Information

9: Leaf Anatomy - Biology


Learning Objectives

Content Objectives

  • Learn the organization of tissues within the leaf
  • Understand how the organization of these tissues changes in adaptation in different environmental conditions
  • Explain how the relationship between surface area and volume relates to water retention in plant tissues
  • Describe the steps in the process of science

Skill Objectives

  • Identify tissues and structures within a variety of leaves
  • Identify whether a leaf is mesophytic, xerophytic, or hydrophytic based on leaf anatomy
  • Predict the drought tolerance of a leaf using anatomical and morphological features
  • Use the process of science to ask and answer scientific questions
  • Collect, analyze, and interpret data

Leaf Anatomy NEET Notes | EduRev

INTERNAL STRUCTURE OF LEAF
Generally leaves divided into two categories - Dorsiventral leaves and isobilateral leaves. The differences in between them as follows :

Dorsiventral or Bi–facial

Iso–bilateral or Equifacial

1. Present at right angle to stem

1. Arranged parallel to stem.

INTERNAL STRUCTURE OF DORSIVENTRAL LEAVES :

Cuticle is present on both surfaces but cuticle on upper surface is more thick.

In dorsiventral leaves stomata more on lower surface and stomata on upper surface is absent or less in number

Mesophyll is differentiated into Palisade tissue and spongy tissue. Palisade tissue is situated towards upper (adaxial) surface.These cells are elongated, arrange vertically and parallel to each other and have more chloroplasts and a large vacuole.

Spongy tissue is situated towards lower (Abaxial) surface. The cells are oval or rounded and between cells large air space are present.

INTERNAL STRUCTURE OF ISOBILATERAL LEAVES

The thickness of cuticle is equal on both surfaces.

Usually stomata on both surface's are equal in number.

Mesophyll is not differentiated into palisade and spongy tissues in isobilateral leaves. Mesophyll cells have only a few intercellular spaces.

(1) In isobilateral leaf, two distinct patches of sclerenchyma are present above and below each of the large vascular bundles and extend up to the upper and lower epidermal layers, respectively.

(2) In dorsiventral leaf, two distinct patches of parenchyma (mainly)/collenchyma are present above and below each of the large vascular bundles and extend up to the upper and lower epidermal layers, respectively. Chloroplasts are absent in bundle sheath extensions.

VASCULAR BUNDLES OF LEAVES :

  • Similar types of vascular bundles are found in both dorsiventral and isobilateral leaves. Vascular bundles of leaves are conjoint, collateral and closed and xylem is endarch (mostly).
  • Protoxylem is situated towards the adaxial surface and protophloem towards the abaxial surface in the vascular bundle. Leaves are devoid of endodermis and pericycle. Vascular bundles are surrounded by a bundle sheath. It is made up of parenchyma (mostly) or sclerenchyma.

1. In the leaves of C4-plants, bundle sheath is chlorenchymatous

2. In grasses, certain adaxial epidermal cells along the veins modify themselves into large, empty, colourless cells. These are called bulliform cells or motor cells . When the bulliform cells in the leaves have absorbed water and are turgid, the leaf surface exposed. When they are flaccid due to water stress, they make the leaves curl inwards to minimize water loss.
Example :- Ammophila, Poa, Empectra and Agropyron etc. are Psammophytic grasses

2. Both upper & lower epidermis of Nerium leaves are multilayered and in Ficus leaves upper epidermis is multilayered. This is an adaptation to reduce transpiration.

1.Xerophytes with isobilateral leaves contain palisade tissue on both sides of leaf.
Example :- Eucalyptus & Nerium.

2. Desert grasses contain palisade like spongy tissue.

3. Unifacial or cylinderical leaf :- In these leaves there occur no differentiation of upper surface and lower surface. Example :- Onion, Garlic.

4. Albascent leaf :- Palisade tissue is restricted in half part of leaf so half part appears more green and other half appears less green. Example : Abutilon

ANOMALOUS PRIMARY STRUCTURE

[1] ANOMALOUS STRUCTURE IN DICOTYLEDON STEM

I. Scattered Vascular Bundles :- In some of dicotyledon stem, vascular bundles are not arranged in a ring, they are scattered in the cortex. Example :- Thallictrum, Nymphaea, Papaver orientale & Peperomia.

II. Phloem on innermost radius :- Generally, phloem is situated in the ring of vascular bundles towards peripheral (outer) radius and xylem towards the inner radius. But anomalously in some plants, the position of phloem is towards the inner side of xylem. Such type of phloem is called Internal phloem. Because, this phloem lies towards the pith, so it is also known as medullary phloem.
This anomalous condition is found in Calotropis, Capsicum, Leptadaenia, etc. plants.

III. Medullary Vascular Bundles :- In addition to normal ring of vascular bundles, some vascular bundles are also present in pith. These are called medullary vascular bundles.
Example : Amaranthus, Boerhaavia, Chenopodium, Mirabilis, Achyranthes, Bougainvillea , Raphanus sativus.

IV Cortical Vascular Bundles :- In addition to normal ring of vascular bundles, some vascular bundles are also present in the cortex. They are known as cortical vascular bundles.
Example :- Casuarina, Nyctanthes and Lathyrus.etc.

V. Polystelic condition :- It is the normal situation in pteridophytes but in some dicotyledons it is present abnormally.
Example :- Primula, Dianthera

VI Exclusively xylem vascular Bundle :- Abnormally, some vascular bundles are only formed by xylem except the normal vascular bundles.
Example :- Paeonia

VII Exclusively phloem vascular Bundle :- Abnormally, some vascular bundles are only formed by phloem except normal vascular bundles in some plants Example :- Cuscuta & Ricinus communis

[2] Anomalous structure in monocot stem :–

Normally vascular bundles are found in monocotyledon stems in scattered form but in the stem of some monocotyledon plants vascular bundles are arranged in rings. Ex. Members of family Gramineae, such as Triticum, Secale, Hordeum, Avena, Oryza etc.

(1) Cricket bat → from Salix [Willow]

(2) Hockey→ from Morus [Mulberry]

(3) Monarch Condition →in Trapa root

(4) Triarch Condition→ in Pisum root

(5) Tetrarch Condition→ in Helianthus annuus (Sunflower) and Cicer arietinum (Gram) root

(6) Waiting meristem concept :- This concept was given by Buvat. According to this, there is an inactive centre in the shoot apex which is known as waiting meristem and it acts as reservoir of active initials and on induction it give rise to reproductive apex.

(7) Tannin is found in latex of banana.When it comes in contact with air it gets oxidised and becomes reddish brown in colour

(8) Chewing gum is made by latex of Achras sapota.

(9) Tannin glands are found in camellia. These glands are schizogenous in origin

(10) Salt glands are found in Tamarix which secretes sodium chloride.

(11) Chalk glands are found in plants of plumbaginaceae family which secretes calcium carbonate.

(12) Multilayered (14 to 15 layers) epidermis is found in Peperomia leaves.

(13) Cystolith containing cells are found in the upper epidermis of Ficus leaf, called lithocytes/Lithocysts.

(14) Lignin is stained by safranin.

(15) Tracheids are the chief water transporting elements in gymnosperms.

(16) Phloem is embedded into the secondary xylem in some plants. Such phloem is called included phloem or interxylary phloem. This is secondary anomalous structure.
Example :- Leptadaenia, Salvadora etc. dicot stem.

(17) Pericycle is absent in roots and stems of some aquatic plants.

(18) In some monocotyledonae roots, pith is sclerenchymatous. Ex. Canna

(19) A nectar secreting gland cell contains granular cytoplasm and a large conspicuous nucleus.

(20) Transition of exarch bundles of root to endarch bundles of stem occurs in hypocotyl.

(21) In leaves, the ground tissue consists of thin walled chloroplast containing cells and is called mesophyll.

(22) The stomatal aperture, guard cells and the surrounding subsidiary cells are together called stomatal apparatus.

(23) The size of vascular bundles are dependent on the size of the veins. The veins vary in thickness in the reticulate venation of the dicot leaves.

(24) The parallel venation in monocot leaves is reflected in the near similar sizes of vascular bundles (except in main veins) as seen in vertical sections of the leaves.


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