Class 11 - Biology -MORPHOLOGY OF FLOWERING PLANTS-Notes -

INTRODUCTION

Morphology is the branch of biology which deals with the study of form, structure and relative position of different organs.
Flowering plants (or angiosperms) are seed bearing plants in which seeds are always enclosed in an ovary inside the fruits and the sporophylls are organized into flowers.
These plants have been classified into monocots and dicots.
Plant morphology refers to the study of external form and structure of plants.
The flowering plants consist of an axis, root system and shoot system.
Shoot system lies above the ground and the root system lies below the ground.
Shoot system bears branches, leaves, flowers and fruits.
The root, leaves and branches constitute the vegetative parts of the plants.
The flowers, fruits and seeds form the reproductive parts of the plants.

ROOT

Root is the descending, non-green, underground part lacking nodes, internodes, leaves and buds.
Root is responsible for nutrition and support.
Radicle comes out/arise from the seed coat in the form of soft structure and moves toward the soil. It develops and forms primary root.

Roots are of two types : tap root and adventitious root.

Tap root : It develops from radicle which is made up of one main branch and other sub-branches.
Tap root forms lateral branches (called secondary roots) which further divide to form tertiary roots.
Tap roots, with the secondary and tertiary roots form tap root system. It is the characteristic of dicot plants.
Adventitious roots : In some plants, after sometime, the growth of tap root stops and then roots develop from other part of plant which are branched or unbranched, fibrous or storage, which are known as adventitious roots.

Adventitious roots are mainly found in monocots.
Adventitious roots can be grouped into 3 types on the basis of their appearance –
- roots arising from the base of the stem, e.g.,Triticum.
- roots arising from leaves, e.g., Bryophyllum.
- roots developing from nodes and internodes of the stem.

REGIONS OF THE ROOT

Root consists of 4 major zones – root cap, meristematic zone, zone of cell elongation and maturation zone.

ROOT CAP

Root cap (also known as calyptra due to its origin from calyptrogen) is a cap like structure made up of thin walled cells that covers the root apex.
It is made of dead cells and protects the young growing cells of the apical region.
It is absent in hydrophytes, epiphytes, parasites and mycorrhiza.

MERISTEMATIC ZONE

It is present just above the root cap.
It is made up of compactly arranged small, thin walled isodiametric and meristematic cells having dense protoplasm and large nucleus.
The cells of this region are in active state of division and so this is the main growing region of the root.

ZONE OF CELL ELONGATION

The region of cell elongation is present above the meristematic zone resulting in an increase in length of the root.
The external cells of this region possess power of absorption of water and mineral salts from the soil.

MATURATION ZONE

Maturation zone is present above the zone of cell elongation.
Secondary growth takes place in this region.
Region of maturation zone is the area of lateral roots.
Its only function is to anchor the plant firmly in the soil.
Unicellular and ephemeral root hairs are formed from the epidermal cells in this zone. Root hairs helps in absorption of water. Root hairs are absent in all the plants where there are no root caps.

MODIFICATION OF ROOTS

Tap and adventitious roots are modified in different forms to perform special functions and are called as modified roots.

MODIFICATION OF TAP ROOTS

Tap root are modified for food storage and respiration.

Modified tap roots for food storage

Fusiform roots : These root are thicker in the middle and tapering on both ends. In this type of roots, both hypocotyl and root help in storage of food. E.g., radish

Conical roots : These roots are thicker at their upper side and tapering at basal end. E.g., carrot.

Napiform roots : These roots become swollen and spherical at the upper end and tapering like a thread at their lower end. E.g., turnip (Brassica rapa), sugarbeet.

Tuberous roots : Such roots do not have a regular shape and get swollen & fleshy at any portion of the root. E.g., Mirabilis.

Nodulated roots : Nodules are formed on branches of roots by nitrogen fixing bacteria (Rhizobium). Nodules are pinkish due to pigment leghaemoglobin which carry oxygen and provide anaerobic environment in nodules for nitrogenase enzyme to fix nitrogen.

E.g., plants of leguminosae family (Papilionaceae) – Pea

Modified tap root for respiration are pneumatophores

The plants grow in marshy areas, where there is scarcity of oxygen. The plants, which grow in this region have some branches of taproot that grow vertically upward and comes on surface of soil. These roots are called pneumatophores. They have minute pores called pneumathodes or lenticels by which air enters inside the plant and gives oxygen for respiration. E.g., Rhizophora, Mangrove, Heritiera.

MODIFICATION OF ADVENTITIOUS ROOTS

Adventitious roots can be modified on the basis of functions like fleshy for storage (e.g., moniliform, annulated, tuberous, fasciculated, palmate, nodulose), mechanical support and for vital functions.
Tuberous adventitious root : When food is stored in these roots, they become swollen and form a bunch. E.g., sweet potato.
Fasciculated roots : These are adventitious roots occurring in clusters and all of them are more swollen. E.g., Asparagus, Dahlia.
Fibrous roots : Roots are very thin and filamentous. E.g., Grass, Wheat.
Nodulose roots : In this type, tips of roots swell up. Eg. Melilotus.
Beaded or moniliform roots : When root swells up like a bead at different places after a regular interval. E.g., Vitis, Momordica (Bitter gourd), Portulaca.
Stilt roots : When root arises from lower nodes and enter inside the soil and form a rope-like structure, it is known as stilt roots. E.g., Maize, Sugarcane, Pandanus (screwpine).

Prop root or pillar roots : When root arises from branches of plant and grows downward towards soil function as supporting stem for the plant. This type of roots is called prop roots. E.g., Banyan.
Buttress root : Such roots appear from the basal part of stem and spread in different directions in the soil. E.g., Terminalia.
Climbing roots : These roots arise from nodes of stem and helps the plant in climbing. E.g., Money plant (pothos), Monstera (Betel), Black pepper.
Respiratory root : When the quantity of oxygen is low in soil then some root comes out from the soil and helps in respiration. E.g., Avicennia, Jussiaea.
Foliar root or Epiphyllous root : When roots arise from leaf they are called as foliar roots. E.g., Bryophyllum, Begonia.
Sucking or haustorial roots or Parasitic roots : In parasitic plants, roots enter in the stem of the host plant to absorb nutrition from host. E.g., Dendrophthoe, Cuscuta, Viscum.
Annulated roots : If the swelling is in a series of rings on the roots, it is called annulated roots. E.g., Ipecac.

STEM

Stem is a part of plant which lies above from surface of soil i.e. it shows negative geotropic growth. It has nodes and internodes. Branches, leaves, flower buds and bracts are developed from nodes.
Stem facilitates conduction of water, minerals and food materials. It also produces and supports leaves and reproductive structures.
Stem develops from the plumule.
Forms of stem are :
- Caudex : It is unbranched, erect, cylindrical stout stem and marked with scars of fallen leaves as in palms. E.g., Palm.
- Culm : Stem is jointed with solid nodes & hollow internodes. E.g., Bamboo (Gramineae).
- Scape : In some monocotyledons there is no aerial stem instead there are aerial shoot (branch) which bears flowers. E.g., Onion and aroids.
- Prostrate or procumbent : If the stem trails on the ground and lies prostrate. E.g., Evolvulus and Oxalis (Wood sorrel).
- Decumbent : Stem trails for some distance and then tends to rise at its apex. E.g., Tridax, Portulaca.
- Diffuse : When the branches of the stem are spread out in all directions on the ground. E.g., Euphorbia, Boerhaavia.
- Climbers : Stem which attach themselves to nearby object by means of some special devices like hooks. E.g., Bougainvillea (by hook), wild pea (by tendrils)

MODIFICATION OF STEM

Modification of stem are of three types – subaerial, underground and aerial modification

SUB-AERIAL MODIFICATION

Sub aerial stems are feeble and weak and aerial part of them grows horizontally on the ground while some parts remain underground and help in vegetative propagation.
It is of 4 types – runner, stolon, sucker and offset.
Runner : In this, stem grows and spread on the surface of soil. Roots are developed at lower side and leaves from upper side from node E.g., Cynodon dactylon (Doob grass), Oxalis.
Stolon : In this, branches are small and stem is condensed and grow in all directions. After sometime of growing, their apical region comes out from the soil. E.g., Fragaria (Wild strawberry), Jasmine, Peppermint.
Sucker : In this, the main stem grows in the soil but branches develop from nodes above the soil. E.g., Mint, Pineapple, Chrysanthemum.
Offset : Generally these are aquatic plants which have a short and fragile stem. E.g., Pistia, Eichhornia.

UNDERGROUND MODIFICATION

Underground stems are non-green stem and this type of modification occurs generally for food storage and vegetative propagation.
Modified underground stems are of four types – tubers, rhizome, corm and bulb.
Tuber : It is irregularly shaped swollen stem. The tips of branches become swollen in the soil. Eyes are found on them which are axillary buds and covered with scaly leaves. E.g., Potato.

Rhizome : It is fleshy and horizontally found below the soil. Small nodes and internodes are found which are covered by scaly leaves. E.g., Ginger, Turmeric, Canna, Water lily.

Corm : It is a condensed structure which grows vertically under the soil surface. It is highly swollen vertical stem.
E.g., Colocasia, Alocasia, Zaminkand, Saffron.

Bulb : This stem has a disc like structure and surrounded with numerous fleshy scaly leaves. Many roots arise from its base. The bulbs are of two types – scaly or imbricate
(e.g., garlic) and tunicate (e.g., onion).

AERIAL MODIFICATION

The aerial modification helps the plants to survive during unfavourable conditions by storing food, propagation and providing support and protection.
Aerial modifications are of following types – stem tendril; phylloclade; prickle and hooks, stem thorn/thorn; and cladode.
Stem tendril : In this type, axillary bud forms tendril in place of branches and helps in climbing of those plants which have a weak stem. E.g., Grapes, Passiflora, Cucumber, Pumpkins, Watermelon.

Phylloclade : Stem is modified into a flat, fleshy and green leaf like structure and carries out photosynthesis like leaf. The leaves are modified into spines. E.g., Opuntia, Euphorbia, Casuarina.

Prickle and hooks : These develop only from cortex and epidermis and found at nodes or internodes. It helps in climbing. It is exogenous in origin. E.g., Rose, Smilax.
Stem thorn/Thorn : It is developed from axillary buds or terminal bud of the stem. It may bear leaves, flowers. It is endogenous in origin. E.g., Catissa (Karonda), Bougainvillea, Pomegranate, Citrus.

Cladode : They are green stems of limited growth generally one or two internode long that perform the function of photosynthesis. E.g., Asparagus (one internode long) and Ruscus (2 internode long).

LEAF

The leaves develop from the nodes. Their main function is photosynthesis and food making. Axillary buds are found in its axil.
Leaf is divided into 3 main parts :

Leaf base (= hypopodium) : The part of leaf attached to stem is leaf base.
Petiole : The part of leaf connecting the lamina with the branch or stem. Petiolated or stalked leaves are known as petiolate and when petiole or stalk is absent then leaves are called sessile. In Eichhornia, petiole swells and in Citrus, it is winged.
Lamina or leaf blade : It is a broad and flattened part of leaf. Photosynthesis and transpiration occur in this.

Depending upon the incision lamina leaf may be simple or compound.
Simple Leaf is a leaf which may be incised to any depth, but not down to the midrib or petiole. E.g., mango, guava, papaya etc.
Compound leaf is a leaf in which the leaf blade is incised up to the midrib or petiole, thus dividing it into several small parts, known as leaflets.

It is of two types : pinnately and palmate compound leaf.

Pinnately compound leaf : In this type of leaf, mid rib is known as rachis. Leaflets are arranged on both sides of rachis. E.g., Neem.
Palmately compound leaf : In this type, incision of leaf is directed from leaf margin to apex of petiole and all leaflets are attached on the upper end of the petiole. E.g., Silk cotton.
Leaves of some plants have lateral appendages on either side of leaf base, known as stipules. If stipules are present in leaf it is called stipulated leaf, if it is absent then the leaf is called ex-stipulated.
Duration of Leaf :
- Persistent/Evergreen : Leaves of such plants are found in all seasons and do not (fall) shed. E.g., Pine, Saraca indica, Date palm.
- Deciduous : All leaves of such plants shed at the same time. E.g., Azadirachta, Ficus.
- Caducous : Leaves are shed as the bud formation takes place. E.g., Rose.

TYPES OF LEAVES

According to the origin and function

Foliage leaf : They are usually green coloured and their main function is photosynthesis.

Fig. : Foliage leaf

Cotyledonary leaf : This leaf comes out during germination and helps in nutrition until the first leaf is not formed.

Fig. : Cotyledonary leaf

Scale leaf : Such leaves are usually dry membrane like and they cannot perform photosynthesis.

Fig. : Scale leaf

Bract : Bracts are the leaves which contain flower in their axil.

Fig. : Bract

Bracteole : These are leaf like structures found on pedicel.
Floral leaf : Sepals, petals, stamens and carpel are found in a flower which are included in this type of leaf.

Fig. : Floral leaf

VENATION OF LAMINA

The arrangement of veins and veinlets in leaves (lamina) is known as venation.
Venation is of 2 types – reticulate and parallel.

RETICULATE VENATION

In reticulate venation, many veins divide into various branches (veinlets) and form a net like structure. It is found in dicots, exception – Calophyllum (It has parallel venation)
Reticulate venation is of 2 types unicostate and multicostate.
Unicostate or pinnate : This type of venation has only one principal vein or midrib that gives off many lateral veins which proceeds toward margin and apex of lamina of the leaf and forms a network. E.g., Mango, Guava, Peepal.
Multicostate or palmate : In this type of venation, many principal veins arise from the tip of petiole and proceed upward. E.g., Camphor, Zizyphus, Tejpata, China rose, plum.

PARALLEL VENATION

In this type of venation, all veins run parallel to each other and they do not form network.

It is found in monocots.

Exception – Smilax (It has reticulate venation).

They are of 2 types – unicostate and multicostate.
Unicostate or pinnate : This type of pattern has only one principal vein, that gives off many lateral veins, which proceed towards the margin of the leaf blade in a parallel manner but they do not have veinlets. E.g., Banana, Ginger, Canna.
Multicostate or palmate : This type has many principal veins arising from the tip of the petiole and proceeding upwards.

PHYLLOTAXY

Phyllotaxy is the arrangement of leaves on both main stem and branches.
Arrangement of phyllotaxy is made to facilitate the leaves to obtain maximum light for photosynthesis.
It is of three types – alternate, opposite and whorled arrangement.

In alternate (spiral) arrangement, only one leaf is borne at a node and leaves are arranged alternately giving a spiral form. E.g., mango, mustard, etc.
In opposite arrangement, each node gives rise to two leaves arranged opposite to each other. Opposite phyllotaxy is of two types – opposite superposed (e.g., lxora etc) and opposite decussate (e.g., Ocimum etc)
In whorled arrangement, more than two leaves are formed from each node, e.g., Nerium etc.

MODIFICATION OF LEAVES

When leaf is modified into different structure, it is called modification of leaves. E.g., leaf tendrils, leaf spine etc.
Leaf tendril : The whole leaf is modified into a wire like structure which is called leaf tendril. E.g., Lathyrus aphaca (wild pea).
Leaf spine : Leaves or any part of leaflets are modified into pointed spine. (E.g., Opuntia, Aloe, Argemone.) either to escape transpiration or for protection.
Leaf scale : Leaves become thin, dry and form a membrane or paper like structure and serve to protect the axillary buds as in Ficus and Tamarix, Ruscus or store food and water as in onion.
Leaf pitcher : Leaves of some plants are modified to pitcher shape. E.g., Nepenthes, Dischidia.
Leaf bladder : In some plants, leaves are modified into bladder like structure, e.g., Utricularia. The bladder like structure is meant for catching the aquatic insects etc. for their nitrogen content.
Leaf Hooks : In some plants, terminal leaflets are modified into curved hooks for helping the plant in climbing. E.g., Argemone,Opuntia,Aloe, Cat’s nail (Bignonia unguis-cati).
Phyllode : In this, petiole becomes flat and functions as a normal leaf. E.g., Australian acacia. It also performs the function of synthesis of food.
Leaflet tendril : When a leaflet is modified into tendril like structure than it is called leaflet tendril. E.g., Pisum sativum (Garden Pea), Lathyrus odoratus (sweet pea).