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subterranean plant organs intro

The document discusses subterranean organs in plants, specifically focusing on subterranean stems and roots, their characteristics, and differences. It highlights types of subterranean stems such as rhizomes, stem tubers, corms, and bulbs, as well as types of roots including primary, secondary, adventitious, and storage roots. The histological structure of these organs is examined, along with anomalous structures that can arise in specific species.

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SUBTERRANEAN ORGANS By Dr. Mostafa Mahmoud Hegazy ( PH.D.)
SUBTERRANEAN ORGANS • These include the following: • 1-Subterranean stems: including rhizomes and others which are related to rhizomes such as corms, stem tubers and bulbs. • 2- Roots: including roots and root tubers. There is no clear division between rhizomes and roots in a commercial sense.
Subterranean organs characters and function • Subterranean organs used in medicine are usually collected from perennial plants. • where they serve as storage organs and thus generally they are full of reserve food materials. • They are usually rich in carbohydrates such as starch (e.g. Filix mas, Rhubarb, and Ipecacuanha); sugars (e.g. Gentian and Squill); inulin (e.g. family Compositae). • they are devoid of chlorophyll. • Underground organs a means of perennation (continuation) of the plant.
SUBTERRANEAN ORGANS A. Subterranean Stem ● The members of this group are characterized Externally by: Bearing: 1-scale leaves, 2-axillary buds, 3-adventitious roots. And 4-Having a growing point protected by scale leaves forming a "terminal" bud which make aerial shoots (not by a cap as in roots). ● Internally - Never show a central solid mass of xylem. - In dicotyledons they have a parenchymatous central pith.
1. Rhizome: • Def.: - A rhizome is an “underground stem with nodes & internodes, terminal buds & aerial shoots.” • The internodes are either long (e.g. American podophyllum) or short (e.g. Indian podophyllum). • Mode of growth: • - Rhizomes usually, grow horizontally, sometimes vertically. Occasionally obliquely. • Mode of branching: • - May be branched, but mostly they branch sympodially as in Ginger and occasionally they branch monopodially. • Monopodial: Main bud "axis" continues growing & branches are shorter than main axis. It has one growing point. • - Sympodial: Destruction of the apical bud forming other things as flowers, then its growth will stop & branches continue growing. It has more than one growing point.
- Types of rhizomes: I. Sobole II. Sucker III. Stolon - long, thin, tough - rapid in growth - with long internodes - e.g. couch grass. -Branch growing under the ground, arising from the stem (as in menthe) or from the top of the root (as in rosa) and giving off roots and aerial shoots at the nodes. horizontal stem that produces rooted plantlets at its nodes and ends, forming near the surface of the ground. It is similar in function to a runner but is much thicker.
• 2. Stem Tuber: • - “A swollen underground stem or part of stem, usually its tip” • - It differs from root tuber in: bearing several small scales and buds forming sometimes what is called eyes, as well as terminal bud on the free end (as in potato). • - It is full of food material and serves in vegetative propagation. • 3. Corm • - “A shortened swollen erect underground stem” • - Covered with brown scale leaves arising at the nodes • - Having usually a large apical bud and small axillary ones (as in colcasia & colchicum). • 4. Bulb • - An underground stem which is reduced to a small flattened disc & is crowned with crowded fleshy scale leaves or scale leaf bases and having adventitious roots arising from the base of the bulb. • - The bulb may be a scaly bulb (imbricate bulb) or a tunicated bulb.
Histology of Subterranean Stem: They generally resemble in structure the aerial stems, although they are either devoid of or containing only very little sclerenchymatous tissue. Dicotyledons Monocotyledons However, the rhizomes and other underground stems show: • Large pith • Wide cortex, formed mostly of thin-walled parenchyma packed with reserve materials. • Endodermis “the inner most layer of cortex" is usually indistinguishable with a few exceptions as the Valerian. • The vascular bundles, are open (contain cambium), generally remain widely separated by wide medullary rays, and form a single ring around the pith. • In some cases as in subterranean stem of liquorice and ipecacuanha, the secondary vascular elements are produced in great amount forming a continuous radiate ring around the pith. The structure differs from that of the aerial stem: • Distinct endodermis separating a clear cortex from the central stele. • The vascular bundles are closed (no cambium) and usually numerous, being scattered in both the cortex and the stele but generally more crowded & smaller within endodermis & larger & fewer at the center.
The tegumentary tissue (outer covering) (protective tissue), either consisting of: Cork Metaderm Epidermis Few regular rows of cork cells (from 2ry thickening), with or without a rhytidoma as in liquorice. Irregularly arranged suberised cells derived from the outer rows of the cortex (1ry origin) forming the metaderm as in white and black hellebore. as in couch grass, colchicum corm, and podophyllum.  Secretory structure are frequently present e g. secretory ducts in Arnica, secretion cells in ginger, laticiferous vessels in dadelion rhizome etc.
B. Roots • Def. : • The root is the part of the plant axis, which grows vertically downwards in seedlings, into the soil and away from light. • It does not show nodes and internodes and does not bear leaves or buds. Its growing point is covered by a special tissue called root-cap or calyptra. • The root bears branches, which are similar in structure and in general characters to the main root. The branching of the root is usually lateral and the lateral branches are thinner and less strong than the parent root. • The root usually consists of four zones: • The growing point, covered by the root cap. • The zone of elongation, where the increase in the length of the root takes place. • The zone of absorption or zone of root hairs. • The zone of lateral roots, which constitutes the majority of the root.
Types of roots: 1-Primary roots: They develop into taproots e.g. Senega. 2-Secondary roots: They are the lateral roots e.g. Krameria. 3-Adventitious roots: They usually arise from the stem e.g. Ipecacuanha and Jalap. 4-Storage roots: They occur when the roots of any of the previously mentioned types become swollen with reserve food materials. • Those, which are very swollen e.g. Jalap, Aconite and Carrot are called root tubers or tuberous or tuberculated roots.
Histology of dicotyledon Roots Young root 1- a piliferous layer 2- cortex: parenchymatous Exodermis the outer row of which is sometimes specially differentiated, being usually formed of small cells with thickened more or less suberised, occasionally lignified walls the endodermis The innermost layer of cortex, generally well marked and has either a strongly developed casparian strip or some special form of thickening. Shows a much wider cortex than the stele. 3- pericycle: surround the stele, Pericylce of young root "which is one row of parenchyma cells" is very important because it will replaced with phellogen. 4- xylem and phloem arranged in a circle, alternating in position so that each lies on a different radius. {Radially arranged} There is no cambium at all in very young root or pith due to secondary thickening Old root (Gymnosperms &Dicots.) 1-Cork 2-Phelloderm, 3-Collapsed primary, 4-phloem, 5-Secondary phloem 6-Cambium 7-Secondary xylem 8-Primary xylem groups in the center NO Central Pith, the monocot usually retain their primary structure
Main differences between roots and subterranean stems: Root Subterranean Stems Morphology: 1-Never develop leaves 2-Shows no nodes or internodes 3-Bears no buds 4-Growing point (calyptra) covered by special tissue called root-cap 5-wrinkles: mainly longitudinal 1-Bears scale leaves 2-Have nodes and internodes 3-Have axillary buds 4-Growing point protected by scale leaves 5-wrinkles: mainly transverse Histologically: 1-Radial vascular bundles (V.B.), masses of phloem and xylem alternating with one another 2-Xylem: exarch 3-No central pith 1-Open Collateral V.B. (Gymnosperms & Dicots.) OR Closed V.B. (Monocots.) 2-Xylem: endarch 3-Shows central pith
Anomalous structure in subterranean organs Many variations in secondary thickening from the normal type of structure may exist in subterranean organs described as anomalous : 1- In subterranean stems: - In Rhubarb "Its Rhizomes" : abnormal bundles are produced in the pith in the form of radiating structures called star spots, consisting of a central dark area of collapsed perimedually phloem surrounded by an abnormally developed cambium which gives phloem to the inside and xylem to the outside with radiating slightly curved brown medullary rays.
Roots 1. In Aconite: the cambium retains the stellate form and only little secondary xylem, but very wide phloem, consisting mostly of parenchyma with little amount of sieve tissues are produced. It also shows wide medullary rays and well developed parenchymatous pith "Parenchyma have stellate form". The tegumentary tissue in Aconite is not formed of cork cells, but of irregularly arranged suberised cells derived from the outer layers of the cortex forming the metaderm. It has narrow cortex and wide stele. 2. In senega, irregularly wedged wood are present due to the formation of exceptionally wide one rarely two or more parenchymatous medullary rays, while the secondary phloem is abnormally developed at one place producing an external ridge, the keel.
3.A- In Jalap, tertiary cambia in the form of circles, curves or concentric rings, producing only parenchyma on both sides, are developed in the wide parenchymatous xylem and enclosing a few of the xylem vessels. 3.B-In Orizaba Jalap: The original cambium is replaced and the root increases in size by successive cambia of a limited period of activity developed in the pericycle, producing vascular bundles in rings. 4- In Belladonna and Gentian: The xylem is well developed and constitutes the main part of the root, but is formed mostly of parenchyma with scattered groups of vessels. Several abnormally developed islands of interxyllary phloem are also present.
5. In Dandelion, the xylem is small, central surrounded by unusually developed phloem, formed of concentric rings of sieve tissue and laticiferous vessels alternating with parenchyma. 6. In ipecacuanha, the xylem is small and dense (non-porous), the phloem is weakly developed as a narrow wedged ring around the xylem, and a very wide starchy parenchymatous phelloderm, constituting the secondary cortex, is present.