Plant Stem : Modification Of Stem

1. Aerial Modification

❐ In certain plants, the aerial stem or buds get modified to perform special functions like climbing, protection, food storage, vegetative propagation etc.

The various aerial modifications are as follows:

i) Stem tendrils:
❐ It is a modification of stem in which axillary bud modifies to form a thin, wiry, and highly sensitive structure called tendril.
❐ Tendrils help the plant to attach itself to the support and climb.
❐ The tendrils are leafless, coiled structures with sensitive adhesive glands for fixation.

Stem tendrils are of four types:
(a) Axillary tendrils­e.g., Passiflora
(b) Extra­axillary tendrils­e.g., Cucurbita, Luffa
(c) Leaf­opposed or Apical bud tendrils, e.g., Grapevine (Vitis)
(d) Floral bud or Inflorescence tendrils – e.g., Antigonon, Cardiospermum (Balloon vine)

ii)Thorn
❐ Thorn is a hard, pointed usually straight structure produced by modification of axillary bud.
❐ Leaves, branches and flowers are developed on thorns at the nodes, indicating that it is a modified stem.
❐ It provides protection against browsing animals
❐ Thorns are used as organs of defense or climbing E.g. Citrus, Bougainvillea, Duranta etc.
❐ In Carrisa, apical bud is modified into thorn.

iii) Phylloclades
❐ The phylloclade is a stem which gets transformed into leaf like structure.
❐ The phylloclade is green, flattened structure with distinct nodes and internodes.
❐ It is thick, fleshy and succulent, in Opuntia or Nagphani,
❐ Cylindrical in Casuarina and Euphorbia tirucalli and ribbon like in Muehlenbeckia.
❐ In xerophytes, leaves get modified into spines or get reduced in size to check the loss of water due to transpiration and thus stem takes up the function of leaf, i.e. photosynthesis.

iv) Cladode
❐ These are green branches of limited growth (usually one internode long) which have taken up the function of photosynthesis.
❐ True leaves are reduced to scales or spines, - e.g. Asparagus

v) Bulibls
❐ When axillary bud becomes fleshy and rounded due to storage of food, it is called bulbils.
❐ It gets detached from the plant, falls on ground and develops into a new plant, - e.g. Dioscorea

2. Sub-Aerial Modification

❐ A plant grows horizontally on the ground and gives off root at each node E.g. Grasses
❐ These fall into four categories
i) Runner
❐ Modified sub aerial stems
❐ Run horizontally on surface of soil
❐ Runner has one or more nodes. The nodes bear scale leaves and axillary buds E.g. Lawn Grass ( Cynodon dactylon)
ii) Suker
❐ Suker is a runner like non-green branch in the underground part of stem
❐ Arise from underground part of plant
❐ Grows obliquely upwards and gives rise to a new plants E.g. Mentha.
❐ The sucker can therefore be called underground runner
iii) Stolon
❐ Stolon is a slender lateral branche that arise from the base of the main stem
❐ Initially stolon grows upwards like an ordinary branch and then bends down and touches the soil where its terminal bud gives rise to a new shoot and adventitious roots E.g. Straberry, Jasmine
iv) Offset 
❐ Its shorter and thicker than runner
❐ Its commonly called the runner of aquatic plants
❐ It helps in the vegetative propagation in aquatic plants
❐ E.g. Pistia, Jalkumbhi, Water Hyacinth

3. Underground Stem

❐ Many plants produce underground stems for perennation and food storage. They produce aerial shoots annually.
❐ Underground stems perform the function
❐ Storage of food
❐ Perennation and vegetative propagation

They can be differentiated from roots by:
❍ Stem like internal structre
❍ Exogenous branching
❍ Presence of nodes and internodes
❍ Occurrence of foliage leaves or scale leaves at the nodes with axillary buds
❍ Absence of root cap

a) Rhizome
❐ Its a thick prostrate and branched stem growing horizontally beneath the soil surface
❐ It has distinct nodes and internodes.
❐ The nodes bear small scale leaves
❐ Axillary bud is present in the axil of scale leaf
❐ The lower surface of the nodes gives out small slender adventitious roots
❐ Arise with the help of lateral bud this type of rhizome is called Sympodial rhizome E.g. Zingiber officinale(Ginger) Curcuma domestica (Turmeric)
❐ In some plant, growth of rhizome occurs with the help of terminal bud these are called monopodial rhizome E.g. Lotus, Pteris(a fern)

b) Tuber
❐ Its the swollen tip of the underground branch.
❐ The growth of these branches is retarted, hence the tips become swollen due to accumulation of food materials
❐ The tubers are rounded or ovel shapes
❐ The tubers show nodes and internodes bear scale leaves with axillary buds, commonly called as eyes.
❐ Under favorable conditions these eyes sprout and produce aerial shoots.
❐ Thus tubers helps in vegetative propagation.
❐ Tubers do  not produce adventitious roots, thus they differ from rhizomes E.g. Potato (Solanum tuberosum)

c)Corm
❐ Corm is a condensed from of rhizome growing vertically down into the soil.
❐ Internodes are usually reduced one or more axillary buds are present in the axil of the scale leaves
❐ Some of these buds grow into daughter corms.
❐ Corms bear adventitious roots either at the base or throughout the surface E.g.Crocus sativus(Saffron), Gladiolus

d) Bulb
❐ It is a highly condensed stem, represented by a short convex or slightly conical disc
❐ On its upper surface are present large number of fleshy scale leaves surrounding a terminal bud at the center of the disc
❐ Few Scale leaves bear bud in their axil
❐ The fleshy leaves of the bulb store food in the form of carbohydrates
❐ A few outer leaves which become dry and scaly, are protective in function
❐ Large number od adventitious roots arise from the lower surface
❐ E.g. Allium cepa, Allium Sativum

Related Items Plant Shoot System