B. Stem

Stem is the ascending part of the plant body that:

Develops from plumule
Bears reproductive units (buds)
Is differentiated into nodes and internodes
Is usually positively phototropic, negatively geotropic, and negatively hydrotropic
Shows different types of buds (axillary, apical, accessory)
At nodes: Produces dissimilar lateral organs (leaves, flowers) and similar organs (branches)
Young stem: Green and capable of photosynthesis

Primary Functions:

Produce and support branches, leaves, flowers, fruits
Conduct water and minerals
Transport food to plant parts

Modifications of Stem

Stems develop modifications to perform additional or specialized functions.

I. Underground Stem (Perennation, Storage, Vegetative Propagation)

In herbaceous plants, stems develop below soil surface and remain dormant during unfavorable conditions. They produce aerial shoots when conditions are favorable.

Rhizome

Appearance: Prostrate, dorsiventrally thickened, brownish
Growth: Horizontally or obliquely beneath soil
Features: Shows nodes, internodes, scale leaves, and terminal and axillary buds at nodes
Buds: Terminal bud produces aerial shoot under favorable conditions (degenerates at end of season)
Growth Pattern:

Sympodial growth: Growth continues with lateral buds (e.g., Ginger, Turmeric, Canna)
Monopodial growth: Terminal bud brings about growth (e.g., Nymphea, Nelumbo/Lotus, Pteris/Fern)

Stem Tuber

Structure: Special underground branches with irregularly swollen tips (due to starch storage)
Distinction: Presence of distinctnodes but NOT internodes
(classifies it as stem)
Features: At nodal parts, scale leaves present with axillary buds called'eyes'
Eyes: Can produce aerial shoots under favorable conditions
Propagation: Vegetative propagation
Examples: Potato (Solanum tuberosum), Matalu (Helianthus tuberosus)
Structure: Two distinct ends:

Rose end (apical): Has more nodes and eyes
Heel end(basal)

Bulb

Structure: Underground, spherical or pyriform stem
Characteristics:

Stem is highly reduced and discoid
Bears whorl of fleshy leaves in concentric arrangement
Outer scale leaves become thin and dry
Reduced stem produces adventitious roots at base
Function: Food storage

Types of Bulbs

Tunicated or Layered Bulb

Fleshy leaves arranged in concentric manner
Outer dry scale leaf present
Example: Onion

Scaly or Compound Tunicated Bulb

Fleshy scales (cloves) arranged in overlapping pattern
Example: Garlic

Corm

Structure: Swollen underground, spherical or subspherical internode of vertically growing stem
Characteristics: Condensed structure with circular or ring-like nodes
Features: Presence of axillary buds and scales
Propagation: Adventitious buds help in vegetative propagation
Roots: Adventitious roots produced at lower part of stem
Examples: Colocasia (Arbi), Amorphophallus (Zamikand/Elephant foot)

II. Sub-aerial Stem (Perennation and Vegetative Propagation)

Weak or straggling stems growing over ground that need support. They show contact with both air and soil. Scale leaves and axillary buds present; latter produce aerial shoots.

Trailer

Growth: Spreads over ground without intervals
Branches:

Procumbent(flat)
Decumbent (partly vertical)

Examples: Euphorbia, Tridax

Runner

Structure: Special narrow, prostrate/horizontal green branches developing at base of erect shoots (crown)
Spread: In all directions
Features:

New crowns
with bunch of adventitious roots produced
Nodes with scale leaves and axillary buds present

Examples: Cynodon (Lawn grass), Centella (Hydrocotyl), Oxalis

Stolon

Structure: Slender lateral branch arising from base of main axis
Growth:

Above ground in some plants (wild strawberry)
Shows upward growth as ordinary branch, then bends to ground
Terminal bud grows into new shoot when touching ground
Adventitious roots develop

Example: Mentha

Sucker

Structure: Non-green runner-like branch developing from underground base of roots
Growth:

Horizontally below soil
Finally emerges above soil to produce new plant

Alternative name: Underground runner
Examples: Chrysanthemum, Banana

Offset

Structure: One internode long runners in rosette plants at ground or water level
Function: Vegetative propagation
Examples: Water hyacinth/Jal kumbhi (Eichhornia), Pistia

III. Aerial Modifications

Stem or its vegetative parts modify to carry out specialized functions. Such modified stems are called metamorphosed stems.

Thorn

Structure: Modification of apical or axillary bud
Features: Hard, pointed, mostly straight (exceptBougainvillea where curved for climbing)
Functions: