Modification of Stem

Modification of Stem 

I.  Aerial modification of stem

1.  Creepers

These are plants growing closer (horizontally) to the ground and produces roots at each node. Example: Cynodon dactylon, Oxalis, Centella

2.  Trailers (Stragglers) 

It is a weak stem that spreads over the surface of the ground without rooting at nodes. They are divided into 3 types,

i.               Prostrate (Procumbent): A stem that grows flat on the ground. Example: Evolvulus alsinoides, Indigofera prostrata.

ii.               Decumbent: A stem that grows flat but becomes erect during reproductive stage. Example: Portulaca, Tridax, Lindenbergia

iii.               Diffuse: Atrailingstemwithspreading branches. Example: Boerhaavia diffusa, Merremia tridentata

3.  Climbers 

These plants have long weak stem and produce special organs for attachment for climbing over a support. Climbing helps to display the leaves towards sunlight and to position the flower for effective pollination.

i.  Root climbers 

Plants climbing with the help of adventitious roots (arise from nodes) as in species of Piper betel, Piper nigrum, Hedera helix, Pothos, Hoya.

ii.  Stem climbers (twiners) 

These climbers lack specialised structure for climbing and the stem itself coils around the support. Example: Ipomoea, Convolvulus, Dolichos, Clitoria, Quisqualis. 

Stem climbers may coil around the support clockwise or anti-clockwise. Clockwise coiling climbers are called dextrose. Example:Dioscorea alata. Anti-clockwise coiling climbers are called sinistrose. Example: Dioscorea bulbifera.

iii.  Hook climbers 

These plants produce specialized hook like structures which are the modification of various organs of the plant. In Artabotrysinflorescence axis is modified into hook. In calamus (curved hook) leaf tip is modified into hook. In Bignonia unguis- cati the leaflets are modified into curved hook (figure: 3.17). In Hugonia the axillary buds modified into hook.

iv.  Thorn climbers 

Climbing or reclining on the support with the help of thorns as in Bougainvillea and Carissa.

v.  Lianas (woody stem climber) 

Woody perennial climbers found in tropical forests are lianas. They twine themselves around tall trees to get light. Example: Hiptage benghalensis, Bauhinia vahlii, Entada pursaetha.

vi.  Tendril climbers 

Tendrils are thread-like coiling structures which help the plants in climbing. Tendrils may be modifications of Stem – as in Passiflora,Vitis and Cissus quadrangularis; Inflorescence axis – Antigonon; Leaf – Lathyrus; Leaflets - Pisum sativum; Petiole – Clematis;Leaftip – Gloriosa; Stipules – Smilax. In pitcher plant (Nepenthes) the midrib of the leaf often coils around a support like a tendril and holds the pitcher in a vertical position.

Phylloclade 

This is a green, flattened cylindrical or angled stem or branch of unlimited growth, consisting of a series of nodes and internodes at long or short intervals. Phylloclade is characteristic adaptation of xerophytes where the leaves often fall off early and modified into spines or scales to reduce transpiration. The phylloclade takes over all the functions of leaves, particularly photosynthesis. The phylloclade is also called as cladophyll. 

Example: Opuntia, Phyllocactus, Muehlenbeckia (flattened phylloclade) Casuarina, Euphorbia tirucalli, Euphorbia antiquorum(cylindrical phylloclade).

Cladode 

Cladode is a flattened or cylindrical stem similar to Phylloclade but with one or two internodes only. Their stem nature is evident by the fact that they bear buds, scales and flowers. Example: Asparagus (cylindrical cladode), Ruscus (flattened Cladode).

Thorns

Thorn is a woody and sharp pointed modified stem. Either the axillary bud or the terminal bud gets modified into thorns. In Carissaapical bud modified into thorns. In Citrus and Atalantia axillary bud is modified into thorns.

II.  Sub aerial stem modifications

Sub aerial stem found in plants with weak stem in which branches lie horizontally on the ground. These are meant for vegetative propagation. They may be sub aerial or partially sub terranean.

1. Runner 

This is a slender, prostrate branch creeping on the ground and rooting at the nodes. Example: Centella (Indian pennywort), Oxalis(wood sorrel), lawn grass (Cynodon dactylon).

2. Stolon 

This is also a slender, lateral branch originating from the base of the stem. But it first grows obliquely above the ground, produces a loop and bends down towards the ground. When touches the ground it produces roots and becomes an independent plantlet. Example:Mentha piperita (peppermint), Fragaria indica (wild strawberry).

3. Sucker 

Sucker develops from a underground stem and grows obliquely upwards and gives rise to a separate plantlet or new plant. Example:Chrysanthemum, Musa, Bambusa.

4. Offset 

Offset is similar to runner but found in aquatic plants especially in rosette leaved forms. A short thick lateral branch arises from the lower axil and grows horizontally leafless for a short distance, then it produces a bunch of rosette leaves and root at nodes. Example:Eichhornia (water hyacinth), Pistia (water lettuce).

III.  Underground stem modifications

Perennial and some biennial herbs have underground stems, which are generally known as root stocks. Rootstock functions as a storage and protective organ. It remains alive below the ground during unfavourable conditions and resumes growth during the favourable conditions.

Underground stems are not roots because they possess nodes, internodes, scale-leaves and buds. Rootstock also lack root cap and root hairs but they possess terminal bud which is a characteristics of stem.

1.  Bulb 

It is a condensed conical or convex stem surrounded by fleshy scale leaves. They are of two types 1. Tunicated (coated) bulb: In which the stem is much condensed and surrounded by several concentric layers of scale leaves. The inner scales commonly fleshy, the outer ones dry. These are two types (a) Simple Tunicated bulb Example: Allium cepa (b) Compound Tunicated bulb. Example:Allium sativum. 2. Scaly bulb: They are narrow, partially overlap each other by their margins only. Example:Tulipa spp.

Pseudobulb is a short erect aerial storage or propagating stem of certain epiphytic and terrestrial sympodial orchids. Example:Bulbophyllum.

2.  Corm

This is a succulent underground stem with an erect growing tip. The corm is surrounded by scale leaves and exhibit nodes and internodes. Example: Amorphophallus, Gladiolus, Colocasia, Crocus, Colchicum

3.  Rhizome

This is an underground stem growing horizontally with several lateral growing tips. Rhizome posses conspiquous nodes and internodes covered by scale leaves. Example: Zingiber officinale, Canna, Curcuma longa, Maranta arundinacea, Nymphaea, Nelumbo.

4.  Tuber 

This is a succulent underground spherical or globose stem with many embedded axillary buds called “eyes”. Example: Solanum tuberosum, Helianthus tuberosus

IV.  Stem Branching

Branching pattern is determined by the relative activity of apical meristems. The mode of arrangement of branches on a stem is known as branching. There are two main types of branching, 1. Lateral branching and 2. Dichotomous branching. Based on growth pattern stems may show indeterminate or determinate growth.

(a)     Indeterminate: The terminal bud grows uninterrupted and produce several lateral branches. This type of growth is also known as monopodial branching. Example: Polyalthia, Swietenia, Antiaris.

(b)    Determinate: The terminal bud caese to grow after a period of growth and the further growth is taken care by successive or several lateral meristem or buds. This type of growth is also known as sympodial branching. Example: Cycas.