stem type

 • A stem:  It develops from the plumule.

 is generally considered to be the central axis of theplant.

 supports the leavesand flowers of a plant.

 has nodes from which new shoots and sometimes new roots canarise.

 is usually found above- ground, but can bemodified and found below-ground as well.The Plant Body: StemsExternal Stem Structures

- Apical meristem - growing point at the tip of

the stem

- Terminal bud - bud at the end of the stem

The apical meristem is the same

type of structure as that found on

the tip of the root, and it is

responsible for growth in the

length of the plant.External Stem Structures (cont.)

 Node = where leaf is attached to stem

 Internode = area between leaves (two nodes)

 Lateral bud = side bud - at node, just above where the leaf is attached

 Bud scales = small protective structures, on outside of terminal and lateral buds

 Leaf scar = Small scar just below the lateral bud from when the leaf falls off of the stem

 Bundle scars: are the broken ends of the vascular bundles passing from the stem into

the leaves. They are identified as small dots, discolorations, or raised bumps on the

surface of the leaf scar.

 Bud scale scar = ring of scars from when the buds sprout each spring & bud scales fall

off

 The distance between bud scale scars represents one year’s growth of the stem.

 Lenticels = small spots on stem, allow a stem to exchange gasesApical Meristem

External Stem Structure

Terminal Bud

Bud Scale

Node

Internode

Axillary/Lateral Bud

Leaf Scar

One Year’s Growth

Lenticel

Bud Scale ScarFUNCTIONS OFSTEMSFUNCTION OFSTEMS

 Stems support leaves and branches. Stems support a display of leaves.

Stems orient the leaves toward the light with minimal overlap among

the leaves.

 Stems transport water andsolutes between roots andleaves.

 Stemsin someplants arephotosynthetic.

 Stemsmaystorematerials necessaryfor life (e.g.,water, starch,sugar).

 In someplants, stemshavebecomeadapted forspecialized functions.Depending upon the presence of mechanical tissues thestems

may be weak, herbaceous, or woody.

1. Weak stems: when the stems are thin and long, they are unable to

stand erect. Theymaybe offollowing types:

(A) Creepers or prostrate stem: when they grow flat on the ground

with or withoutroots. Eg:Grasses

(B) Climbers: these are too weak to stand alone. They climb on the

support with the help of tendrils, hooks, prickles or roots.

Eg: vitis, piperbetel, piper longum.

(C)Twinners: these coil the support and grow further. Theyare thin and

wiry.

Eg: Ipomoea,Phaseolus.creeper

climber

twinner2. Herbaceous or woody stems: these are the normal stems and may be soft or hard or

woody.

Eg: sun flower, sugarcane,Mango.Various types of aerial stemmodifications.

a)Tendrils

b) Thorns

c)Hooks

d) Phylloclade

e) Cladode

f) Bulbila) Tendrils: Wiry, coiled, sensitive structures produced by weak

stemmed plants are called tendrils.

Terminal bud is modified into tendil inVitis,

Axillary bud is modified into tendril inPassiflora.b) Thorns: Hard, woody, pointed structures developed from Terminal

/Axilary bud is called thorn. They give protection.

Eg:-In Bougainvillea–axillarybud is modified into thorns.

In Carissa -terminal bud is modified into pair of thorns.Comparison between Thorns vs Spines vs Prickles:

Spines

Thorns

Definition

Thorns are modified

branches or stems,

Spines are modified

leaves.

Prickles

Prickles

are

extensions of the

plants cortex and

epidermis

(the

outerlayer of the

stem).

Functions

•Protection

•Also serves as a climbing

organ of the plant

Example Bougainvillea

Location

Vascular

Bundles

Thorns can be located

along the branches and

twigs of a tree or they

can be found along the

main trunk

Have vascular bundles

inside

•Protection

•Prevent

transpiration

Many cacti

It usually forms just

below a bud or branch

or on leaf

Have vascular bundles

inside

•Protection

•Function

like

excessive grappling hooks for

stems to climb over

neighboring objects

Roses

Prickles can occur

anywhere on the twig

or

leaf

Are absentc) Hooks: Woody curved sensitive structure formed from Axillary/ Terminal

bud is called Hook.

They help in climbing.

In Hugoniaaxillary bud is modified intohook.

InArtabotrys apicalbudis modifiedinto an inflorescence& peduncle into

ahook.d) Phylloclade: In xerophytes the leaves are modified into scales/spines to reduce transpiration.

The main stem is modified into flattened structure to perform photosynthesis.

Phylloclade is a portion of stem or branch (several nodes and internodes) flattened and

expandedto serve the functionsofa leafEg:A)OpuntiaB)CocolobaC)Casuarina

Cladode is a single node or internode of stem or branch that is flattened and expanded

to serve the functions of a leaf (eg: Asparagus)e) Bulbils: In someplant species the vegetative andfloral budsare modified into

condensed branches. They store food materials. These modified buds are

called bulbils. They

help in vegetative propagation.

Ex: Dioscorea – Bulbils present in the axils of leaves.

Agave&Globba–Bulbilspresent in the axilsvarious types of sub-aerial stem modifications

• In some weak stemmed plants thestem

remains partly underground.

• Thesestems are called sub-aerial stems.

These are four types:

a)Runners

b)Stolons

c)Suckers

d) Offsetsa) Runners: In someweak stemmed plants the stemcreeps on the

soil and adventitious roots develop at every node.

Whenever internodes break off, nodes lead an

independent life. Ex:Oxalisb) Stolons: In some plants the basal branches of the stem grow obliquely

downwards

and

touch

the

soil.

There they produce adventitious roots. These branches are called

stolon. They help in vegetative propagation when they are separated from

mother plant, they live asindependent plants.

Ex:Jasminum,Nerium.c) Suckers: In some plants a part of the stem is underground. The underground

branches grow obliquely upwards from the axillary buds of nodes

present below the soil. These branches produce roots on the

lowersurface are calledsuckers.

Ex:Chrysanthemum,Mentha.Offset: These are found in free floating water plants.

The stem is reduced to a disc like structure.

Many leaves are developed from this stem in rosette manner.

The axillary buds of these leaves develop into short, slender branches of usually

one internodal length and grow horizontally above the water. These branches

are called offsets. When these offsets break-off they develop into newplants.

Ex: EiachhorniaMultipurpose stemmodifications(Underground)

1. The underground stems storefood.

2. They help in vegetativepropagation.

3. They are protected from herbivorousanimals.

4. They act as organs of perennation.

As theyperform all these above characters they are called multipurpose

stem modifications.

They are of followingtypes.

1)Rhizome

2)Corm

3) Stemtuber

4)BulbMultipurpose stemmodifications(Underground)Rhizome:

It is an underground stem which grows horizontally below the soil

It contains nodes and internodes.

 Scale leaves are present at the nodes.

It is dorsiventrally differentiated and produces aerial branches as well as

reproductive organs from the dorsal side and adventitious roots from the

ventral side.

Thebranches developed from the axillarybudsnot only store foodmaterials

but maintain horizontal growth of the rhizome.

 Terminal bud develops and produces aerial branches.

The underground rhizome remains alive even in droughtconditions.

Ex:-Zingiber officinalis,Curcuma longa, Musa paradisiaca.Corm:

It grows vertically in soil.

It bears nodes, internodes and scale leaves.

Axillary buds produce daughter corms.

Terminal bud is big and produce aerial shoot.

Ex: Amorphophallus.

In Amorphophallus, special adventitious roots called pull roots keepthe corm at a

particular depth insoil.c) Stem tuber:

The apices of underground branches that store food and become tuberous

are called stem tubers.

The stem tuber is covered by brown periderm.

It bears many eyes that represent nodes.

Each eye contains semi lunar leaf scar and an axillary bud in its axil.

Eyes help in vegetative propagation.d) Bulb:

It is special underground stem modification that does not store food materials in stem.

The stem is reduced to a biconvex shaped disc like structure.

It bears adventitious roots on its lower side and leaves on upper side.

Leaf bases (scale leaves) store food and water.

Axillary budsproduce daughter bulbs and terminal budpresent at the centre

of the bulb develops into an aerial shoot producing inflorescence.

It is two types:

a) Tunicated bulb: The entire bulb is covered by dry membraneous scale leaves called

tunic. The fleshy leaf bases overlap one above the other in concentriccircles.

Eg: Alliumcepa.b) Scaly/ imbricate/ naked bulb: Fleshy scale leaves are arranged

loosely and tunic is absent.

Ex:

In Allium sativum a number of

fleshy scaleleaves called cloves

are arranged as in

scaly bulb but a group of

such clovesare enclosed in a

whitish, skinny tunic.Stem Branching

The types are:

(1) Lateral or Axillary Branching and

(2) Dichotomous Branching.

A. Lateral orAxillary Branching:

 Branching from lateral buds is the rule among flowering

plants.

 As the lateral buds are usually axillary, this may also be called

axillary branching.

 Branching is racemose or cymose according as the lateral

buds are less vigorous or more vigorous than the apical bud.Lateral or Axillary Branching

1. Racemose or Monopodial Type

 The growth of the main stem is indefinite, i.e., it continues to grow

indefinitely by its terminal bud and gives off branches in acropetal

succession.

 Here the lower branches are older and longer than the upper branches,

e.g., Polyalthia, Casuarina, Eucalyptus, etc..

• Type # 2. Cymose Type:

 The main axis or the stem does not grow indefinitely due to the limited

growth of the terminal or apical bud.

 Here the growth is definite, and the main stem produces one or more

lateral branches which grow more vigorously than the terminal one. The

process may be repeated again and again.

The may be sub-divided as follows:

1. Uniparous cyme and

2. Biparous cyme.

3. Multiparous cymeCymose branching

1. Uniparous Cyme

In such type, only one lateral branch is produced at a time and the branching

is known as uniparous or monochasial.

It is further sub-divided into two types:

(a) Helicoid cyme: Here the successive branches develop on one side

only, e.g., in Saraca indica.

(b) Scorpioid cyme: Here the successive lateral branches develop on

alternate sides, forming a zig-zag, e.g., in Vitis vinifera..

2. Biparous Cyme:

• In such type of cymose branching, two lateral axes develop at a

time, and it is called biparous or dichasial cyme,

• e.g., in Mirabilis, Viscum, Stellaria, etc.

3. Multiparous Cyme:

If there be more than two leaves at each node and if the apical

bud does not develop, there will be

more than two branches at

ore than two branches at

every step.

In Nerium odorum (oleander) leaves are borne in whorls of three

and the branching is of the trichasium type.

Polychasium may be seen in Croton bonplandianum..B. Dichotomous Branching:

• Although this type of branching is practically confined to the Cryptogams.

• The growth of such a plant is by a single apical cell or by a group ofsuch cells

which bifurcates or splits giving rise to two apical branches.

•

•

The tips of these branches again bifurcate and this splitting goes on in

branches of higher orders. This is normal or true dichotomy .

In other cases, after the bifurcation of the tip, one half grows normally while

the other half is practically suppressed.

• This is called sympodial dichotomy as this type of branching gives rise to a

false axis formed of the bases of successive branches (sympodium) just as in

the case of uniparous cymose branching..

• If the branches on one side only are suppressed the plant

should take up a coiled structure and the branching is called

helicoid dichotomy.

• On the other hand, if alternate branches on the left and the

right are suppressed, the zigzag appearance causes the name

scorpioid dichotomy. In nature, however, these curved stems

straighten up forming false axes or sympodia.

• The types of dichotomous branching should be compared with

similar cymose forms.Dichotomous Branching