BOT117_Unit IV LEAF.pdf

Unit IV
Leaf: morphology, anatomy and
modifications

• The leaf is a flattened, lateral outgrowth of the stem in
the branch, developing from a node and having a bud in
its axil.
• It is normally green in colour and manufactures food
for the whole plant.
• The leaves take up water and carbon dioxide and
convert them into carbohydrates in the presence of
sunlight and chlorophyll.
• Leaves always follow an acropetal development and are
exogenous in origin.

Types of Leaves:
On the basis of origin point:
Radical:
Proceeding from or near the root, e.g., onion,
radish, etc.
Cauline:
Pertaining to the stem, e.g., palms.
Cauline and Ramal:
Pertaining to the main stem as well as its branches,
e.g., mango.

Types of leaves
1. Foliage Leaves: They are most common types
of leaves for photosynthesis.

2. Cotyledons:
• They are seed leaves &
they are 1 inmonocots
& 2 in dicots.
• Cotyledons nourish the
embryo.
• After germination
cotyledons manufacture
food by becoming 1st
foliage leaves.

3. Scale
leaves/cataphylls:
• They are non-green,
brown,
• Small, thin sessile &
membranous
structures.
• eg. Bud scales, scales
on aerial stem like
Ruscus.

4. Bracts/Hypsophylls:
• They are specialized
leaves which bears
flowers in their axils.
• They protect the
young floral buds &
when coloured they
attract the insects.
• Eg. Bougainvillea

5. Bracteoles:
• They are two small leaf like structures which are
found attached to the stalk of flower, aggregate to
form whorls of epicalyx below the sepals.

6. Prophylls:
• They are the 1st one /two leaves of the branch
which differ from other leaves.
• They form spines in Aegle (Bael) & Citrus.

7. Floral Leaves:
• They are special leaves which represent vegetative parts of
flowers eg. Sepals, petals.
8. Sporophylls:
• They are modified spore bearing leaves of flowers eg.
Stamens & carpels.
9. Stipules:
• They are two small leaf like lateral appendages present at
leaf base.
10. Ligules:
• They are minute scale-like outgrowths present at the upper
end of leaf sheath in grasses.

IV. Apex:
The apex of the lamina may be :
(1) Acute—when pointed and narrow as in mango.
(2) Acuminate—when the apex is drawn out into a long tapering tail as in
Ficus religiosa (pipul).
(3) Obtuse—when the apex is broad angled and blunt as in banyan.
(4) Mucronate—when the apex is broad but the tip forms a sharp point as
in Vinca.
(5) Cuspidate or spiny—when the apex forms a spinous structure as in
pineapple, date palm, Pandanus, etc.
(6) Tendrillar—when the apex forms a tendril for climbing as in Gloriosa.
(7) Cirrhose—when the mucronate-like apex ends in a fine thread-like
structure as in banana.
(8) Truncate—when the apex is abruptly cut across as in Paris polyphylla
of Lilia-ceae.
(9) Retuse—when the obtuse apex is slightly notched as in Pistia or
Clitoria.
(10) Emarginate—when the obtuse apex is deeply notched as in Bauhinia.

• In many plants, leaves get modified to
perform some special functions other than the
normal ones, such as photosynthesis and
transpiration.
• Sometimes these modifications are in
response to certain environmental conditions.

1. Storage Leaves:
• Some plants of xerophytic habitats
and members of the family
Crassulaceae generally have highly
thickened and succulent leaves with
water storage tissue.
• These leaves have large
parenchymatous cells with big
central vacuole filled with
hydrophilic colloid.
• This kind of adaptation helps plants
to conserve very limited supply of
water and resist desiccation (drying
up).

2. Leaf Tendrils:
In weak- stemmed plants, leaf or a part of leaf gets
modified into green threadlike structures called tendrils
which help in climbing around the support.
• The parts of leaf which get modified into tendrils are as
follows:
• (i) Entire Leaf is Modified into Tendril, e.g., Lathyrus
aphaca (wild pea) (Fig. 4.19).
• (ii) Upper Leaflets Modified into Tendrils, e.g, Pisum
sativum (pea) (Fig. 4.20), Lathyrus odoratus (sweet pea).
• (iii) Terminal leaflets Modified into Tendrils, e.g.,
Naravelia

• (iv) Leaf Tip Modified into Tendril, e.g.,
Gloriosa (Glory lily)
• (v) Petiole Modified into Tendril, e.g., Clematis
• (vi) Stipule Modified into Tendril, e.g., Smilax

3. Leaf-spines:
• Leaves of certain plants become wholly or partially modified
for defensive purpose into sharp, pointed structures known as
spines.
• Thus, in prickly pear (Opuntia) the minute leaves of the
axillary bud are modified into spines.
• The leaf-apex in date-palm, dagger plant (Yucca) etc., is so
modified,
• while in plants like prickly or Mexican poppy (Argemone),
Amercian aloe (Agave), Indian aloe (Aloe), etc., spines develop
on the margin as well as at the apex.
• In barberry the leaf itself becomes modified into a spine;
while the leaves of the axillary bud are normal.

4. Scale-leaves:
• Typically these are thin, dry, stalkless,
membranous structures, usually brownish
in colour or sometimes colourless.
• Their function is to protect the axillary bud
that they bear in their axil.
• Sometimes scale-leaves are thick and
fleshy, as in/onion; then their function is
to store up water and food.
• Scale-leaves are common in parasites,
saprophytes, underground stems, etc.
They are also found in Casuarina,
Asparagus etc.

5. Leaflet Hooks:
• In Bignonia unguiscati the three terminal
leaflets of leaf get modified into claw like
hooks which help in climbing
6. Leaf Roots:
• In case of Salvinia three leaves are present at
one node. Out of these two leaves are normal
and third gets modified into adventitious roots
which help in floating over the surface of
water

8. Insect Catching Leaves:
(i) Leaf-Pitcher:
This is a device to catch insects for fulfilling the deficiency of
nitrogen in the medium where plant is growing.
In case of Nepenthes, Dischidia and Sarracenia leaf-lamina is
modified into pitcher-like structure called leaf-pitcher.

(ii) Leaf Bladder:
• Some of the segments of the leaf are modified
into bladders or utricles.

(iii) In Drosera
• The lamina possesses numerous spine-like
hairs spreading all around. Each hair has a
shining sticky globule at its tip which contains
digestive enzymes.

Epidermis:
1. Two epidermal layers are present, one each on upper
and lower surfaces.
2. Uniseriate upper and lower epidermal layers are
composed of more or less oval cells.
3. Few big, motor cells or bulliform cells are present in
groups here and there in the furrows of upper epidermis.
4. Stomata, each consisting of a pore, guard cells and a
stomatal chamber, are present on both the epidermal
layers.
5. A thick cuticle is present on the outer walls of
epidermal cells.
6. Bulliform cells help folding of leaves.

Mesophyll:
7. It is not clearly differentiated into palisade and spongy
parenchyma but the cells just next to the epidermal layers are a
bit longer while the cells of the central mesophyll region are oval
and irregularly arranged.
8. The cells are filled with many chloroplasts.
9. Many intercellular spaces are also present in this region.
10. Sub-stomatal chambers of the stomata are also situated in
this region.

Vascular System:
11. Many vascular bundles are present. They are arranged in a
parallel series.
12. The central vascular bundle is largest in size.
13. Vascular bundles are conjoint, collateral and closed.
14. Each vascular bundle remains surrounded by a double-
layered bundle sheath.
15. Outer layer of bundle sheath consists of thin-walled cells
while the inner layer is made up of thick-walled cells.
16. On the upper as well as lower surfaces of large vascular
bundles are present patches of sclerenchyma which are closely
associated with the epidermal layers.
17. Xylem occurs towards the upper surface and phloem towards
lower surface.

18. Xylem consists of vessels and tracheids.
Sometimes small amount of xylem parenchyma
is also present.
19. Phloem consists of sieve tubes and
companion cells.
Xerophytic Characters:
(i) Thick cuticle on epidermis.
(ii) Presence of motor cells.
(iii) Sclerenchyma patches are present.
(iv) Stomata in furrows.

Identification:
(a) 1. Presence of upper and lower epidermal
layers.
2. Mesophyll is present.
3. Each vascular bundle is surrounded by bundle
sheath…………. Leaf
(b) 1. Many vascular bundles are arranged
parallaley.
2. Absence of cambium.
3. Vascular bundles are collateral and closed.
4. Stomata on both the surfaces.
Isobilateral monocot leaf.

Epidermis:
1. An epidermal layer is present on the upper as
well as lower surfaces.
2. One-celled thick upper and lower epidermal
layers consist of barrel-shaped, compactly
arranged cells.
3. A thick cuticle is present on the outer walls of
epidermal cells. Comparatively, thick cuticle is
present on the upper epidermis.
4. Stomata are present only on the lower
epidermis.

Mesophyll:
5. It is clearly differentiated into palisade and spongy parenchyma.
6. Palisade lies just inner to the upper epidermis. It is composed of
elongated cells arranged in two layers.
7. The cells of palisade region are compactly arranged and filled
with chloroplasts. At some places the cells are arranged loosely and
leave small and big intercellular spaces.
8. Palisade cells are arranged at a plane at right angle to the upper
epidermis, and the chloroplasts in them are arranged along their
radial walls.
9. Parenchymatous cells are present above and below the large
vascular bundles. These cells interrupt the palisade layers and are
said to be the extensions of the bundle sheath.
10. Spongy parenchyma region is present just below the palisade
and extends upto the lower epidermis.
11. The cells of spongy parenchyma are loosely arranged, filled with
many chloroplasts and leave big intercellular spaces.

Vascular Region:
12. Many large and small vascular bundles are present.
13. Vascular bundles are conjoint, collateral and closed.
14. Each vascular bundle is surrounded by a bundle sheath.
15. Bundle sheath is parenchymatous and in case of large
bundles it extends upto the epidermis with the help of thin-
walled parenchymatous cells.
16. The xylem is present towards the upper epidermis and
consists of vessels and xylem parenchyma. Protoxylem is present
towards upper epidermis while the metaxylem is present
towards the lower epidermis.
17. Phloem is present towards the lower epidermis and consists
of sieve tubes, companion cells and phloem parenchyma.

Identification:
(a) 1. Presence of expanded portion or blade.
2. Presence of mesophyll.
3. Bundle sheath is present………………………………….
leaf
(b) 1. Upper and lower epidermal layers are clearly
distinguishable.
2. Mesophyll is clearly differentiated into palisade
and spongy parenchyma.
3. Stomata only on the lower surface- dicot leaf

BOT117_Unit IV LEAF.pdf

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BOT117_Unit IV LEAF.pdf