Shoot system (Stem)

The plumule of the embryo of a germinating seed grows into stem. The epicotyl elongates after embryo growth into the axis (the stem) that bears leaves from its tip, which contain the actively dividing cells of the shoot called apical meristem. Further cell divisions and growth result in the formation of mass of tissue called a leaf primordium. The point from which the leaf arises is called node. The region between two adjacent nodes is called internode.

I. Characteristic features of the stem

1. The stem is usually the aerial portion of the plant

2. It is positively phototropic and negatively geotropic

3. It has nodes and internodes.

4. Stem bears vegetative bud for vegetative growth of the plant, and floral buds for reproduction, and ends in a terminal bud.

5. The young stem is green and thus carries out photosynthesis.

6. During reproductive growth stem bears flowers and fruits.

7. Branches arise exogenously

8. Some stems bears multicellular hairs of different kinds.

II. Functions of the stem

Primary functions

1. Provides support and bears leaves, flowers and fruits.

2. It transports water and mineral nutrients to the other parts from the root.

3. It transports food prepared by leaves to other parts of the plant body.

Secondary functions

•            Food storage- Example: Solanum tuberosum, Colocasia and Zingiber officinale

•            Perennation / reproduction – Example: Zingiber officinale, Curcuma longa

•            Water storage – Example: Opuntia

•            Bouyancy – Example: Neptunia

•            Photosynthesis – Example: Opuntia, Ruscus, Casuarina, Euphorbia, Caralluma.

•            Protection – Example: Citrus, Duranta, Bougainvillea, Acacia, Fluggea, Carissa.

•            Support   -   Example:   Passiflora, Bougainvillea, Vitis, Cissus quadrangularis.

1.  Buds

Buds are the growing points surrounded by protective scale leaves. The bud primordium matures into bud. They have compressed axis in which the internodes are not elongated and the young leaves are closed and crowded. When these buds develop, the internodes elongate and the leaves spread out. Buds have architecture identical to the original shoot and develop into lateral branches or may terminate by developing into a flower or inflorescence. Based on Origin Buds are classified into (a) Terminal or Apical bud (b) Lateral or Axillary or Axil bud. Based on Function Buds classified into (a) Vegetative bud (b) Floral or Reproductive bud

1.        Terminal bud or apical bud: These buds are present at the apex of the main stem and at the tips of the branches.

2.        Lateral bud or Axillary bud: These buds occur in the axil of the leaves and develop into a branch or flower.

2.        Extra axillary bud : These buds are formed at nodes but outside the axil of the leaf as in Solanum americanum.

3.        Accessory bud : An extra bud on either side (collateral bud) or above (superposed bud or serial bud) the axillary bud. Example: Citrus and Duranta

4.        Adventitious buds: Buds arising at any part other than stem are known as adventitious bud. Radical buds are those that arises from the lateral roots which grow into plantlets. Example: Millingtonia, Bergera koenigii (Murraya koenigii), Coffea arabica and Aegle marmelos. Foliar buds are those that grow on leaves from veins or from margins of the leaves. Example: Begonia (Elephant ear plant) and Bryophyllum (Sprout leaf plant). Cauline buds arise directly from the stem either from cut, pruned ends or from branches. Adventitious buds function as propagules which are produced on the stem as tuberous structures. Example: Dioscorea, Agave.

5.        Bulbils (or specialized buds) : Bulbils are modified and enlarged bud, meant for propagation. When bulbils detach from parent plant and fall on the ground, they germinate into new plants and serve as a means of vegetative propagation. In Agave and Allium proliferum floral buds get modified into bulbils. In Lilium bulbiferum and Dioscorea bulbifera, the bulbils develop in axil of leaves. In Oxalis, they develop just above the swollen root.

2.  Types of Stem

Majority of angiosperm possess upright, vertically growing erect stem. They are (i) Excurrent, (ii) Decurrent, (iii) Caudex, (iv) Culm.

i.  Excurrent

The main axis shows continuous growth and the lateral branches gradually becoming shorter towards the apex which gives a conical appearance to the trees. Example: Polyalthia longifolia, Casuarina.

ii.  Decurrent

The growth of lateral branch is more vigorous than that of main axis. The tree has a rounded or spreading appearance. Example: Mangifera indica, Azadirachta indica, Tamarindus indicus, Aegle marmelos

iii.  Caudex

It’s an unbranched, stout, cylindrical stem, marked with scars of fallen leaves. Example: Cocus nucifera, Borassus flabelliformis, Areca catechu

iv.  Culm

Erect stems with distinct nodes and usually hollow internodes clasped by leaf sheaths. Example: Majority of grasses including Bamboo.

3.  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 Artabotrys inflorescence 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 Carissa apical 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.