NEET Biology Anatomy of Flowering Plants Important Questions Set 01

These Class 11 NEET Biology important questions on Anatomy of Flowering Plants give 45 questions with step-by-step answers, arranged as very short, short and long answer types, as asked in school and board examinations. They help NEET aspirants revise the definitions, structures, differences and key terms of the chapter.

Very Short Answer Type Questions

Question 1. What is the name given to the specialised regions of active cell division in the plants?
Answer: Meristems

Question 2. In which two forms sclerenchyma exists in the body of plants?

Answer:

(i) Sclerenchyma fibres

(ii) Sclereids

Question 3. What is the location of protoxylem in a dicot stem?
Answer: Protoxylem lies towards the centre (pith).

Question 4. Which phloem element closely associates with the sieve tube elements?
Answer: Companion cells

Question 5. Define epidermis.
Answer: Epidermis is the outermost layer of the primary plant body.

Question 6. Which type of arrangement of vascular bundles is seen in the roots of angiosperms?
Answer: Radial arrangement

Question 7. Name the material deposited on the radial and tangential walls of endodermal cells.
Answer: Suberin

Question 8. Which type of growth takes place in the plants with the help of lateral meristems?
Answer: Secondary growth

Question 9. Give one function of the heartwood.
Answer: It gives mechanical support to the stem.

Question 10. Which type of cells are found in the secondary cortex?
Answer: Parenchymatous cells

Short Answer Type Questions

Question 11. What are the different types of meristems found in flowering plants w.r.t. position? Name them and describe any one of them.

Answer:

(i) Apical meristems

(ii) Intercalary meristems

(iii) Lateral meristems

The meristems which occur at the tips of roots and shoots and produce primary tissues are called apical meristems, e.g., root apical meristem which occupies the tip of a root and is responsible for the increase in length of the root.

Question 12. Give a short description of the parenchymatous cells.
Answer: The cells of the parenchyma are generally isodiametric. They may be spherical, oval, round, polygonal or elongated in shape. Their walls are thin and made up of cellulose. They may either be closely packed or have small intercellular spaces.

Question 13. What do you mean by sclereids?
Answer: The sclereids are spherical, oval or cylindrical, highly thickened dead cells with very narrow cavities (lumen). Their cell walls have lignin. These are commonly found in the fruit walls of nuts; pulp of fruits like guava, pear and sapota; seed coats of legumes and leaves of tea, etc.

Question 14. What are the main water transporting elements in flowering plants?
Answer: Tracheids and vessels are the main water transporting elements in the flowering plants. Both are the components of xylem, the conducting tissue of plants. While tracheids are the elongated cells, vessels are the tube-like structures made up by fusion of many cells called vessel members.

Question 15. What do you understand by protoxylem and protophloem?
Answer: The first formed primary xylem is called protoxylem. It lies towards the pith in stems and towards the periphery in roots. The first formed primary phloem is called protophloem which consists of narrow sieve tubes.

Question 16. Give a brief description of the guard cells.
Answer: Guard cells are a component of stomatal apparatus. These are the living cells and possess chloroplasts. These cells regulate the opening and closing of stomatal pores hence control the exchange of gases between plant and external environment and the rate of loss of water in the form of vapours, i.e., transpiration. The guard cells are bean-shaped cells but in grasses, these are dumb-bell shaped.

Question 17. What are trichomes? Write their function in angiosperms.
Answer: Trichomes are epidermal appendages. They may be unicellular or multicellular. In the shoot system, they are usually multicellular. They may be branched or unbranched and soft or stiff structurewise. The trichomes help in preventing water loss caused due to transpiration and also provide protection to the delicate tissues underneath it from the heat, dryness, etc.

Question 18. Discuss about the location of conjunctive tissue. Which cells constitute this tissue?
Answer: The xylem and phloem are arranged in an alternate manner on different radii in the roots of angiosperms. Such an arrangement of vascular bundles is called radial arrangement. The tissue which is present between xylem and phloem in the roots, in the form of patches is called conjuctive tissue. It is composed of parenchymatous cells.

Question 19. Why do the monocotyledonous roots not undergo any secondary growth? Discuss in brief.
Answer: Unlike dicotyledonous roots, the monocotyledonous roots do not undergo any secondary growth because of the absence of vascular cambium of secondary origin.

Question 20. Why endodermis is also called the starch sheath in dicot stem?
Answer: Endodermis is the innermost layer of the cortex. The cells of endodermis are living and are rich in the starch grains. Due to the abundance of starch grains, endodermis is also called the starch sheath.

Question 21. Give two differences between the internal structure of dicot and monocot stem w.r.t. hypodermis and vascular bundle.

Answer:

Dicotyledonous stem Monocotyledonous stem
(i) T h e h y po d e r m i s c o n s i s t s o f (i) T h e h y p o d e r m i s c on s i s t s o f collenchymatouscells. sclerenchymatous cells.
(ii) Vascular bundles are arranged in a ring - (ii) Vascular bundles are scattered in the

Question 22. Comment upon the pericycle present in stems and roots of angiosperms.
Answer: Pericycle is a component of ground tissue system of angiosperms which lies inner to the endodermis. In roots of angiosperms (dicots as well as monocots), it is made up of parenchymatous cells. In dicotyledonous stems, it lies on the inner side of endodermis and above the phloem in the form of semi-lunar patches of sclerenchyma while it is absent in the monocotyledonous stems.

Question 23. What are the two types of epidermis found in leaves of flowering plants?
Answer: The epidermis which covers the upper surface of the leaf is called adaxial epidermis and which covers the lower surface of the leaf is called abaxial epidermis. The abaxial epidermis generally bears more stomata than the adaxial epidermis in dorsiventral leaves while in an isobilateral leaf, the stomata are present on both the surfaces of the epidermis in near equal numbers.

Question 24. Which tissue in the leaves performs the photosynthesis?
Answer: Mesophyll performs the photosynthesis in leaves. It is the tissue present between the upper and lower epidermis in leaves. The cells of mesophyll possess chloroplasts. Mesophyll is divided into palisade and spongy parenchyma in dorsiventral leaves while in isobilateral leaves, mesophyll is not differentiated into palisade and spongy parenchyma.

Question 25. Give a short description of the vascular system of dorsiventral leaves.
Answer: Vascular system includes vascular bundles, which can be seen in the veins and the midrib. The size of the vascular bundles are dependent on the size of the veins. In dorsiventral leaves, the vascular bundles are surrounded by a layer of thick-walled bundle sheath cells. The bundle sheath cells are parenchymatous cells. Xylem is responsible for the distribution of water and minerals and phloem is responsible for the conduction of food.

Question 26. What are bulliform cells? What role do they play?
Answer: Bulliform cells are large, empty, colourless cells found on the adaxial epidermis of leaves of grasses. When the bulliform cells in the leaves have absorbed water and are turgid, the leaf surface is exposed. When they are flaccid due to water stress, they make the leaves curl inwards to minimise the water loss.

Question 27. Differentiate between primary and secondary growth of plants.

Answer:

The growth of roots and stems in length with the help of apical meristems is called primary growth. This type of growth is exhibited by all flowering plants.

The increase in diameter (girth) of roots and shoots with the help of lateral meristems is called secondary growth. Most of the dicotyledonous plants exhibit this type of growth.

Question 28. What do you understand by secondary medullary rays?
Answer: During secondary growth, at some places, the cambial ring forms a narrow band of parenchyma, which passes through the secondary xylem and the secondary phloem in the radial directions. These band-like structures which pass through secondary vascular bundles are called secondary medullary rays.

Question 29. Why are annual rings formed in the flowering plants of temperate regions?
Answer: Annual rings are formed due to the differential activity of the vascular cambium due to non-uniform climatic conditions in the temperate regions. In the spring season, cambium is very active and spring wood is produced which is lighter in colour while in winters, cambium is less active and autumn wood is produced which is darker in colour. These lighter and darker concentric rings are called annual rings.

Question 30. Define periderm.
Answer: Periderm is the protective tissue that replaces the epidermis after secondary growth is initiated. Phellogen (cork cambium), phellem (cork) and phelloderm (secondary cortex) are collectively known as periderm. It is formed when due to increase in girth, the outer cortical and epidermis layers get broken and need to be replaced to provide new protective cell layers.

Long Answer Type Questions

Question 31. Differentiate between collenchyma and sclerenchyma.

Answer:

Collenchyma and sclerenchyma both are the simple permanent tissues in plants which can be differentiated as

Collenchyma Sclerenchyma
(i) It is a living tissue in which the cells possess (i) This tissue consists of the cells that lose their protoplasm. protoplasm at maturity. Hence mature sclerenchyma is composed of deadcells.
(ii) Collenchymatous cells may be oval, spherical (ii) Sclerenchymatouscells may be elongated and or polygonal in shape. narrow or spherical, oval or cylindrical in shape.
(iii) Collenchyma is not present in the form of fibres (iii) On the basis of variation in form, structure, or sclereids. origin and development, sclerenchyma may be present eitherasfibresorassclereids.
(iv) The walls of collenchymatous cells are highly (iv) The walls of sclerenchymatous cells are not thickened at the corners due to a deposition of thickened only at the corners rather these are cellulose, hemi-cellulose and pectinleaving no thickened completely and are lignified (having intercellular spaces. a deposition of lignin alongwith cellulose) which makes them hard. Thickened corners Protoplasm Lumen Pits Vacuole Cell wall Thick cell wall Lumen

Thick cell wall A sclereid A fibre
(v) Collenchyma provides mechanical support to (v) Sclerenchyma provides mechanical support to the growing parts of the plant such as young the plant organs. It is commonly found in fruit stem and petiole of a leaf. It may also perform walls of nuts, pulp of fruits like guava, pear etc.; photosynthesis and assimilate food when its seed coats of legumes, leaves of tea, etc. cellspossess chloroplasts.

Question 32. Write a descriptive note on phloem and its elements in angiosperms.

Answer:

Phloem is a complex tissue found in plants. Phloem is a conducting tissue that transports food materials, usually from leaves to other parts of the plant.

companion cells. Their end walls are perforated in a sieve-like manner to form the sieve plates. A mature sieve element possesses a peripheral cytoplasm and a large vacuole but lacks a nucleus.  The companion cells are specialised parenchymatous cells, which are closely associated with sieve tube elements. The sieve tube elements and companion cells are connected by pit fields present between their common longitudinal walls. The companion cells help in maintaining pressure gradient in the sieve tubes.  Phloem parenchyma is made up of elongated, tapering cylindrical cells which have dense cytoplasm and nucleus. The phloem parenchyma stores food material and other substances like resins, latex and mucilage. It is absent in most of the monocotyledons.  Phloem fibres are made up of sclerenchymatous cells. These are much elongated, unbranched and have pointed, needle-like apices. The cell walls of the phloem fibres is quite thick. At maturity, these fibres lose their protoplasm and become dead. Phloem fibres of jute, flax and hemp are used commercially.

Question 33. Discuss epidermal tissue system in detail.
Answer: The epidermal tissue system forms the outermost covering of the whole plant body and comprises epidermal cells, stomata and the epidermal appendages - the trichomes and hairs.  The epidermis is the outermost layer of the primary plant body. It is made up of elongated, compactly arranged cells, which form a continuous layer. Epidermis is usually single-layered. Epidermal cells are parenchymatous with a small amount of cytoplasm lining the cell wall and a large vacuole. The outside of the epidermis is often covered with a waxy thick layer called the cuticle which prevents the loss of water.  Stomata are present in the epidermis of leaves. They regulate the process of transpiration and gaseous exchange. Each stoma is composed of two bean-shaped cells called guard cells which enclose a stomatal pore. In grasses, the guard cells are dumb-bell shaped. The guard cells possess chloroplasts and regulate the opening and closing of stomata.  The cells of epidermis bear a number of hairs. The root hairs are unicellular elongations of the epidermal cells and help absorb water and minerals from the soil. On the stem, the epidermal hairs are called trichomes. They may be branched or unbranched and soft or stiff. They may even be secretory. The trichomes help in preventing the water loss due to transpiration.

Question 34. Write short notes on the following :
(a) Lateral meristems
(b) Vessels
(c) Subsidiary cells
(d) Ground tissue

Answer:

(a) Lateral meristems : The meristem that occurs in the mature areas of roots and shoots of many plants, particularly, those that produce woody axis (dicotyledons and gymnosperms) and appear later than primary meristem is called lateral meristem. This is also called secondary meristem. They are cylindrical meristems. These are responsible for producing the secondary tissues like secondary xylem, secondary phloem, secondary medullary rays, secondary cortex and cork. The examples of lateral meristems are fascicular vascular cambium, interfascicular cambium and cork cambium.
(b) Vessels : Vessel is a long cylindrical tube-like structure made up of many cells called vessel members, each with lignified walls and a large central cavity. The vessel cells are devoid of protoplasm. Vessel members are interconnected through perforations in their common walls. The presence of vessel is a characteristic feature of angiosperms. Vessels
(c) Subsidiary cells : These are the specialised epidermal cells present in the vicinity of the guard cells. These differ morphologically from other epidermal cells in their shape and size. They are also called accessory cells as they are associated with the guard cells. They form a part of the stomatal apparatus and provide support to the guard cells. Subsidiary cells do not possess the chloroplasts.

Epidermal cells Subsidiary cells Chloroplast Guard cells

Stomatal pore

(d) Ground tissue : Ground tissue is the part of ground tissue system. All the tissues except epidermis and vascular bundles constitute the ground tissue. It comprises cortex, pericycle, pith and medullary rays. Cortex may be further differentiated into hypodermis, general cortex and endodermis. Ground tissue consists of simple tissues such as parenchyma, collenchyma and sclerenchyma. It forms the main framework of body of plants. The ground tissue in leaves is called mesophyll.

Question 35. Observe the structures given below and answer the questions following the figures :
B
A (i)
A
B (ii)
B (iii) A B (iv) A
(a) Name the structures depicted in the figures.
(b) Identify A and B in each of the given figure.
(c) Mention the functions of the given structures.

Answer:

(i) (a) Structure in the given figure is a sclereid.

(b) A shows the lumen of the sclereid and B represents the pits present in it.

(c) Sclereids are spherical, oval or cylindrical, highly thickened dead cells with very narrow cavities. These are present in pulp of fruits like guava, pear and sapota and is responsible for the gritty nature of these fruits that can be felt while eating them. These are also found in the hard fruit walls of nuts, seed coats of legumes and leaves of tea and provide mechanical support to these organs, i.e., fruits, leaves and seeds.

(ii) (a) This is the figure of phloem.

(b) A in the given figure is sieve tube element and B is the phloem parenchyma.

(c) Phloem is the vascular tissue of the plants which transports the synthesised food materials from their sites of synthesis to all other parts of the plant.

(iii) (a) Given figure is the diagrammatic representation of stomatal apparatus in grasses.

(b) A represents the guard cell and B represents the subsidiary cell.

(c) The stomatal apparatus, constituted by guard cells, subsidiary cells and stomatal aperture; performs gaseous exchange and transpiration. The gases enter and exit through the stomatal aperture. The water is lost in the form of water vapours through stomatal apertures.

(iv) (a) This is the diagrammatic representation of radial vascular bundle.

(b) A represents phloem and B represents the xylem.

(c) This kind of vascular bundles are found in roots. They help in the conduction of water and minerals through xylem and of food material through phloem.

Question 36. Draw a well labelled diagram of the T.S. of a dicotyledonous root and mention the various tissues that comprise its ground tissue system.

Answer:

A well-labelled diagram should show these parts: Root hair; Epidermis; Cortex; Endodermis; Pericycle; Protoxylem; Metaxylem; Pith; Phloem; T.S. : Dicot root (primary).

The ground tissue is differentiated into :
(i) Cortex : The cortex consists of several layers of thin-walled parenchyma cells with intercellular spaces. The cells may be circular or polygonal in shape. The innermost layer of the cortex is called endodermis. It comprises a single layer of barrel-shaped cells without any intercellular spaces. The tangential as well as radial walls of the endodermal cells have a deposition of water-impermeable, waxy material-suberin in the form of casparian strips.
(ii) Pericycle : Next to endodermis lies a few layers of thick-walled parenchymatous cells referred to as pericycle. Initiation of lateral roots and vascular cambium during the secondary growth takes place in these cells.
(iii) Pith : The pith is small or inconspicuous. It may even be absent. When present, it occupies the central portion of the root and comprises rounded, parenchymatous cells.
(iv) Conjuctive tissue : The parenchymatous cells which lie between the xylem and the phloem are called conjuctive tissue.

Question 37. Differentiate between the anatomy of a dicotyledonous and monocotyledonous root.

Answer:

Question 38. Carefully study the figure given below and answer the following questions :
A B C
D
(a) Which structure is represented by the given figure?
(b) Identify A, B, C and D.
(c) Give one characteristic feature of each of them.
Answer: (a) Given figure is the diagrammatic representation of the transverse section (T.S.) of a dicotyledonous stem.
(b) A – Endodermis B – Pericycle C – Vascular bundle D – Medullary ray
(c) Features of A, B, C and D – Endodermis : It is the innermost layer of the cortex, which comprises a single layer of barrel-shaped cells. The cells of the endodermis are rich in starch grains and the layer is also referred to as the starch sheath. Pericycle : Pericycle lies on the inner side of the endodermis and above the phloem in the form of semi- lunar patches of sclerenchyma. Vascular bundle : A large number of vascular bundles are arranged in a ring-like arrangement. Each vascular bundle is conjoint, open (having cambium) and with endarch protoxylem. Medullary rays : In between the vascular bundles, there are few layers of radially placed parenchymatous cells, which constitute the medullary rays.

Question 39. Identify the structure given below and explain it.

Answer:

This is the diagrammatic representation of the transverse section of a monocotyledonous stem. It shows the following characteristics :
(a) Epidermis : It is the outermost layer which is made up of single layer of cells. The outer walls of the epidermal cells are covered with a thick cuticle which prevents the loss of water.

(b) Hypodermis : It lies below the epidermis and is made up of sclerenchymatous cells.

(c) Ground tissue : The entire mass of parenchymatous cells next to hypodermis, forms the ground tissue. It is not differentiated into cortex, endodermis, pericycle, pith and pith rays. The cells contain reserve food materials. Vascular bundles are embedded in this tissue.

(d) Vascular bundles : A large number of vascular bundles are present in the stem which are scattered in the ground tissue. Vascular bundles are conjoint and closed. Each bundle is surrounded by a sclerenchymatous bundle sheath. The peripheral vascular bundles are generally smaller than the centrally located ones. The phloem parenchyma is absent and rest of the phloem elements, i.e., sieve tubes, companion cells and phloem fibres are present. Water-containing cavities are present within the vascular bundles.

Question 40. Explain the functions of the following :
(a) Parenchyma
(b) Xylem parenchyma
(c) Medullary rays
(d) Mesophyll

Answer:

(a) Parenchyma : Parenchyma forms the major component within the organs. This tissue is present in all organs of the plants, i.e., roots, stems, leaves, flowers, fruits and seeds. Its functions are –

(i) The main function of parenchyma is storage of food.

(ii) Those parenchymatous cells which possess chloroplasts in them perform photosynthesis e.g., palisade and spongy parenchyma in leaves.

(iii) This tissue also helps in the secretion of resins.

(b) Xylem parenchyma : It is the living component of xylem.

(i) Its cells store food materials in the form of starch or fat and other substances like resins, latex and tannins.

(ii) Those parenchymatous cells which are radially present are called ray parenchymatous cells and help in the radial conduction of water.

(c) Medullary rays : In most dicotyledonous stems, the peripheral layers of pith extend between the vascular bundles and are in contact with pericycle. These extensions of parenchymatous cells are called medullary rays. They help in the formation of interfascicular cambium and hence the cambial ring during the secondary growth. The cambial ring gives rise to secondary xylem and phloem.

(d) Mesophyll : The ground tissue of the leaf is called mesophyll. It is made up of parenchymatous cells that possess chloroplasts and carry out photosynthesis. It is the main photosynthesizing tissue of leaves that synthesize and store the food materials.

Question 41. (a) Draw well labelled diagrams of a section of –
(i) Dicotyledonous leaf
(ii) Monocotyledonous leaf
(b) Give three differences between them.

Answer:

(a) (i) Bundle sheath Xylem Adaxial epidermis Phloem

Palisade mesophyll

Air cavity

Spongy mesophyll Sub-stomatal cavity

Stoma Abaxial epidermis

Dicotyledonous leaf

Adaxial epidermis (ii) Xylem Mesophyll

Sub-stomatal cavity

Abaxial epidermis Stoma Phloem Monocotyledonous leaf

(b)

Question 42. Discuss the formation and activity of cambial ring in dicot stems.

Answer:

Formation of cambial ring In dicot stems, the cells of cambium present between primary xylem and primary phloem is the intrafascicular cambium. The cells of medullary rays, adjoining these intrafascicular cambium become meristematic and form the interfascicular cambium. Thus, a continuous ring of cambium is formed which is called cambial ring. Activity of the cambial ring

The cambial ring becomes active and begins to cut off new cells, both towards the inner and the outer sides. The cells cut off towards pith, mature into secondary xylem and the cells cut off towards periphery mature into secondary phloem. The cambium is generally more active on the inner side than on the outer. As a result, the amount of secondary xylem produced is more than secondary phloem and soon forms a compact mass. The primary and secondary phloems get gradually crushed due to the continued formation and accumulation of secondary xylem. The primary xylem however remains more or less intact, in or around the centre. At some places, the cambium forms a narrow band of parenchyma, which passes through the secondary xylem and the secondary phloem in the radial directions. These are the secondary medullary rays.

Question 43. Explain the following :
(a) The functions of sieve tubes are controlled by the nucleus of the companion cells.
(b) The guard cells regulate the opening and closing of stomata.
(c) The size of vascular bundles in a dorsiventral leaf are dependent on the size of veins.
Answer: (a) Sieve tubes elements are arranged longitudinally one above the other to form the sieve tubes. A mature sieve tube element possesses a peripheral cytoplasm and a large vacuole but lacks a nucleus. These are closely associated with the companion cells which possess the nucleus. The functions of the sieve tubes are controlled by the nucleus of companion cells as these are connected by the pit fields present between their common longitudinal walls. These pit fields behave like pores between sieve tube elements and companion cells.
(b) Guard cells are the epidermal cells which are bean-shaped (dumb-bell shaped in grasses). Two guard cells enclose a stomatal aperture. When guard cells absorb water, they swell up and cause opening of the aperture and when they shrink by losing water, they cause closing of the aperture. This aperture is meant for the entry and exit of gases and for transpiration. Hence the guard cells control opening and closing of stomata.
(c) The dorsiventral leaves are found in dicotyledons. These have reticulate venation in which the veins vary in thickness. In this type of venation, the veinlets (branches of veins) form a network and their thickness vary in different parts of lamina of leaf. As the veins differ in their thickness, the vascular bundles also differ in their size. Broader veins possess larger bundles and narrower possess smaller bundles.

Question 44. (a) What is cork?
(b) Name the various kinds of layers which constitute the bark.

Answer:

(a) Cork is also called phellem. It is the protective layer that develops in the stems during secondary growth. Cork cells are formed as a result of divisions of the phellogen cells or cork cambium towards the outer face. The cells of phellogen are compactly arranged and have thin cellulosic walls in the beginning. As they mature, there is gradual loss of living matter. The cell walls become thick because of deposition of a fatty substance called suberin. The cork becomes impervious to water due to suberin deposition in the cell wall.

(b) Bark is a non-technical term that refers to all tissues exterior to vascular cambium. The tissues that lie exterior to vascular cambium after the secondary growth are secondary phloem, primary phloem, pericycle, primary cortex, secondary cortex, cork cambium and cork. Last three constitute the periderm. Hence bark constitutes all the layers discussed above.

Question 45. Comment upon the following :
(a) Tissues present in stele.
(b) Ring arrangement of vascular bundles.
(c) Origin of axillary buds.
(d) Structure and function of heartwood.
Answer: (a) Stele is the axial portion of the plant axis that anatomically comprises all the tissues present on the innerside of the endodermis such as pericycle, vascular bundles and pith. Hence endodermis is the border between cortex and stele. Pericycle is the outermost layer of stele. It is noteworthy that stele comprises tissues of ground tissue system as well as vascular tissue system.
(b) The 'ring' arrangement of vascular bundles is a characteristic of dicot stem. In this type of arrangement, the vascular bundles are arranged around the central pith in a ring - like manner with their xylems directed towards the pith and phloems towards the periphery. The vascular bundles in dicot stems possess vascular cambium between xylem and phloem.
(c) Origin of axillary buds : Axillary buds are present in the axils of leaves and are capable of forming a branch or a flower. The shoot apical meristem gives rise to the axillary buds as during the formation of leaves and elongation of stems, some cells that 'left behind' from shoot apical meristem, constitute the axillary bud.
(d) Structure and function of heartwood : In old trees, the greater part of secondary xylem is dark brown due to deposition of organic compounds like tannins, resins, oils, gums, aromatic substances and essential oils in the central or innermost layers of the stem. These substances make it hard, durable and resistant to the attacks of microorganisms and insects. This region comprises dead elements with highly lignified walls and is called heartwood. The heartwood does not conduct water but it gives mechanical support to the stem.