon dioxide, oxygen and large surface area for absorption of carbon dioxide into cells, they also contain chloroplasts).

In the center we can observe veins. The veins contain vascular tissues. Veins support the leaf and are filled with vessels that transport food, water, and minerals to the plant.

Anatomical structures of leaves are of several types: isolateral (common for monocot and dicot plants); bifacial (common for dicot); radial (common for gymnosperm).

A cross-section of a pine leaf sample is composed of many cells. Transparent cells are in pith, xylem, and endodermis. Green cells compose mesophyll tissues. Cross-sections of stomata are only found in the edges, or epidermis. Resin canals are found on the surface of the mesophyll tissues. The resin canals function as water pipes for leaf.

 

http://www.enchantedlearning.com/subjects/plants/leaf/

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Leaf Structure:
Most food production takes place in elongated cells called palisade mesophyll. Gas exchange occurs in the air spaces between the oddly-shaped cells of the spongy mesophyll.