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FAQs
Q. No. 1. Describe briefly the various parts of a leaf?
Ans. A structurally complete leaf of an angiosperm consists of
a petiole (leaf stalk), a lamina (leaf blade), and stipules (small
structures located to either side of the base of the petiole).
Not every species produces leaves with all of these structural
components. In certain species, paired stipules are not obvious
or are absent altogether. A petiole may be absent, or the blade
may not be laminar (flattened). The petiole mechanically links the
leaf to the plant and provides the route for transfer of water and
sugars to and from the leaf. The lamina is typically the location of
the majority of photosynthesis.
In monocots, the leaf is almost always broadly sheathed at the
base, with edges either fused or overlapping. In taxa such as
grasses (Poaceae) and gingers (Zingiberaceae), there is an abaxial
flap or ligule at the junction of sheath and blade.
Q. No. 2. What is stipule and stipulation?
Ans. A stipule, present on the leaves of many dicotyledons, is an
appendage on each side at the base of the petiole resembling a
small leaf. Stipules may be lasting and not be shed (a stipulate
leaf, such as in roses and beans), or be shed as the leaf expands,
leaving a stipule scar on the twig (an exstipulate leaf).
The situation, arrangement, and structure of the stipules is
called the “stipulation”. It may be:
o
free
o
adnate : fused to the petiole base
o
ochreate : provided with ochrea, or sheath-formed
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stipules, e.g. rhubarb,
o
encircling the petiole base
o
interpetiolar : between the petioles of two opposite leaves.
o
intrapetiolar : between the petiole and the subtending
stem
Q. No. 3. Elucidate the various functions of leaf.
Ans. Various functions of leaf are:
• Leaves are the physical “platform” for the process of photosynthesis.
• The cells in the leaf contain chloroplasts; these contain chlorophyll which converts carbon dioxide and water to simple
glucose sugars in the presence of sunlight.
•
•
•
•
This food (carbohydrates) is used and stored by the plant to
grow and reproduce. As plants are able to produce “food”
they are normally referred to as “producers”.
To increase the photosynthesising surface area of the plant
to maximise the production of food.
To provide a large surface area for transpiration and gaseous
exchange (through the stomata).
To assist in the collection of water for the plant; some leaves
are funnel-shaped in order to actively collect water which is
then directed to the roots. Leaves also form a canopy of the
tree collecting rainwater from a larger area which is then led
(through run-off) to the root zone of the plant.
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•
The leaves provide shade and shelter to the stem and root
system below.
Q. No. 4. What are deciduous plants and what is their distribution?
Ans. Plants which shed their leaves in autumn of every year are
called deciduous plants. The shed leaves may be expected to
contribute their retained nutrients to the soil where they fall. In
contrast, many other non-seasonal plants, such as palms and
conifers, retain their leaves for long periods; Welwitschia retains its
two main leaves throughout a lifetime that may exceed a thousand
years. Deciduous plants occur mostly in frigid or cold temperate
regions of the world, whereas in areas with a severe dry season,
some plants may shed their leaves until the dry season ends.
Q. No. 5. Distinguish between simple and compound leaves.
Ans. Two basic forms of leaves can be described considering the
way the blade (lamina) is divided. A simple leaf has an undivided
blade. However, the leaf shape may be formed of lobes, but the
gaps between lobes do not reach to the main vein.
A compound leaf has a fully sub-divided blade, each leaflet of
the blade is separated and possess the secondary vein. Because
each leaflet can appear to be a simple leaf, it is important to
recognize where the petiole occurs to identify a compound leaf.
Q. No. 6. What are the various types of compound leaves?
•
•
Ans. Palmately compound leaves have the leaflets radiating
from the end of the petiole, like fingers of the palm of a hand,
e.g. Cannabis (hemp) and Aesculus (buckeyes).
Pinnately compound leaves have the leaflets arranged along
the main or mid-vein. These are of two types:
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o
o
•
•
odd pinnate: with a terminal leaflet, e.g. Fraxinus (ash).
even pinnate: lacking a terminal leaflet, e.g. Swietenia
(mahogany).
Bipinnately compound leaves are twice divided: the leaflets are
arranged along a secondary vein that is one of several branching
off the rachis. Each leaflet is called a “pinnule”. The pinnules on
one secondary vein are called “pinna”; e.g. Albizia (silk tree).
Trifoliate (or trifoliolate): a pinnate leaf with just three leaflets,
e.g. Trifolium (clover), Laburnum (laburnum).
Q. No. 7. What are the various kinds of petioles? Cite an example
of each type.
Ans. Petiolated leaves have a petiole (leaf stem). Sessile leaves
do not have a petiole, and the blades are directly attached to the
stem. In clasping or decurrent leaves, the blade partially or wholly
surrounds the stem, often giving the impression that the shoot
grows through the leaf. When this is the case, the leaves are called
“perfoliate”, such as in Claytonia perfoliata. In peltate leaves, the
petiole attaches to the blade inside from the blade margin.
In some Acacia species, such as the Koa Tree (Acacia koa), the
petioles are expanded or broadened and function like leaf blades;
these are called phyllodes. There may or may not be normal
pinnate leaves at the tip of the phyllode.
Q. No. 8. Describe leaf vennation and its types in brief.
Ans. The pattern or arrangement of veins in the leaf blade is known
as leaf venation. If there is only one prominent vein in a leaf, it
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is called mid-vein or primary vein; branches from this vein are
called secondary veins. Tertiary veins usually link the secondaries,
forming a ladder-like (scalariform) or netlike (reticulate) pattern.
Most frequent types of venations are as:
•
•
•
•
•
•
Unicostate pinnate venation– The veins arise pinnately
from a single mid-vein and subdivide into vein-lets. These,
in turn, form a complicated network. This type of venation is
typical for dicotyledons.
Multicostate or palmate venation- Here more than one
equally strong veins entering the leaf blade. These are of
following two types:
Three main veins branch at the base of the lamina and
run essentially parallel subsequently, as in Ceanothus. A
similar pattern (with 3-7 veins) is especially conspicuous in
Melastomataceae.
Several main veins diverge from near the leaf base where the
petiole attaches, and radiate toward the edge of the leaf, e.g.
Acer (maples).
Parallel-veined– veins run parallel for the length of the
leaf, from the base to the apex. Commissural veins (small
veins) connect the major parallel veins. Typical for most
monocotyledons, such as grasses.
Dichotomous – There are no dominant bundles, with the
veins forking regularly by pairs; found in Ginkgo and some
pteridophytes.
Q. No. 9. Name the group of plants in which the following types
of leaves are found:
fronds, needles, standard leaves, microfhylls and
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sheathed leaves.
Ans.
•
Ferns have fronds
•
Conifer leaves are typically needle-, awl-, or scale-shaped
•
Angiosperm (flowering plant) leaves: the standard form
includes stipules, a petiole, and a lamina
•
Lycophytes have microphyll leaves.
•
Sheath leaves (type found in most grasses)
•
Other specialized leaves (such as those of Nepenthes)
Q. No. 10. Compare the following leaf arrangements:
a. Alternate
b. Opposite
c. Whorled
Ans.
Alternate – Here the leaves are borne singly and are
usually arranged in spiral pattern along the stem. Alternate
leaves are some times places along the two sides of the stem
(2-ranked or distichous), or on three sides of the stem (3-ranked
or tristichous).
Opposite – In contrast two leaves are borne on opposite sides at
each node on the stem. Opposite leaves may be spiraled, as in red
Mangroves (Rhizophora); 2-ranked as in many Zygophyllaceae;
or decussate (the leaves of adjacent nodes rotated at 90°. The
decussate condition is the most common arrangement among
vascular plant species.
Whorled – Three or more leaves are attached at each node on
the stem. As with opposite leaves, successive whorls may or may
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not be decussate. Opposite leaves may appear whorled near the
tip of the stem.
Rosulate – leaves form a rosette
Q.No. 11. Describe the epidermal complex of leaf.
Ans. The epidermis includes several differentiated cell types such
as: epidermal cells, epidermal hair cells (trichomes), stomata
complex; guard cells and subsidiary cells. The epidermal are
typically more elongated in the leaves of monocots than in those
of dicots.
The epidermis is studded with pores called stomata, part of a
stoma complex consisting of a pore surrounded on each side by
chloroplast-containing guard cells, and two to four subsidiary cells
that lack chloroplasts. Opening and closing of the stoma complex
regulates the exchange of gases and water vapor between the
outside air and the interior of the leaf and plays an important
role in allowing photosynthesis without letting the leaf dry out.
In a typical leaf, the stomata are more numerous on the abaxial
(lower) epidermis than on the adaxial (upper) epidermis and more
numerous in plants from cooler climates.
The epidermis is usually transparent (epidermal cells lack
chloroplasts) and coated on the outer side with a waxy cuticle
that prevents water loss. The cuticle is in some cases thinner on
the lower epidermis than on the upper epidermis, and is generally
thicker on leaves from dry climates as compared with those from
wet climates.
Q. No. 12. What is the role of mesophyll in leaves?
Ans.
The
mesophyll
cells
are
responsible
for
photosynthesis
because
they
contain
chloroplasts.
The spongy mesophyll, together with the intercellular air spaces,
allow for the interchange of gases.
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Q. No. 13. Describe briefly spongy and palisade parenchyma.
Ans. The upper palisade layer is a tightly packed layer composed
of vertically elongated cells, one to two cells thick, directly beneath
the adaxial epidermis. Its cells contain many more chloroplasts
than the spongy layer. These long cylindrical cells are regularly
arranged in one to five rows. Cylindrical cells, with the chloroplasts
close to the walls of the cell, can take optimal advantage of light.
Beneath the palisade layer is the spongy layer. The cells of
the spongy layer are more rounded and not so tightly packed.
There are large intercellular air spaces. These cells contain fewer
chloroplasts than those of the palisade layer. The pores or stomata
of the epidermis open into sub-stomatal chambers, which are
connected to the air spaces of the spongy layer.
Q. No.14. What is aerenchyma?
Ans. The parenchymatous tissue which encloses the air spaces
is called aerenchyma. It provides the bouncy to the plants. In
aquatic and marsh plants the two different layers of the mesophyll
are absent instead for their gaseous exchanges they use a
homogeneous aerenchyma (thin-walled cells separated by large
gas-filled spaces).
Q. No.15. Describe briefly the leaf vasculature.
Ans. The veins are the vascular tissue of the leaf and are located
in the spongy layer of the mesophyll. The veins are made up of:
•
•
Xylem: vascular tissue that bring water and minerals from the
roots into the leaf.
Phloem: vascular tissue that usually move sap, with dissolved
sucrose, produced by photosynthesis in the leaf, out of the leaf.
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The xylem typically lies on the adaxial side of the vascular
bundle and the phloem typically lies on the abaxial side. Both
are embedded in a dense parenchyma tissue, called the pith
or sheath, which usually includes some structural collenchyma
tissue.