Biology Today — January 2018

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