Shoot System: Internal Structure of Leaf, General and Special Features (For CBSE, ICSE, IAS, NET, NRA 2022)

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Internal Structure of Leaf

General Features

Leaves of most dicot plants are dorsiventral (oriented horizontally, with differentiated mesophyll) whereas those of monocots are isobilateral (oriented vertically, mesophyll undifferentiated) as seen in transverse section shows the following features.

  • Epidermis: the upper and lower surfaces of the leaf are covered by the upper and lower epidermis, single layered cells, lack chloroplasts, some epidermal cells form stomata which help in exchange of gases for photosynthesis, respiration and evaporation of water vapour during transpiration.
  • Mesophyll: the entire tissue between the upper and lower epidermis, containing parenchyma (Chlorenchyma) and is responsible for carrying out photosynthesis and divided into two regions; palisade and spongy cells. Palisade cells: occurs beneath the upper epidermis, abundant chloroplasts so upper surface of the leaf appears dark green, cells are arranged parallel and radially elongated. Spongy cells: occurs below the palisade cells with loosely arranged and irregularly shaped cells with intercellular spaces, contain fewer chloroplasts and helping in the distribution and diffusion of gases.
  • Vascular Bundles: They are conjoint, collateral, Endarch and closed. The xylem is situated towards the upper epidermis (adaxial surface) and phloem lies towards the lower epidermis (abaxial surface) . Each vascular bundle is surrounded by a layer of thick-walled cells arranged compactly and known as the bundle sheath.
Image Showing T. S. Of Leaf; (A) Dicot (B) Monocot

Structure of Stomata

Each stoma is surrounded by two guard cells. The kidney-shaped guard cells contain chloroplasts. A respiratory cavity or chamber is found under each stoma. The mechanism of the closing and opening of the stomata depends upon the presence of sugar and starch in the guard cells.

Distribution of Stomata

Table Showing Distribution of Stomata
PlantsStomatal charactersExamples
DicotsGuard cells semicircular, present on lower surface of leafMango, Neem
MonocotsGuard cells dumbbell shaped, equally distributed on both the surfacesMaize
XerophytesTo reduce transpiration- occur only on lower surface, are less in number, may be sunkenNerium
Hydrophyte

-With floating leaves

-With submerged leaves

Occur only on upper surface

Stomata absent

Lotus

Hydrilla

Difference between Internal Structure of Dicot and Monocot Leaf

Table Showing Difference between Dicot and Monocot Leaf
TissueDicot leafMonocot leaf
CuticleCuticle thick at upper epidermis and thin at lower epidermisUniform cuticle on both the surface
Stomatamore on lower surfaceEqual number of stomata on either side
Mesophylldifferentiated into palisade parenchyma and spongy parenchymadifferentiated into palisade and spongy parenchyma
Bulli form cellsAbsentPresent
Vascular systemin the form of network, vascular bundle in mid rib region is large, rest smallin rows, vascular bundle generally of same size

Special Features

  • Bulli form cells: In monocots, a few large, empty and colorless cells are present at intervals on the upper surface is called the Bulli form cells are found most commonly seen in the upper epidermis and in the rolling and unrolling of leaves in response to their water status.
  • Hairs: Present on leaves of plants which are growing in dry conditions. They check the rate of transpiration and protect the leaf from high temperature.
  • Hydathodes (water stomata) : Loss of water through water stomata (Hydathodes) is called guttation. Guttation mainly occurs when transpiration is difficult due to high air humidity or when the plant is placed in a watery soil.
Image Showing Hydathode and Stomata

Difference between Stomata and Hydathode

Table Showing Difference between Stomata and Hydathode
CharactersHydathodeStomata
SizeLargeSmall
LocationOn the tips of young leaves at vein endingsIn epidermis of leaves, stems and fruits.
Structurealways openopening and closing by guard cells
Loss of waterloss of water in the liquid formloss of water in the gaseous form
OccurrenceFound in plants of humid areasIn plants occurring in all climates
processGuttationTranspiration
surrounded byring of cuticularized achlorophyllous cellspairs of achlorophyllous guard cells

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