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Tree roots
The first structures to appear from a germinating seed in the form of a radicle are the tree roots. It is geotropic and goes deep down into the soil in the form of tap root. The tree roots are fixing the plant with the substratum and absorbing water and nutrients from the soil.
Definition of tree roots (plant roots): –
The tree roots are the descending axis of the plants that develops from the radical, reforming the function of absorption of water and nutrients and fixation of the plant to the substratum.
Characteristics of tree roots: –
i) The root is the descending axis of the plant developing from the radicle.
ii) The tree roots are positively geotropic and hydrotropic and negatively phototropic in nature.
iii) They are colourless or non-green, due to absence of chloroplast.
iv) They contain leucoplast which helps in storage of food.
v) The tree roots never contain leaves, buds and flowers.
vi) They are undifferentiated into nodes and Internodes.
vii) Two types of tree roots are there are, those are tap roots and adventitious roots.
viii) Tree roots are differentiated into root-cap, growing region, root hair zone and the permanent zone.
ix) The root caps of the tree root protect the root apices.
x) The meristematic cells contain by the zone of elongation and it helps in the growth of root.
xi) The unicellular, exogenous tubular projection of epiblema of the root hair zone called root hairs help in the absorption of water.
xii) The permanent tissue contains by the permanent region is not in the active state of division.
xiii) The normal function of the tree roots are attachment, absorption and storage.
xiv) The special function of the tree roots include assimilation, absorption of air, supporting mechanically, providing epiphytic or parasitic adjustment.
Types of tree roots: –
The tree roots may be classified into two major types on the basis of the origin of roots, those are (1) true roots and (2) adventitious roots.
What are the true roots?
Tree roots which develop directly from the radicle or its branches are known as true roots. Sometimes it may say as normal roots. From the primary root the radicle are going deep into the soil in dicot plants, the primary roots give rise to lateral branches known as secondary roots and they also give rise to the tertiary roots .
What are the adventitious roots?
Tree roots which do not develop from the radicle, but it developed from other parts of the tree are known as adventitious roots. In such case the radicle degenerates at an early age and is replaced by the tuft of adventitious roots. The adventitious roots are of three types. Those are the Couline roots, fibrous roots and foliar roots.
What is Cauline roots?
The Adventitious tree roots that develop solitarily or in groups either from the internodes of stems are called true adventitious roots or in the other name the cauline roots. Example : Banyan, Hydrocotyle , etc.
What are Fibrous roots?
A bunch of fibre-like tree roots, that develop from the base of the plumule after the death of primary roots are called fibrous roots. Example: Grasses. The primary tap roots developing from the radicle dies at early stage of development in the monocot plants. In this case, the primary roots developing from the base of the radicle and it is called seminal root.In case of wheat, paddy, etc., these seminal roots die off with the development of the plant and is replaced by the bunch of fibrous roots.
What are foliar roots?
The adventitious tree roots that develop from the various parts of leaves like leaf margin and petiole are called foliar roots. Example: Bignonia.
Uptake of Water and Minerals in Roots
In the first section of this tutorial, we looked at the structure of the dicotyledonous root and stem and compared the different cells in the specialised tissues of the plant root and stem. Now we will look at how these specialised cells help the plant to absorb water from the soil and transport it to the stem, where it can then be transported to the rest of the plant.
Movement of water through the dicotyledonous root
Water is found in the spaces between the soil particles. Water and mineral salts first enter through the cell wall and cell membrane of the root hair cell by osmosis. Root hair cells are outgrowths at the tips of plants’ roots (see figure below). They function solely to take up water and mineral salts. Root hair cells do not perform photosynthesis, and do not contain chloroplasts as they are underground and not exposed to sunlight. These cells have large vacuoles which allow storage of water and mineral salts. Their small diameter (5-17 micrometres) and greater length (1500 micrometres) ensure they have a large surface area over which to absorb water and mineral salts. Water fills the vacuole of the root hair cell.
The following list summarises how the root hair is adapted to absorb water from the soil:
- There are many, elongated root hairs to increase the total root surface area for water absorption.
- They have thin walls to speed up the intake of water by osmosis.
- They have large vacuoles to absorb water quickly and transport it to the next cells.
- The vacuoles have salts, which speed up water absorption from soil water.
- Root hairs do not have cuticles, as this would prevent water absorption.
Water can now move from the root hair cells and across the parenchyma cells of the cortex in two major ways. Some water passes through the cells by osmosis. Most water travels either in, or between the cell walls (of the parenchyma cells) by simple diffusion. The water must pass through the endodermis to enter the xylem. Once water is in the xylem of the root, it will pass up the xylem of the stem.
ROOTS AND WATER
Drought is the major limitation to the growth of crops and the distribution of natural plant communities globally, and several papers address the physics and physiology of water uptake by roots. Zarebanadkouki et al. (2016) remind us how little is known about basic aspects of water uptake. Hydraulic conductance is a critical parameter for understanding water uptake by roots, yet few models take into account the variation in conductance that is likely to occur along roots and radially within roots. This uncertainty was examined by mapping the spatial variation of water uptake in lupin (Lupinus albus) roots. Using neutron radiography the authors demonstrate that water uptake varies significantly between different types of roots as well as along roots. This information is useful for developing realistic models for water uptake from soil. Pioneering work by Gardner (1960) considered the moisture gradients in soil immediately adjacent to roots, but a clear understanding of the behaviour of water at that interface is still lacking. This topic was addressed by Carminati et al. (2016), who developed a biophysical model to explain the changes that occur in the xylem water potential as the soil dries. Their model provides a framework for estimating the water potential across the rhizosphere and for predicting moisture levels directly adjacent to the root surface.
Root hydraulic conductivity in a drying soil was also investigated experimentally by Henry et al. (2016) using drought-susceptible and drought-tolerant cultivars of rice (Oryza sativa). Bleeding rates (sap exuded from a cut stem) were compared with vapour pressure deficit and transpiration rates during diurnal and seasonal cycles. They show how changes in these environmental factors affect root hydraulic conductivity and suggest a link between suberin content of roots and drought tolerance. Meng et al. (2016) investigated how rice plants balance the demand for water from the shoots with the supply of water by the roots. They show that changes in water demand alter the expression of aquaporins in the roots. Removing the shoots rapidly decreased the hydraulic conductance of the roots and decreased transcript levels of six aquaporin genes. These responses were reduced if xylem tension was maintained after excision by quickly applying a vacuum to the cut stems. The authors propose that xylem water tension acts as a signal between shoots and roots to coordinate water supply with demand. Aquaporin function was investigated in barley (Hordeum vulgare) roots by Sharipova et al. (2016) with an immunochemical approach. They applied abscisic acid (ABA) treatments to wild-type plants and an ABA-deficient mutant and successfully linked local ABA concentrations with the abundance of aquaporins and changes in root hydraulic conductivity.
Many fundamental questions concerning how water moves from the soil to the shoots remain to be answered. The studies mentioned above share the central objective of understanding how water movement is controlled by roots, but the scale of the investigations varies widely, from analysis of the root–soil interface and specific transporter proteins on root-cell membranes, to total hydraulic conductivity. In situ measurements using tracers showed fine spatial control of water flow along roots that was influenced by age and soil moisture. This likely reflects further levels of regulation. These studies confirm roles for hormones, protein transporters and cell-wall structure in water uptake, indicating the necessity for complex signals to coordinate these factors.
Root Function
The roots may have two types of functions, those are, normal root function and special root function.
Normal root function:
The normal root function includes the mechanical root functions and physiological root functions.
What is the normal mechanical root function?
The mechanical root function includes the attachment and fixation of the plants to the soil and they form a close network to root system which forms a close association with these soil particles.
What is the normal physiological root function?
The absorption of water and mineral nutrients by active and passive means are the most important physiological functions of the root. Conduction is another physiological function. The absorb water and nutrients are conducted through the xylem elements of the root to different parts of the plant body. The trans-location of the prepared food takes place through the phloem elements from the source of leaves to this sink or root. And the food storage is also a physiological root function. The food prepared which is excess is stored in the tap root which may be swollen as in conical of carrot. The excess of starch is stored as storage starch or fat. Due to the presence of this stored food the structure become very useful organs which surpass the unfavorable condition and germinate produce new plants.
Special root function: –
This special root function may also be divided into two types those are mechanical function and physiological function. Now we have some discussion:
What is the special mechanical root function?
The prop roots support like a pillar to the heavy branches specially which are more horizontal in nature such as banyan tree. The plants which have the week stem and bend towards the soil and hence supported by the stilt roots from one site of the leaning stem such as screw pine.
The climbing roots develop from the notes of creepers and help it climbed around a solid support to words the light, such as betel. Some roots contract from one point and develop in some other area keep the stem idiot and those are called contractile roots, such as onion. There have other types of root which are called floating root. These roots contain parenchyma and provide buoyancy to floating hydrophytes, such as the Jussiaea.
What is the special physiological root function?
One of the special physiological root functions is the storage root. This type of root such as sugar beet, radish and sweet potato are helping in storage of food. The assimilatory roots contain chlorophyll and it helped to prepare food by photosynthesis. Tinospora is the example of such root. There is also having the breathing roots. These roots exhibit positive phototropic movement to absorb air for respiration as these soil in which they grow, are inadequately aerated.
Stem & Root Anatomy
V ascular plants contain two main types of conduction tissue, the xylem and phloem. These two tissues extend from the leaves to the roots, and are vital conduits for water and nutrient transport. In a sense, they are to plants what veins and arteries are to animals. The structure of xylem and phloem tissue depends on whether the plant is a flowering plant (including dicots and monocots) or a gymnosperm (polycots). The terms dicot, monocot and polycot are summarized in the following table.
Flower parts in 3's or multiple of 3's one cotyledon inside seed parallel leaf venation includes Lilium , Amaryllis , Iris , Agave , Yucca , orchids, duckweeds, annual grasses, bamboos and palms.
Flower parts in 4's or 5's 2 cotyledons inside seed branched or net leaf venation contains the most species of flowering herbs, shrubs and trees includes roses ( Rosa ), buttercups ( Ranunculus ), clover ( Trifolium ), maple ( Acer ), basswood ( Tilia ), oak ( Quercus ), willow ( Salix ), kapok ( Ceiba ) and many more species.
Gymnosperms include pines ( Pinus ), spruce ( Picea ), fir ( Abies ), hemlock ( Tsuga ) and false hemlock ( Pseudotsuga ). Some of the coniferous genera (division Coniferophyta) are the most important timber trees in the world. Since these species have several cotyledons inside their seeds, they are conveniently referred to as polycots.
X ylem and phloem tissues are produced by meristematic cambium cells located in a layer just inside the bark of trees and shrubs. In dicot stems, the cambium layer gives rise to phloem cells on the outside and xylem cells on the inside. All the tissue from the cambium layer outward is considered bark, while all the tissue inside the cambium layer to the center of the tree is wood. Xylem tissue conducts water and mineral nutrients from the soil upward in plant roots and stems. It is composed of elongate cells with pointed ends called tracheids, and shorter, wider cells called vessel elements. The walls of these cells are heavily lignified, with openings in the walls called pits. Tracheids and vessels become hollow, water-conducting pipelines after the cells are dead and their contents (protoplasm) has disintegrated. The xylem of flowering plants also contains numerous fibers, elongate cells with tapering ends and very thick walls. Dense masses of fiber cells is one of the primary reasons why angiosperms have harder and heavier wood than gymnosperms. This is especially true of the "ironwoods" with wood that actually sinks in water.
A recent article in Science Vol. 291 (26 January 2001) by N.M. Holbrook, M. Zwieniecki and P. Melcher suggests that xylem cells may be more than inert tubes. They appear to be a very sophisticated system for regulating and conducting water to specific areas of the plant that need water the most. This preferential water conduction involves the direction and redirection of water molecules through openings (pores) in adjacent cell walls called pits. The pits are lined with a pit membrane composed of cellulose and pectins. According to the researchers, this control of water movement may involve pectin hydrogels which serve to glue adjacent cell walls together. One of the properties of polysaccharide hydrogels is to swell or shrink due to imbibition. "When pectins swell, pores in the membranes are squeezed, slowing water flow to a trickle. But when pectins shrink, the pores can open wide, and water flushes across the xylem membrane toward thirsty leaves above." This remarkable control of water movement may allow the plant respond to drought conditions.
S piral thickenings in the secondary walls of vessels and tracheids gives them the appearance of microscopic coils under high magnification with a light microscope.
| Magnified horizontal view (400x) of an inner perianth segment of a Brodiaea species in San Marcos showing a primary vascular bundle composed of several strands of vessels. The strands consist of vessels with spirally thickened walls that appear like minute coiled springs. Although this species has been called B. jolonensis by San Diego botanists for decades, it appears to be more similar to B. terrestris ssp. kernensis . This species contains at least 3 strands of vessels per bundle, while B. jolonensis only has one strand per bundle. |
T he water-conducting xylem tissue in plant stems is actually composed of dead cells. In fact, wood is essentially dead xylem cells that have dried out. The dead tissue is hard and dense because of lignin in the thickened secondary cell walls. Lignin is a complex phenolic polymer that produces the hardness, density and brown color of wood. Cactus stems are composed of soft, water-storage parenchyma tissue that decomposes when the plant dies. The woody (lignified) vascular tissue provides support and is often visible in dead cactus stems.
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| Left: Giant saguaro ( Carnegiea gigantea ) in northern Sonora, Mexico. The weight of this large cactus is largely due to water storage tissue in the stems. Right: A dead saguaro showing the woody (lignified) vascular strands that provide support for the massive stems. |
P hloem tissue conducts carbohydrates manufactured in the leaves downward in plant stems. It is composed of sieve tubes (sieve tube elements) and companion cells. The perforated end wall of a sieve tube is called a sieve plate. Thick-walled fiber cells are also associated with phloem tissue.
I n 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.
| Microscopic view of the root of a buttercup ( Ranunculus ) showing the central stele and 4-pronged xylem. The large, water-conducting cells in the xylem are vessels. [Magnified Approximately 400X.] |
I n dicot stems, the xylem tissue is produced on the inside of the cambium layer. Phloem tissue is produced on the outside of the cambium. The phloem of some stems also contains thick-walled, elongate fiber cells which are called bast fibers. Bast fibers in stems of the flax plant ( Linum usitatissimum ) are the source of linen textile fibers. Gymnosperms generally do not have vessels, so the wood is composed essentially of tracheids. The notable exception to this are members of the gymnosperm division Gnetophyta which do have vessels. This remarkable division includes Ephedra (Mormon tea), Gnetum , and the amazing Welwitschia of Africa's Namib Desert.
P ine stems also contain bands of cells called rays and scattered resin ducts. Rays and resin ducts are also present in flowering plants. In fact, the insidious poison oak allergen called urushiol is produced inside resin ducts. Wood rays extend outwardly in a stem cross section like the spokes of a wheel. The rays are composed of thin-walled parenchyma cells which disintegrate after the wood dries. This is why wood with prominent rays often splits along the rays. In pines, the spring tracheids are larger than the summer tracheids. Because the summer tracheids are smaller and more dense, they appear as dark bands in a cross section of a log. Each concentric band of spring and summer tracheids is called an annual ring. By counting the rings (dark bands of summer xylem in pine wood), the age of a tree can be determined. Other data, such as fire and climatic data, can be determined by the appearance and spacing of the rings. Some of the oldest bristlecone pines ( Pinus longaeva ) in the White Mountains of eastern California have more than 4,000 rings. Annual rings and rays produce the characteristic grain of the wood, depending on how the boards are cut at the saw mill.
| Microscopic view of a 3-year-old pine stem ( Pinus ) showing resin ducts, rays and three years of xylem growth (annual rings). [Magnified Approximately 200X.] |
| A cross section of loblolly pine wood ( Pinus taeda ) showing 18 dark bands of summer xylem (annual rings). |
A ngiosperms typically have both tracheids and vessels. In ring-porous wood, such as oak and basswood, the spring vessels are much larger and more porous than the smaller, summer tracheids. This difference in cell size and density produces the conspicuous, concentric annual rings in these woods. Because of the density of the wood, angiosperms are considered hardwoods, while gymnosperms, such as pine and fir, are considered softwoods.
T he following illustrations and photos show American basswood ( Tilia americana ), a typical ring-porous hardwood of the eastern United States:
| A cross section of the stem of basswood ( Tilia americana ) showing large pith, numerous rays, and three distinct annual rings. [Magnified Approximately 75X.] |
| A cross section of the stem of basswood ( Tilia americana ) showing pith, numerous rays, and three distinct annual rings. The large spring xylem cells are vessels. [Magnified Approximately 200X.] |
In the tropical rain forest, relatively few species of trees, such as teak, have visible annual rings. The difference between wet and dry seasons for most trees is too subtle to make noticeable differences in the cell size and density between wet and dry seasonal growth. According to Pascale Poussart, geochemist at Princeton University, tropical hardwoods have "invisible rings." She and her colleagues studied the apparently ringless tree ( Miliusa velutina ) of Thailand. Their team used X-ray beams at the Brookhaven National Synchrotron Light Source to look at calcium taken up by cells during the growing season. There is clearly a difference between the calcium content of wood during the wet and dry seasons that compares favorably with carbon isotope measurements. The calcium record can be determined in one afternoon at the synchrotron lab compared with four months in an isotope lab.
M onocot 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. They cannot increase in girth by adding lateral layers of cells as in conifers and woody dicots. Instead, they have scattered vascular bundles composed of xylem and phloem tissue. Each bundle is surrounded by a ring of cells called a bundle sheath. The structural strength and hardness of woody monocots is due to clusters of heavily lignified tracheids and fibers associated with the vascular bundles. The following illustrations and photos show scattered vascular bundles in the stem cross sections of corn ( Zea mays ):
| A cross section of the stem of corn ( Zea mays ) showing parenchyma tissue and scattered vascular bundles. The large cells in the vascular bundles are vessels. [Magnified Approximately 250X.] |
U nlike most monocots, palm stems can grow in girth by an increase in the number of parenchyma cells and vascular bundles. This primary growth is due to a region of actively dividing meristematic cells called the "primary thickening meristem" that surrounds the apical meristem at the tip of a stem. In woody monocots this meristematic region extends down the periphery of the stem where it is called the "secondary thickening meristem." New vascular bundles and parenchyma tissue are added as the stem grows in diameter.
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| The massive trunk of this Chilean wine palm ( Jubaea chilensis ) has grown in girth due to the production of new vascular bundles from the primary and secondary thickening meristems. |
T he scattered vascular bundles containing large (porous) vessels are very conspicuous in palm wood. In fact, the vascular bundles are also preserved in petrified palm.
| Cross section of the trunk of the native California fan palm ( Washingtonia filifera ) showing scattered vascular bundles. The large cells (pores) in the vascular bundles are vessels. |
| The trunk of a California fan palm ( Washingtonia filifera ) in Palm Canyon, Anza-Borrego State Park. The palm was washed down the steep canyon during the flash flood of September 2004. The fibrous strands are vascular bundles composed of lignified cells. |
| Right: Cross section of the trunk of a California fan palm ( Washingtonia filifera ) showing scattered vascular bundles that appear like dark brown dots. The dot pattern also shows up in the petrified Washingtonia palm (left). The pores in the petrified palm wood are the remains of vessels. The large, circular tunnel in the palm wood (right) is caused by the larva of the bizarre palm-boring beetle ( Dinapate wrightii ) shown at bottom of photo. An adult beetle is shown in the next photo. |
| A beautiful cutting board made from numerous flattened strips of bamboo ( Phyllostachys pubescens ) glued together. Through a specialized heating process, the natural sugar in the wood is caramelized to produce the honey color. Vascular bundles typical of a woody monocot are clearly visible on the smooth cross section. The transverse surface of numerous lignified tracheids and fibers is actually harder than maple.
A 270 Million-Year-Old Petrified Tree Fern D uring the Carboniferous Era, approximately 300 million years ago, the earth was dominated by extensive forests of giant lycopods (division Lycophyta), horestails (division Sphenophyta) and tree ferns (division Pterophyta). Much of the earth's coal reserves originated from massive deposits of carbonized plants from this era. Petrified trunks from Brazil reveal cellular details of an extinct tree fern ( Psaronius brasiliensis ) that lived about 270 million years ago, before the age of dinosaurs. The petrified stem of Psaronius does not have concentric growth rings typical of conifers and dicot angiosperms. Instead, it has a central stele composed of numerous arcs that represent the vascular bundles of xylem tissue. Surrounding the stem are the bases of leaves. In life, Psaronius probably resembled the present-day Cyathea tree ferns of New Zealand. Types of Water Absorption in PlantsPlants typically absorb water by the following two methods: Active Absorption of WaterThis type of water absorption requires the expenditure of metabolic energy by the root cells to perform the metabolic activity like respiration. Active absorption in plant occurs in two ways, namely osmotic and non-osmotic absorption of water.
Passive Absorption of WaterThis type of water absorption does not require the use of metabolic energy. The absorption occurs by metabolic activity like transpiration. Passive absorption is the type where the water absorption is through the transpiration pull. This creates tension or force that helps in the movement of water upwards into the xylem sap. Higher is the transpiration rate, and higher is the absorption of water. Role of Root Hairs in Water Absorption
A root contains some tubular, hair-like and unicellular structures called Root hairs. In the root system, the region from which the root hairs protrude out is termed as Root hair zone. The zone of root hair is the only region that participates in water absorption activity. Root hair zone is the water-permeable region. Root hairs are the outgrowths, which arise from the epidermal layer called the piliferous layer.
Root cells, nucleus, and vacuole or cell sap are present inside the cytoplasmic membrane. Soil aggregates contain small droplets of water carried away by the root hairs into the root xylem through different mechanisms, out of which osmosis is most common. 7: Roots and the Movement of Water - Root structure and anatomy - BiologyTHE MORPHOLOGY AND ANATOMY OF A FLOWERING PLANT The following web page represents a copy of my notes that formed the basis of lectures given during the first portion of the Biology of Plants (BOT 1103) lecture course. Please refer to your own notes, handouts, and to the textbook ( Stern, K., R., J. E. Bidlack, and S. H. Jansky. 2008. Introductory Plant Biology, McGraw-Hill. 616 pp. - reading assignments are in the syllabus) for additional information. This web page does not include information found in various handouts and related materials (e.g., films, charts, overhead projections, etc.) that you will receive during the course of the semester. You will be evaluated over this information as well. If you note any errors in the following document, I'd appreciate it if you would bring this to my attention. Email address: [email protected] ROOTS: Organization and Anatomy
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