BOTANY Higher Secondary Second Year - Textbooks Online

2.4. Anatomy of a dicot and monocot leaves
Leaves are very important vegetative organs because they are mainly
concerned with photosynthesis and transpiration. Like stem and roots,
leaves also have the three tissue systems - dermal, ground and vascular.
The dermal tissue system consists of an upper epidermis and lower
epidermis. Stomata occur in both the epidermis but more frequently in the
lower epidermis. The ground tissue system that lies between the epidermal
layers of leaf is known as mesophyll tissue. Often it is differentiated into
palisade parenchyma on the adaxial (upper) side and spongy parenchyma
on the abaxial (lower) side.
A leaf showing this differentiation in mesophyll is designated as
dorsiventral. It is common in dicot leaves. If mesophyll is not differentiated
like this in a leaf (i.e., made of only spongy or palisade parenchyma) as in
monocots, it is called isobilateral. The mesophyll tissue, especially spongy
parenchyma cells enclose a lot of air spaces. The presence of air spaces is
a special feature of spongy cells. They facilitate the gaseous exchange
between the internal photosynthetic tissue (mesophyll) and the external
atmosphere through the stomata.
The vascular tissue system is composed of vascular bundles. They
are collateral and closed. The vascular tissue forms the skeleton of the
leaf and they are known as veins. The veins supply water and minerals to
the photosynthetic tissue. Thus the morphological and anatomical features
of the leaf help in its physiological functions.
Anatomy of a dicot leaf - Sunflower leaf
Internal structure of dicotyledonous leaves reveals epidermis, mesophyll
and vascular tissues.
Epidermis
A dicotyledonous leaf is generally dorsiventral. It has upper and
lower epidermis. The epidermis is usually made up of a single layer of
cells that are closely packed. The cuticle on the upper epidermis is thicker
than that of lower epidermis. The minute openings found on the epidermis
are called stomata. Stomata are more in number on the lower epidermis
than on the upper epidermis. A stoma is surrounded by a pair of bean
shaped cells called guard cells.
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