Plant Stem : Characteristics And Different Types Of Stem

Characteristics Of Stem

✽ The aerial part of the plant body is collectively described as shoot system
✽ Main axis of this shoot system is called the stem
✽ Stem is the ascending part of the plant axis developing from the plumule
✽ Stem is differentiated into nodes and internodes. Its bears leaves and branches at the nodes
✽ Its positively phototropic
✽ Negatively geotropic
✽ The young stem is green and is performing photosynthesis

Types Of Stem

1. Strong Stem

✽ Strong, Stout & Erect
✽ Cylindrical in shape E.g. Palm Tree
✽ There are 4 types of strong stems
i)Excurrent
The main axis show continues growth and the lateral branches develop regularly giving a conical appearance to the tree
E.g. Polyalthia longifolia, Casuarina
ii) Deliquescent
The growth of lateral branch is more vigorous than that of main axis. The tree has a rounded or spreading appearance
E.g. Mongifere
iii) Caudex
Its an unbranched, stout, Cylindrical stem, marked with scars of fallen leaves
E.g. Cocos
iv) Culm
Erect stems with distinct nodes and internodes. stem shows jointed appearance
E.g. Bambusa arundinacea

2.Weak Stems

There are following types

i) Trailer
✽ It is weak stem that spreads over the surface of the ground without rooting at the nodes.
✽ These fall into three categories
a) Procumbent : A stem that lies flat on the ground E.g. Evalvulus
b) Decumbent : A stem that lies flat but its apex is raised E.g. Tridax
c) Deffuse : A trailing stem with spreading branches E.g. Boerhaavia

ii) Creeping

✽ A plant grows horizontally on the ground and gives off root at each node E.g. Grasses
✽ These fall into four categories
a) Runner
✽ Modified sub aerial stems
✽ Run horizontally on surface of soil
✽ Runner has one or more nodes. The nodes bear scale leaves and axillary buds E.g. Lawn Grass ( Cynodon dactylon)
b) Sucker
✽ Sucker is a runner like non-green branch in the underground part of stem
✽ Arise from underground part of plant
✽ Grows obliquely upwards and gives rise to a new plants E.g. Mentha.
✽ The sucker can therefore be called underground runner
c) Stolon
✽ Stolon is a slender lateral branches that arise from the base of the main stem
✽ Initially stolon grows upwards like an ordinary branch and then bends down and touches the soil where its terminal bud gives rise to a new shoot and adventitious roots E.g. Straberry, Jasmine
d) Offset 
✽ Its shorter and thicker than runner
✽ Its commonly called the runner of aquatic plants
✽ It helps in the vegetative propagation in aquatic plants
✽ E.g. Pistia, Jalkumbhi, Water Hyacinth

iii) Climber
✽ This weak stem climbs a support by means of some special structure. help plant to climb on any neighboring objects
✽ These are divided into following types
a) Twiners : Its is a weak, long and slender stem that climbs by twining its body around the support E.g. Dalichose (Dextrorse) Convolvulus ( Sinistrorse)
b) Lianas : Its a long woody perennial twiner E.g. Bauhinia vahlii
C) Tendril : Its a weak stem climbing by its slender, leafless, spirally coild structure, known as tendrils, E.g. Lathyrus aphaca , L.odoratus
D) Root Climbers: Such climbers give out adventitious roots at each node which strick to the support E.g. Pothos scandens (Money Plant) Piper Betle (Betel)

3. Underground Stem

✽ Many plants produce underground stems for perennation and food storage. They produce aerial shoots annually.
✽ Underground stems perform the function
✽ Storage of food
✽ Perennation and vegetative propagation

They can be differentiated from roots by:
❍ Stem like internal structure
❍ Exogenous branching
❍ Presence of nodes and internodes
❍ Occurrence of foliage leaves or scale leaves at the nodes with axillary buds
❍ Absence of root cap

a) Rhizome
✽ Its a thick prostrate and branched stem growing horizontally beneath the soil surface
✽ It has distinct nodes and internodes.
✽ The nodes bear small scale leaves
✽ Axillary bud is present in the axil of scale leaf
✽ The lower surface of the nodes gives out small slender adventitious roots
✽ Arise with the help of lateral bud this type of rhizome is called Sympodial rhizome E.g. Zingiber officinale(Ginger) Curcuma domestica (Turmeric)
✽ In some plant, growth of rhizome occurs with the help of terminal bud these are called monopodial rhizome E.g. Lotus, Pteris(a fern)

b) Tuber
✽ Its the swollen tip of the underground branch.
✽ The growth of these branches is restarted, hence the tips become swollen due to accumulation of food materials
✽ The tubers are rounded or ovel shapes
✽ The tubers show nodes and internodes bear scale leaves with axillary buds, commonly called as eyes.
✽ Under favorable conditions these eyes sprout and produce aerial shoots.
✽ Thus tubers helps in vegetative propagation.
✽ Tubers do  not produce adventitious roots, thus they differ from rhizomes E.g. Potato (Solanum tuberosum)

C)Corm
✽ Corm is a condensed from of rhizome growing vertically down into the soil.
✽ internodes are usually reduced one or more axillary buds are present in the axil of the scale leaves
Some of these buds grow into daughter corms.
✽ Corms bear adventitious roots either at the base or throughout the surface E.g.Crocus sativus(Saffron), Gladiolus

D) Bulb
✽ It is a highly condensed stem, represented by a short convex or slightly conical disc
✽ On its upper surface are present large number of fleshy scale leaves surrounding a terminal bud at the center of the disc
✽ Few Scale leaves bear bud in their axil
✽ The fleshy leaves of the bulb store food in the form of carbohydrates
✽ A few outer leaves which become dry and scaly, are protective in function
✽ Large number old adventitious roots arise from the lower surface E.g. Allium cepa, Allium Sativum

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