Morphology of Flowering Plant

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:
• Reduce transpiration
• Provide protection against browsing animals
• Examples:
• Apical bud develops into thorn:Carrisa
• Axillary bud develops into thorn:

  Duranta,Citrus,

  Bougainvillea

Phylloclade

• Structure: Modification of stem into leaf-like photosynthetic organ

• Features:
• Possesses nodes and internodes (being stem)
• Thick, fleshy, succulent
• Contains mucilage for retaining water
• Examples:
• Flattened:Opuntia

• Cylindrical: Casuarina

• Ribbon-like:Muehlenbeckia

Cladodes

• Structure: Branches of limited growth

  (one internode long)

• Function: Photosynthetic function
• Note: True leaves reduced to spine or scales
• Example: Asparagus

Cladophylls

• Structure: Leaf-like structures borne in axil of scale leaf
• Features:
• Floral bud and scale leaf in middle
• Upper half is leaf, lower half is stem
• Example: Ruscus

Stem Tendrils

• Structure: Thin, wiry, photosynthetic, leafless coiled structures
• Function: Provide additional support to developing plant
• Features: Possess adhesive glands for fixation
• Types and Examples:

Axillary tendril (Passiflora): Axillary bud modified into tendril

 

Apical tendril (Vitis quadrangularis): Apical bud modified; further growth by axillary bud (dichotomous branching)

 

Extra-axillary tendril (Cucurbita): Outside axil; modified into tendril

 

Floral bud modified into tendril (Antigonon): Usually floral buds produce flowers, but here produce tendrils.

 

Bulbils

• Structure: Axillary bud becomes fleshy and rounded due to food storage
• Reproduction: When it falls off, it produces new plant
• Function: Vegetative propagation
• Example: Dioscorea

---