9. Morphology of Flowering Plants -part 01 - ROOT

 

9. Morphology of Flowering Plants -part 01 - ROOT

Angiosperms :

• Angiosperms are one of flowering plants from phanerogams.
  

Types of Angiosperms [on the basis of habitat] :

1. Hydrophytes - Growing in aquatic habitat e.g. H ydrilla
2. Xerophytes - Growing in regions with scanty or no rainfall like desert e.g. Opuntia
3. Psammophytes - Growing in sandy soil e.g. Elymus
4. Lithophytes - Growing on rock e.g. Couchidium
5. Halophytes - Growing in saline soil e.g. Mangroove plants like Rhizophora

• In angiosperms seed germinates under favourable environmental conditions and produces a seedling which develops into a new plant.

 Morphology :

• Morphologically plant shows vegetative structures like -  root, stem, leaf and reproductive structures such as flowers, fruits and seeds.

Root : 

• Root is descending axis of plant body which is positively geotropic and hydrotropic but negatively phototropic and aerotropic. 
• Root grows beneath the soil surface towards gravity. 
• Roots are generally non-green, cylindrical and without nodes and internodes.

Typical Root Structure :

• A typical root has different regions :-

1. Root cap
2. Region of cell division or meristematic region
3. Region of elongation
4. Region of root hair / absorption/piliferous zone
5. Region of cell maturation or differentiation

 1. root cap :

• A parenchymatous multicellular structure in the form of cap, present over young growing root apex is known as root cap. 
• Cell of root cap secrete mucilage for lubricating passage of root through the soil. 
• Cells of root cap show presence of starch granules which help in graviperception and geotropic movement of root. 
• Usually single root cap is present in plants. But in plants like Pandanus or screw pine multiple root caps are present.
• In hydrophytes root caps are replaced by root pocket e.g. Pistia, Eichhornia etc.

2. region of cell division or meristematic region :

• Due to presence of root cap the growing apex of root is subterminal in position. 
• The apex of the root is a growing point about 1 mm in length protected by root cap. This region is called as region of cell division or meristematic region. 

3. Region of elongation :

• The structure is developed by compactly arranged thin walled actively dividing meristematic cells.
• These cells bring about longitudinal growth of root. 
• It is followed by Region of elongation. This region of cells is present just above zone of cell division. 

4.  A Region of root hair / absorption/piliferous zone :

• The cells are newly formed and show rapid elongation to bring about increase in length of the root. 
• The cells help in absorption of mineral salts. 
• A Region of root hair / absorption/piliferous zone is made up of numerous hair like outgrowths. 

5.  Region of cell maturation or differentiation :

• The epiblema or piliferous layer produces tubular elongated unicellular structures known as root hair. 
• They are in close contact with soil particles and increase surface area for absorption of water.
• Root hair are short lived or ephimeral and are replaced after every 10 to 15 days. 
• Region of cell maturation or differentiation is major portion of root is developed by this region.
• The cells of this region are quite impermeable to water due to thick walled nature. 
• The cells show differentiation and form different types of tissues. This region helps in fixation of plant and conduction of absorbed substances.
• Development of lateral roots also takes place from this region.

Function of Root :

• Roots carry out several functions which can be categorized into primary and secondary functions. 
• Primary functions of  root are - 

1. fixation or anchorage of plant body in the soil
2. absorption of water and minerals from soil and 
3. conduction of absorbed materials up to the stem base etc.

Types of Root :

• On the basis of origin, roots can be classified as -

1. Tap roots or true roots and
2. Adventitious roots.

a. Tap root : 

• The root which develops from the radicle of an embryo during seed germination is known as tap root or true root. 
• The main root is called as primary root; its branches of first order are called as secondary roots whereas branches of second order are called as tertiary roots e.g. Pea, Bean, Sunflower etc.
• The main root with all its branches is known as tap root system. 
• Tap root system is commonly seen in dicotyledonous plants.

b. Adventitious roots : 

• A root that develops from any part other than radicle is known as adventitious root. 
• Such root may develop from the base of the stem, nodes or from leaves. 
• In monocots, radicle is short lived and from the base of stem a thick cluster of equal sized roots arise. This is adventitious root system. eg.Maize, Wheat, Sugarcane etc. 
• It is also known as fibrous root system as they look like fibre.
• The growth of roots is superficial. 
• Adventitious root in some plants are used for vegetative propagation. eg. Euphorbia, Carapichea ipecacuanha (Ipecac) etc.

Modification of root : 

• When roots have to perform some special type of function in addition to or instead of their normal function they develop some structural changes. Such roots are called as metamorphosed roots.

Modifications of tap root
a. Food storage : 

• When tap root stores food it becomes swollen fleshy and also develops definite shape. 
• Main or primary root is the main storage organ but sometimes hypocotyl part of embryo axis also joins the main root.
• Secondary roots remain thin. Stem in such cases remain reduced, discoid and leaves are radicle leaves.
•  On the basis of shape swollen tap roots are classified as - 

1. Fusiform
2. Conical and 
3. Napiform.

Fusiform root  :

• It is swollen in the middle
• tapering towards both ends forming spindle shaped structure. 
• e.g. Radish (Raphanus sativus) 

Conical root :

• It is broad at its morphological base 
• narrows down towards its apex is called as conical root. 
• e.g. Carrot (Daucus carota) 

Napiform root :

• The base of root is highly swollen
•  almost spherical in shape and 
• abruptly narrows down towards its apex. 
• e.g. Beet (Beta vulgaris)

b. For Respiration : Pneumatophores or Respiratory Roots : 

• Halophytes are the plants which grow in - 

1. saline swamps
2. marshy places and 
3. salt lakes. 

• These plants produce special kind of roots called as pneumatophores or breathing roots. 
• The main root system of these plants do not get sufficient air for respiration as soil is water logged. 
• Due to this, mineral absorption of plant also gets affected.
• To overcome this problem underground roots develop special roots which are negatively geotropic; growing vertically upward. 
• These roots are conical projections present around main trunk of plant.
• The roots show presence of lenticels i.e. minute pores for gaseous exchange ( Pneuamatic - Hollow, phore - stalk) 
• e.g. Rhizophora, Avicennia, Sonneratia, H eritiera (ver. sundri) etc.

Modifications of Adventitious Roots :
a. Food storage : 

• Fibrous roots also show food storage like tap root. 
• The main difference is that fibrous root usually do not develop definite shape.
• These roots are further classified as -

1. Simple tuberous
2. Fasciculated tuberous,
3. Beaded and 
4. Nodulose roots.

Simple tuberous roots

• become swollen and do not show definite shape. 
• They are produced singly. 
• The roots arise from nodes over the stem and penetrate into the soil. 
• E.g. sweet potato or shakarkand (I pomoea batatas).

Fasciculated tuberous root :

• A cluster of roots arising from one point which becomes thick and fleshy due to storage of food is known as fasciculated tuberous root. 
• These clusters are seen at the base of the stem. 
• E.g.Dahlia, Asparagus, etc. 

Beaded roots :

• Beaded roots are also called as moniliform roots. 
• These roots are swellings at regular intervals like beads of a necklace. 
• e.g. Spinacia oleracea (Indian Spinach).

Nodulose roots

• The cluster of long slender roots become enlarged at the tips forming nodules is known as nodulose roots. 
• E.g. Arrow (Maranta root) Amhaldi (Curcuma amada)

do you know :

• A banyan tree growing in the Indian Botanical Garden, howrah (Kolkata) has nearly 1700 such prop roots. 
• The crown of tree has a large circumference. 
• The tree is about 200 years old.

b. For mechanical support :

1. Prop roots : 

• These roots arise from horizontal branches of tree like Banyan tree (Ficus benghalensis) and grow vertically downwards till they penetrate the soil. 
• These prop roots show secondary growth, become thick.
• Act like pillars to provide mechanical support to the heavy branches.

2. Stilt roots : 

• These roots normally arise from a few lower nodes of a weak stem in some monocots shrubs and small trees. 
• They show obliquely downward growth penetrating soil and provide mechanical support to the plant. 
• In the members of family Poaceae, the plants like Maize, Jowar, Sugarcane etc. produce stilt root in whorl around the node.
• These roots provide additional support to the plant body. 
• In Screwpine or Pandanus (Kewada), stilt roots arise only from the lower surface of obliquely growing stem for additional support. 
• These roots show multiple root caps.

3. Climbing roots :

• Different climbers with weak stem produce roots at their nodes by means of which they attach themselves to support and there by raise themselves above the ground 
• e.g. Betel leaf or Pan, black pepperor Piper nigrum (Kali Mirch), Pothos or money plant.

4. Clinging Roots :

• These tiny roots develop along internodes, show disc at tips, which exude sticky substance. 
• This substance enables plant to get attached with walls of buildings.
• They do not damage substratum. 
• e.g. English Ivy (Hedera helix).

5. Plank Roots/Buttresses :

• Often develop at the base of large trees form plank like extensions around stem. e.g. Silk cotton, Pipal etc.

6. Buoyont roots : 

• Roots developed at the nodes of aquatic herbs like (Jussiaea repens), become highly inflated and spongy providing buoyancy and helping the plant to float.

c. For special functions :
1. Epiphytic roots : 

• Small epiphytic plants such as orchids growing on the branches of huge trees in dense rain forests and are unable to obtain soil moisture. 
• They produce specialized root to hang in the air. 
• The roots are provided with a spongy membranous absorbent covering of the velamen tissue. 
• The cells of velamen that absorb moisture from air. 
• A tissues are hygroscopic and have porous walls The roots may be silvery white or green but without root cap. 
• e.g. Vanda, Dendrobium etc.

2. Sucking roots or Haustoria : 

• Specialised microscopic sucking roots developed by parasitic plants to absorb nourishment from the host. 
• Viscum album is a partial parasite.
• It develops haustoria which penetrate into xylem of host plant for absoptional food. 
• In Cuscuta reflexa or Dodder (Amarvel) haustoria penetrates vascular strand and suck food from phloem, water and minerals from xylem.
• Cuscuta is leafless plant with yellow stem. It is a total parasite.