Botany session 3

© Copyright PCNM 2011
Botany and Pharmacognosy
Session 3

Review
Last week we covered:
Angiosperm morphology
Stems
Roots

This Session
During this session we will
cover:
Leaves

Leaves
• Function:
• The main functions of
leaves are:
• Photosynthesis
• Transpiration.

Leaves
• Photosynthesis is the process by
which carbon dioxide from the air and
water drawn from the ground by the
roots are converted into sugar and
oxygen, in the presence of light
absorbed by chlorophyll in the
chloroplasts of leaves and other green
structures.
• The oxygen is released into the
atmosphere; the sugar can be used for
immediate energy production or some
biosynthetic pathways. If not used
immediately it is converted to starch
and stored in different parts of the
plant.

Leaves
• To maximise photosynthesis leaves
are arranged so that they receive
sunlight. The water and nutrients
from the roots are carried in the
plantís vascular tissue of the stem
through the petiole into the veins of
the leaf and then to all the tissues of
the leaf.
• Carbon dioxide is drawn in and
oxygen given out through pores
known as stomata on the surfaces of
the leaf. In general, the underside of
leaves contain more stomata than
the upper surface.

Leaves
• Leaves are typically broad and flat.
This large surface area allows
maximum sunshine to fall on it, and
allows for rapid diffusion of carbon
dioxide and oxygen through all the
tissues of the leaf.

Leaves
• Transpiration is the process
whereby liquid water contained
in plants is converted to water
vapour and released to the
atmosphere. It is difficult to
separate the processes of
transpiration and evaporation,
so sometimes the entire process
is called evapotranspiration.
Evapotranspiration is a vital part
of the water cycle, also known
as the hydrological cycle,
moving water from the soil to
the atmosphere.

Leaf Structure
• In dicotyledons, a typical leaf consists
of a blade or lamina, the flat portion of
the leaf, and a petiole, the stalk
attaching the blade of the leaf to the
stem of the plant. Some dicot leaves do
not have a petiole, the blade arises
directly from the stem; they are said to
be sessile. Some dicot leaves have
two stipules which are small leaf-like
outgrowths on either side of the stem
end of the petiole. Grapefruit, roses
and clover plants possess stipules. In
some plants the stipules are modified
to form tendrils or spines. The West
Australian native prickly Moses, Acacia
pulchella, has stipules modified to
spines.

Leaf Structure
• The petiole has vascular bundles or veins running through it to the
blade. Dicot leaves have a main vein, the mid-rib, with smaller
branches arising from the midrib. These branch again and again,
forming a network of veins. This is net or reticulate venation.
• Monocotyledons have a different leaf structure to dicotyledons.
• In most monocots, a sheathing leaf base, the leaf sheath, replaces
the petiole. The leaf sheath wraps around the stem and is attached
to the stem at the node.
• The leaf blade of the monocot is usually long and narrow with
parallel veins. These veins are usually about the same size. There
are veins connecting the parallel veins but they are very small and
not easily seen.

Leaf Structure
• At the junction of the leaf blade and the leaf sheath there is a small
tongue-like outgrowth, the ligule. Projecting from the base of the leaf
blade are the auricles. Auricles vary in size and shape and are
absent in some grasses.

Checkpoint!

Simple and compound leaves
• Leaves are either simple or compound. The blade of a compound
leaf is divided into several leaflets. The main stem or axis of the
compound leaf is called the rachis. The individual ëleavesí are
called leaflets. Leaflets are easily pulled off individually. Roses have
compound leaves, as does the pecan tree.
• The blade of a simple leaf is all one structure. It may be in one
piece, such as a:
• camellia or a eucalyptus tree, or it may be divided into lobes. The
divisions of a divided simple leaf do not go all the way down to the
midrib. Lobes of a divided simple leaf may be very variable on the
same leaf and on different leaves of the same plant. Unlike the
leaflets of a compound leaf, lobes are not easily removed.
Tomatoes, grevillea and marigolds have divided simple leaves.

Types of compound leaves
• There are two main categories of compound leaves, pinnate and
palmate.
• Pinnate compound leaves have leaflets arising along the length of
the rachis. Wisteria and cassia have pinnate compound leaves.
• Palmate compound leaves have leaflets arising from the same point,
at the apex of the petiole. Palmate leaves are found on clover,
strawberries and lupins.
• Leaves can be described according to their arrangement on the
stem, their shape, their margin, apices and bases as well as their
surfaces. For the purposes of identification of herbs and other plants
it is necessary to be familiar with the terms used. These are covered
in the text, section 4.5, figures 4.10-4.14

Modified leaves
• In some plants leaves are highly modified for specific purposes:
• Leaf tendrils are modified leaves or parts of leaves which coil round
to hold a plant up. Note that we have previously examined tendrils
from modified buds. Leaf tendrils can vary in appearance depending
on which part of the leaf has been modified.
• Spines are slender, sharp structures. Remember that thorns are
modified shoots but spines are modified leaves. Spines are often
found on cacti where they provide defence for the plant but also
water conservation, as transpiration is greatly reduced. The prickly
pear is noted for its spines.

Bud scales and scale leaves
• The overlapping scales that protect young apices in vegetative and
floral buds are modified leaves. Bud scales are often tough and
fleshy. They can be seen on deciduous plants in the winter before
budburst.
• Scale leaves found on underground rhizomes are usually thin, dry
and brown in colour, but are still modified leaves.
• Bulb scales are fleshy modified leaves or leaf bases found in bulbs,
as in onions and garlic. Within the axils of these fleshy scales are
axillary buds that can grow into new plants.
• A phyllode is a leaf-like structure modified from a petiole. Australian
acacias show phyllode development. These shrubs and trees only
produce true bipinnate leaves at the seedling stage, after which they
produce phyllodes.

Modifications for special nutrition
• Some plants living in areas with very nutrient-deficient soils have
leaves modified to trap insects which are then digested to provide
food for the plant. The pitcher plants have leaves modified into a
receptacle with a lid. When an insect ventures into the pitcher it
drowns in the small amount of liquid in the pitcher. The body is then
digested, either by bacterial action or by enzymes secreted by the
plant.
• The Drosera species, the sundews, have leaves covered with many
glandular hairs which secrete a sticky substance. These glitter in the
sun and attract insects which then become trapped on the leaves
and digested.
• The Venus flytrap has modified leaves which can snap shut when
an insect lands on them. The insect is then digested.

Inflorescences
• Angiosperm flowers can be solitary, that is, borne singly. They arise
from the shoot tip or a leaf axil. They may be on a stalk, the pedicel,
or they may be sessile and not have a stalk. Solitary flowers are
often large. Examples are violas and pansies, petunias, hibiscus
and fuchsias.
• An inflorescence is a cluster of flowers arranged in a specific way on
the same main stalk. Strictly speaking, it is part of the shoot of seed-
bearing plants where flowers are formed and which is accordingly
modified. The modifications can involve the length and the nature of
the internodes, and the phyllotaxis (the arrangement of the leaves
on the plant stem) as well as other variations.

Inflorescences
• The stem holding the whole inflorescence is called a peduncle. The
main stem holding the flowers or more branches within the
inflorescence is the rachis. The stalk of each individual flower is the
pedicel. A flower without a pedicel is said to be sessile.
• When an inflorescence reaches the fruiting stage, after flowering, it
is called an infructescence.
• Inflorescences are described by many different characteristics,
including how the flowers are arranged on the peduncle, the
blooming order of the flowers, and how different clusters of flowers
are grouped. It should be noted however that some plants can have
a combination of types.

Bracts
• Bracts are modified leaves which appear different to the leaves on
the rest of the plant. A bract is usually located at the node where the
main stem of the inflorescence forms, joined to the main stem of the
plant. Other bracts, often but not always smaller than the first bracts,
are found just below each flower in the inflorescence. They are often
called bracteoles. These bracteoles may be of different shapes,
sizes and colour.

Bracts
• In Bougainvillea the
bracteoles are large and
brightly coloured, and are the
most conspicuous part of the
inflorescence.

Bracts
• Inflorescences may develop at the tip of a shoot from a terminal bud,
in which case they are called terminal inflorescences, or arise from
the axil of a leaf. Inflorescences arising from the axil of a leaf are
axillary inflorescences.
• Some plants may develop both types of inflorescences.
• Some inflorescences have the oldest flowers opening at the bottom
and younger flowers opening as they ascend, so that the youngest
flowers are opening at the top of the inflorescence. Sometimes there
are young flowers opening at the tops, mature flowers in the middle
and fruit already set at the base of the main axis. Members of the
Fabaceae family, the legumes, have indeterminate inflorescences,
as do gladioli. Indeterminate inflorescences are also known as
racemose inflorescences. There are many types of racemose
inflorescences. See the set text, for the different types and plant
examples of each.

Bracts
• Determinate inflorescences are
also known as cymose
inflorescences. The oldest flowers
are at the tip of the main axis and
the younger flowers open below
them. The set text shows the
variations of determinate
inflorescences.

Summary
Today we have covered;
• Leaves

Next Session
We will cover:
Flowers & Fertilization

Preparation
Brief Notes
• Don’t forget to log on to the LMS and download and print off your
brief notes and handouts for the next session.

Botany session 3

More Related Content

Botany session 3