9. Morphology of Flowering Plants - part 02 - STEM

 

9. Morphology of Flowering Plants - part 02 - STEM

Stem : 

• The aerial part of the plant body is know as shoot system. 
  
• Stem is the main axis of this shoot system. 
• Stem is the ascending part of the plant body which develops from plumule and reproductive units and is differentiated into nodes and internodes. 
• It is usually positively photorophic, negatively geotropic and negatively hydrotropic. 
• It shows different types of buds (axillary, apical, accessory, etc.).
• At nodes it produces dissimilar organs such as leaves and flowers and similar organs such as branches. 
• Young stem is green and capable of photosynthesis.
• The primary functions of the stem are -

1. To produce and support branches, leaves, flowers and fruits; 
2. conduction of water and minerals and 
3. transportation of food to plant parts.

Modifications of stem : 

• Stem develops some modifications for additional or accessory functions. To perform such function stem shows different modifications :

a. Underground stem : 

• In some herbaceous plants stem develops below the soil surface called as underground stem. 
• Underground stem remains dormant during unfavourable condition and on the advent of favourable condition produces aerial shoots. 
• Underground stem is known to store food, helps in perinnation and vegetative propagation.

1. Rhizome : 

• It is prostrate dorsiventrally thickened and brownish in colour. 
• It grows either horizontally or obliquely beneath the soil.
• Rhizome shows nodes and internodes, bears terminal and axillary buds at nodes. 
• Terminal bud under favourable conditions produces aerial shoot which degenerates at the end of favourable condition.
• Growth of rhizome takes place with lateral buds such growth is known as sympodial growth. 
• e.g. Ginger (Zingiber officinale), Turmeric (Curcuma domestica), Canna etc. 
• In plants where rhizomes grows obliquely, terminals bud brings about growth of rhizomes. This is known as monopodial growth. 
• e.g. Nymphea, Nelumbo (Lotus), Pteris (Fern) etc.

2. Stem Tuber : 

• Special underground branches of stem at their tips become swollen due to storage of food material which is mostly starch.
• Presence of distinct nodes but not internodes classifies tuber as stem. 
• At nodal part scale leaves are present with axillary buds commonly known as ‘eyes’. 
• ‘Eyes’ can produce aerial shoots under favourable conditions. 
• Tubers are porpogated vegetatively e.g. Potato (Solanum tuberosum), Matalu (H elianthus tuberosus).
• Tuber has two distinct ends viz. apical end and basal end called as rose and heel end respectively. 
• The number of nodes and eyes is more towards rose end.

3. Bulb : 

• Bulb is an underground spherical or pyriform stem. 
• Stem is highly reduced and discoid. It bears a whorl of fleshy leaves. 
• The scale leaves or fleshy leaves show concentric arrangement over the stem. These store food material. 
• Some outer scale leaves become thin and dry. 
• The reduced stem produces adventitious roots at its base. 
• The bulb is of different types Tunicated or layered bulb is made up of fleshy leaves arranged in concentric manner with outer dry scale leaf. 
• e.g. Onion.
• In garlic the bulb is scaly or non-tunicatied.The fleshy scales are arranged in overlapping pattern.

4. Corm : 

• Corm is swollen underground spherical or subspherical vertically growing stem. 
• It is condensed structure with circular or ring like nodes. 
• Presence of axillary buds and scales is observed. 
• Adventitious buds are produced which help in vegetative propagation.
• Adventitious roots are produced at lower part of stem 
• e.g. Colocasia (Arbi), Amorphophallus (Zamikand or Elephant foot) etc.

b. Sub aerial stem : 

• The stems are generally weak or straggling stems growing over the ground and need support for perpetuation.
• Sometimes these stems are found to grow beneath the soil surface also. Thus they show contact with both air and soil. 
• Sub aerial stems are meant for perennation and vegetative propagation. 
• Scale leaves and axillary buds are present over stem surface. The later produces aerial shoots.

Different types of sub aerial shoots ::

1. Trailer : 

• The shoot spreads over the ground without intervals. 
• The branches are either flat i.e. procumbent or partly vertical i.e. documbent
• e.g. Euphorbia, tridax etc.

2. Runner : 

• They are special narrow, prostrate or horizontal green branches which develop at the base of erect shoots known as crown.
• Runners spread in all directions to produce new crowns with bunch of adventitious roots.
• Presence of nodes with scale leaves and axillary buds is observed. 
• Eg. Cynodon (Lawn grass) Centella (Hydrocotyl), Oxalis etc.

3. Stolons : 

• The slender lateral branch arising from the base of main axis is known as stolon. 
• In some plants it is above ground (wild strawberry). 
• Primarily stolon shows upward growth in the form of ordinary branch, but when it bends and touches the ground terminal bud grows into new shoot and adventitious roots 
• e.g. Jasmine, Mentha etc.

4. Sucker :

•  It is non green runner like branch of stem. It which develops from underground base of roots. 
• It grows horizontally below soil and finally comes above the soil surface to produce a new plant. 
• Sucker can be termed as underground runner 
• eg. Chrysanthemum, Banana etc.

5. Offset : 

• These are one internode long runners in rosette plants at ground or water level. 
• Offset helps in vegetative propagation
• e.g. Water hyacinth or Jal kumbhi (Eichhornia) and Pistia.

c. Aerial modification : 

• Stem or it's vegetative part modify to carry out specialized functions.
• They develop various modifications for this purpose. Such modified stems are called as metamorphosed stems.

Different  modifications  :

1. Thorn : 

• It is modification of apical or axillary bud. 
• Thorn is hard pointed and mostly straight structure (except Bougainvillea where it is curved and useful for climbing) 
• It provides protection against browsing animals and also helps in reducing transpiration. 
• Apical bud develops into thorn in Carrisa whereas axillary bud develops into thorn in Duranta, Citrus, Bougainvillea, etc

2. Phylloclade : 

• Modification of stem into leaf like photosynthetic organ is known as phylloclade. 
• Being stem it possesses nodes and internodes. 
• It is thick, fleshy and succulent, contains mucilage for retaining water 
• e.g.Opuntia, cylindrical in Casuarina and ribbon like in Muehlenbeckia.

3. Cladodes :

• The branches of limited growth i.e. one internode long and performing photosynthetic function are called as cladodes.
• True leaves are reduced to spine or scales.
• E.g.Asparagus.

4. Cladophylls :

• These are leaf like structures bore in the axil of scale leaf. 
• It has floral bud and scale leaf in the middle i.e. upper half is leaf and lower half is stem. 
• e.g. Ruscus.

Stem tendrils :

• Tendrils are thin, wiry, photosynthetic, leafless coiled structures. 
• They give additional support to developing plant. 
• Tendrils have adhesive glands for fixation.
• Apical bud in Vitis quadrangularis gets modified in to tendril. The further growth is carried out by axillary bud. This branching pattern is termed dichotomous.
• Axillary tendril in Passiflora axillary bud gets modified in tendril.
• Extra axillary bud is the one which grows outside the axil. This bud in cucurbita gets modified in to tendril.
• Normally floral buds are destined to produced flowers.But in plants like Antigonon they produce tendrils.

5. Bulbils : 

• In plants like Agave, Dioscorea etc. axillary bud becomes fleshy and rounded due to storage of food called as bulbil. 
• When it falls off it produces new plant and help invegetative propagation.