Modications of Aerial Stem
- Aerial stem modifications can be recognised by their – differentiation of nodes
- axillary position – formation of flowers
- exogenous origin – presence of leaves or buds
- occurrence of occasional branching – internal structure
Aerial stem
modications
Stem thorns
- They are stiff, sharp structures
which have lost their growing
point and become hard. Thorns
not only reduce transpiration but
also check browsing by animals.
E.g., axillary stem thorns occur
in Citrus, Bougainvillea, etc. Fig.:Stem thorn
Thorn
Cladodes
- They are green stems of limited
growth (usually one internode
long) which have taken over the
function of photosynthesis from
the leaves. The true leaves are
reduced to scales or spines, e.g.,
Ruscus aculeatus, Asparagus, etc.
Fig.:Cladode ofRuscus
Cladode
Thalamus
- Flower is a specialised reproductive
shoot which possesses a highly
condensed axis called thalamus or
torus. Thalamus bears four types
of floral organs (sepals, petals,
stamens and carpels), each from
their own nodes.
Thalamus
Sepal
Ovary
Stigma Style
Anther
Filament
Petal
Fig.:Thalamus of flower
Phylloclades
- Plants growing in dry and xerophytic
conditions have modified green
stems of unlimited growth called
as phylloclades which reduce
the transpiration. E.g., Opuntia,
Euphorbia royleana, etc.
Phylloclade
Flower
Fig.:Phylloclades ofOpuntia
Stem
Node
Internode
Leaf spine
Stem tendrils
- Tendrils are thread-like sensitive structures
which can coil around a support and
help the plant in climbing. They may be
branched or unbranched. Stem tendrils
may be axillary, (e.g., Passiflora), extra-
axillary, (e.g., Cucurbita, Luffa), leaf
opposed, (e.g., grapevine), etc.
Leaf opposed
stem tendril
Sympodial
axis
Scale leaf
Fig.: Stem tendril of grape vine
Modications of Underground Stem
- The underground or subterranean stem lies below the surface of soil and is non-green. It bears buds and roots as well
as aerial shoots or leaves at intervals during favourable seasons. It stores food and takes part in vegetative propagation
of plants.
Dierentiation between underground stem and roots
Despite being non-green and underground, the underground stem can be differentiated from roots by presence of following
characteristics:
- Absence of root cap and root hair
- Presence of terminal bud
- Presence of nodes and internodes
- Occurrence of foliage or scale leaves and axillary buds on nodes
- Exogenous branching