Morphology of Flowering Plants

Introduction

• Angiosperms are flowering, fruit bearing, spermatophytic plants that are well adapted for land. These plants are the most advanced and evolved group that exists today.
• Angiosperms appeared in lower cretaceous period of mesozoic era but flourished in tertiary period of coenozoic era. They all have enclosed seeds and bear flowers. The flowering plants vary in size and ranging from small grasses to tall Eucalyptus or giant banyan trees.
• Angiosperms lack archegonium which is replaced by pistil. They have double fertilization and endosperm is triploid.
• The male gametes are nonmotile and carried by pollen tube to avoid dependence on water for fertilization.
• These plants have been broadly classified into monocots and dicots.
• Monocots are more advanced than dicots and have probably evolved from primitive dicots.
• Plant morphology refers to the study of external form and structure of plants.
• There are two major systems in flowering plants i.e., root system and shoot system.

THE ROOT

• The main characteristics of root are as following :

• It is the underground system, usually below the soil and originates from the radicle.
• In generally it is positively geotropic, negatively phototropic and positively hydrotropic.
• The roots grow in downward direction for the purpose of anchorage and absorption of water and nutrients from the soil.
• It lacks nodes, internodes, leaves and buds.
• Unicellular hair and a terminal root cap or calyptra is present produced by calyptrogen.
• The roots may be absent in some hydrophytes, but when present they are poorly developed and lack hair.

Connecting Concept

• In few hydrophytes root cap is absent and instead root pockets are present. A root pocket differs from a cap in its mode of development. In Pandanus (screwpine), multiple root caps are present.

Functions of root

• The primary functions of root are fixation of plant firmly in soil, absorption of water and conduction of mineral nutrients.
• Roots also prevent soil erosion.
• The secondary or special functions of root include food storage, providing additional mechanical support, photosynthesis, respiration and vegetative multiplication, etc.

Connecting Concept

• In a hydrophyte Podostemon, root performs the function of assimilation and reproduction. It is a plant that is all root.
• Roots are absent in some angiosperms, e.g., Utricularia, Wolffi , Balanophora, Cuscuta, etc.

Types of Roots

• Roots are divided into two types:
  1. Tap roots
  2. Adventitious roots
• Tap root : Tap root or primary root develops from the radicle. It form lateral branches or secondary roots which are further branched to form tertiary roots. Thus tap root along with its branches, i.e., secondary and tertiary roots from tap root system. It is generally found in dicotyledons.
• Adventitious roots : These roots develop from any part of the plant instead of radicle. Adventitious root can be originate from base of stem, leaf, nodes and internodes.

1. Root arising from the base of the stem, e.g., maize, wheat, rice, onion, etc.
2. Roots arising from the leaf, e.g., Bryophyllum, Pogostemon etc.
3. Roots developing from the nodes and internodes of the stem, e.g., Ficus (banyan), Pothos (money plant), etc.

Root (A) Tap root (B) Fibrous or adventitious root

Regions of the Root

• A typical root can be differentiated into five distinct regions namely region of root cap, meristematic region, region of elongation, region of root hair and mature region.
• The first three regions take up little space whereas the last region forms the major part of the root.
• Root cap (calyptra): Root cap is derived from calyptrogen. It covers root tip and protects it against friction from soil particles. Root cap being rich in starch grains responsible for geotropic response. Root cap is absent in epiphytes, parasites, hydrophytes and mycorrhizal roots.
• Region of active cell division or meristematic region: This region is only 1 mm long. The growing point produces new cells for root cap and basal parts. It is, therefore, the zone of cell division. The cells of this region are thin and non-vacuolated.
• Region of elongation: This region is 4-8 mm long behind the tip. The cells elongate rapidly due to vacuolization. This zone is responsible for growing of root in length.
• Region of root hair: This region is 1-6 cm long. Outer cells just above the region of elongation give rise to lobular unbranched unicellular root hairs for increasing the absorptive area. Root hairs are absent in parasitic and mycorrhizal roots.
• Region of cell maturation: The outermost layer of this region has thick walled or impermeable cells. So this region cannot help the root in water absorption. Its only function is to anchor the plant firmly in the soil. Lateral roots also arise from the interior of this region.

Zones or regions of a typical root

Modification of Roots

• Roots are modified to carry out some special functions, other than normal functions assigned to them like storage of food, additional mechanical support, photosynthesis, respiration and vegetative multiplication, etc.

Modified tap root

• Tap roots are modified for the purpose of storage of food and are, therefore, fleshy and swollen.
• The modified tap roots are classified according to the shape they assume.
• Fusiform: It is a spindle-shaped root. It is swollen in the middle and tapering at both the ends e.g., radish (Raphanus sativus).
• Napiform: It is much swollen at the upper end and abruptly tapering towards the lower end e.g., turnip (Brassica rapa) and beet.
• Conical: The conical root is broadest at the top (base) and gradually tapering towards the lower end. The hypocotyl does not contribute to the swollen root e.g., carrot (Daucus carota).
• Tuberous: Tap root is irregularly swollen except at the base, e.g., 4 o’clock plant (Mirabilis jalapa).
• Nodulated (tuberculated roots): In legumes, the roots are symbiotically associated with the nitrogen-fixing bacterium, Rhizobium leguminosarum. This results in the formation of several nodules in the branches of the roots. Nodules are pinkish due to pigment leghaemoglobin which carry oxygen and provide anaerobic environment in nodules for nitrogenase enzyme to fix nitrogen.

(A) conical root of Carrot. (B) fusiform root of Radish. (C) Napiform root of Turnip (D) Tuberous root of Mirabilis. (C) Nodulated root of a legume.

Modified adventitious roots

• Tuberous : These roots arise from nodes of stem and one root out of bunch become tuberous and fleshy for storage of food, e.g., Sweet potato ( (Ipomoe a batatas ).
• Fasciculated : Fasciculated roots arise in bunches (fascicles) from lower nodes of stem and become thick and fl eshy for storage of food, e.g., Asparagus, Dahlia.

(A) Tuberous roots of Ipomoea batatas (sweet potato) (B) Fasciculated roots of Dahlia

• Beaded or Moniliform roots : Due to food storage these roots swells at different places forming a beaded structure e.g., Vitis, Momordica and some grasses.
• Nodulose : In these roots terminal portions swell up, e.g., Curcuma amada.

(A) Moniliform roots of Grass (B) Nodulose roots of Curcuma amada

• Annular: In these roots swelling at different places takes place in such a way that it looks like stack of rings, e.g., Psychortia.
• Prop roots: Some plants are profusely branched and branches are thick and heavy. From these branches roots arise and hang downwards in the air and later penetrate the soil and function as prop (for support) to the branch, e.g., Ficus bengalensis (Banyan).

(A) Annular roots of Psychortia (B) Prop roots of Banyan

• Stilt roots : In some plants roots arise from lower nodes of stem and enter the soil and become stronger. They protect the plant against winds, e.g., sugarcane, maize, screwpine.
• Climbing roots : In Pothos, Piper beetle, adventitious roots arise from nodes and help the plant in climbing. These roots produce a viscous substance which dries in the air and so the roots get attached to substratum.
• Buttress roots : These are horizontal roots arise from basal part of main stem and spread in different directions in the soil. They are irregular, thick, broad, like planks of wood, e.g., Bombax, Ficus.

(A) Stilt roots of sugarcane (B) Climbing roots of Piper beetle (C) Root buttress of Bombax

• Contractile roots : Underground stem of plants, like Chrocus and Onion roots are meant for keeping storage stems at a proper depth in the soil. Their apical part contracts and fixes the plant in the soil.
• Sucking roots: Parasitic plants like , Cuscuta roots arise from the stem and enter the host plant developing contact with xylem and phloem of the host stem. Such roots are called haustoria (= sucking roots)

(A) Contractile roots of Chrocus (B) Sucking root of Cuscuta

• Epiphytic or Hygroscopic roots :These aerial hanging roots present in epiphytes (plants growing on other plants for space only). These roots have a spongy tissue called velamen (outside the cortex). Cell of velamen are dead and have pores for the absorption of atmospheric moisture. Root cap and root hairs are absent, e.g., Orchids.
• Floating root : These are adventitious roots present in some floating plants like Jussiaea. Spongy aerial roots filled with air help the plant in floating.

(A) An orchid showing epiphytic and clinging roots (B) Floating roots of Jussiaea

• Assimilatory or Photosyntheitc roots : Roots of some plants develop chlorophyll and perform photosynthesis, e.g., Trapa, Tinospora
• Reproductive roots : In sweet plants, Asparagus, Dahlia vegetative buds arise on adventitious roots which after separation from parent plant form new plants.
• Mycorrhizal roots : Roots of some plants which grow in humus soil have fungal hyphae. Root of such plants do not have root hair and fungal hypae function as root hair, e.g., Pinus.
• Roots thorns : Plants like Pothos, roots near the soil become hard like thorns. These thorns serve as protective organ.
• Clinging roots : These roots arise from node of a stem and pierce the stem of host plant and thus help in fixing the plant to the host Orchid and Ivy.

(A) Assimilatory roots of Trapa (B) Reproductive roots of Trichosanthes (C) Mycorrhizal roots of Pinus (D) Root thorns of Pothos (E) Clinging roots of Ivy

• Leaf roots : In some plants adventitious roots are produced on the margin of leaves, e.g., Bryophyllum and Bignonia.
• Pneumatophores : Pneumatophores or respiratory roots are short, vertical and negatively geotropic (grow in an upward direction). It occur in certain halophytes, which grow in saline marshes (mangroves). They bear numerous minute pores or special lenticels (pneumatothodes) in their terminal parts that help in taking atmospheric oxygen which is completely absent in the saline water e.g., Rhizophora, Heritiera (‘sundari’), Avicennia (‘bina’).

(A) Leaf root of Bryophyllum (B) Leaf root of Rhizophora

SHOOT SYSTEM

• It is an aerial system, usually above the soil and originates from the plumule. It consists of stem, branches, leaves, flowers, fruits and seeds.

Characteristic features of the stem

• It is a direct prolongation of the plumule.
• It is negatively geotropic and positively phototropic.
• It bears branches, leaves and flowers.
• It is distinguishable into nodes and internodes. It lacks a terminal cap.
• A node is formed at the place from where a leaf arises i.e., the place of origin of a leaf on the stem apex is differentiated as the node.
• The space between two successive nodes is called internode.
• The branches and leaves arise from the nodes. It often bears multicellular hairs.
• The growth of shoot system is accomplished by buds. The buds may be vegetative or flowering.

Functions of stem

• Stem facilitates conduction of water, mineral and food material. It also produces and supports leaves and reproductive structure.

Modifications of Stem

• The stem and branches of certain plants undergo various types of modifications to perform special type of functions such as:
  1. perennation by underground modifi cations;
  2. vegetative propagation by sub-aerial modifi cations and
  3. specialised functions by aerial modifications.

Underground modifications of stems

• Some perennial herbs develop their stems underground for the purpose of perennation during unfavourable conditions. The stem produces aerial branches every year when conditions become favourable.
• The underground stems are usually very thick as a result of heavy deposit of food within them and hence they also act as storage organs.
• Besides they also help in vegetative propagation by means of their buds.
• These stems are non-green and leafl ess like roots but differ from them in the following way :
  1. Presence of nodes and internodes, scale-leaves, axillary and terminal buds.
  2. Absence of root hair and root cap.

The various types of underground modifi cations of stem are as follows :

Rhizome

• Rhizomes are colourless and apparently leafl ess and, therefore can be mistaken as roots.It is a thick and prostrate stem. It grows horizontally forward under soil surface.E.g., Rhizome of Ginger
• Nodes and internodes are very distinct.
• Generally branched.
• Bears several axillary and terminal buds.
• It bears adventitious roots all over.
• E.g., Musa (plaintain), Canna, Zingiber (ginger), Curcuma (turmeric), ferns etc.

Rhizome of Ginger