Chapter 5- Morphology of flowering plants- Modifications of Roots (Notes)
Posted on : 19-09-2018 Posted by : Admin

Types of root systems

Introduction

Plants cannot survive unless they get enough light, water, minerals and air. Plants struggle for their existence in different habitats. The factors in these habitats may or may not help the plant to survive. For example, if the plant grows in marshy soil, they get more water and less air. On the contrary, if the plants grow in deserts they get less water and more air. So to make a living in all such kinds of environments plants must modify themselves. These plant organs perform some additional functions along with the normal functions. As a result some permanent changes occur in these plant organs. These permanent changes are called modifications.

Depending on the need, the entire organ or a part of the organ is modified. When permanent change occurs in the structure of a root to perform new functions suitable for the environment, it is called root modification. Roots in some plants change their shape and structure and become modified to perform functions other than absorption and conduction of water and minerals. They are modified to give support, store food and for respiration. The following is the description of the types of root systems,

Storage roots: Some amount of the food synthesized by the plant is utilized for the immediate requirements and the remaining food is stored in their roots for future purposes. Due to the accumulation of the stored food, the roots get swollen to become fleshy and tuberous. Hence these roots are also called as tuberous roots.

The roots of most of the biennial plants (plants which grow for two years or two seasons) are modified into storage roots. Depending on the shape they attain after the storage, they are named as one of the following,

Spindle shaped- Also called as fusiform shaped. Example: Raphanus sativus (Radish)

Cone shaped- Also called as conical shaped. Example: Daucus carota (Carrot)

Top shaped- Also called as Napiform shaped. Example: Beta vulgaris (Beet root)

Moniliform- Also called as beaded roots. Example: Momordica (Bittergourd)

Annulated- These roots are seen in Psychotria

Nodulose- These roots are seen in Curcuma amada (Mango ginger) and Costus

Exceptions-

• In Ipomea batatus (sweet potato) a single tuberous root is found at each node.
• In dicot plants like Dahlia and Ruellia tuberosa and monocot plants like asparagus a number of tuberous roots are found in clusters at the base of the stem. These are called as fasciculated roots.
• Almost in all the plants, the stored food material is in the form of starch. Exceptionally, in Beta vulgaris the stored food material is in the form of sugar while in Dahlia it is in the form of inulin.

 

 Types of root systems

  

Epiphytic roots: The other names of these roots are Velamen roots, hygroscopic roots and aerophytic roots. The smaller plants in the evergreen forests do not get sufficient sunlight for their growth. These plants take shelter on the branches of big trees to obtain more sunlight. Such plants which take shelter on the big plants are called epiphytes and their roots are called epiphytic roots.

Example: Members of orchidaceae family (Vanda)

The epiphytes have two kinds of roots namely velamen roots and clinging roots

Velamen roots- They are long, thick and branched roots. They hang freely in air. These roots do not produce root hairs. The velamen roots contain a special, multilayered, dead hygroscopic epidermal tissue called velamen tissue. This tissue absorbs moisture from the atmosphere just like blotting paper

Clinging roots- They are short, comparably thin and branched roots. They are useful for fixing the plant on the branches of the host roots.

Epiphytic plants do not have direct contact with the soil, so they do not get continuous supply of water. Therefore epiphytes show many xerophytic adaptations. Plants growing in the habitats where there is scarcity of water and also the soil is physiologically dry.

Photosynthetic roots: In some epiphytes like Taeniophyllum the stems and leaves are absent. The roots of these plants become aerial, flattened, ribbon-like and green colored structures. These modified roots absorb moisture from the atmosphere and also synthesize food materials by carbon assimilation. Hence, these roots are also known as assimilatory roots.

Example: Podostemon, Tinospora and Trapa

Respiratory roots: The other names of these roots are breathing roots, Knee-roots and pneumatophores. Mangroves live in marshy and saline soil, so their roots will not get enough oxygen supply for respiration. Also mangroves suffer from lack of oxygen because these soils are water logged. So mangrove plants produce special aerial branches from the roots. These aerial roots possess minute pores which help in the absorption of oxygen from air. These pores are called pneumatothodes and the roots bearing these pores are called pneumatophores. They are also called as respiratory roots or breathing roots.

Example: Rhizophora and Avicennia

*Another aquatic example is a hydrophyte Jussiaea. Here the aerial roots become spongy and porous. They act as breathing roots apart from helping the plants to float on water. These roots are also known as spongy respiratory roots.

Nodular roots: The roots of leguminous plants show small nodule like structures called as nodular roots. The nodules are inhabited by colonies of rod shaped, nitrogen fixing bacteria called Rhizobium. Normally higher plants cannot utilize atmospheric nitrogen directly as it is diatomic. So this bacterium absorbs atmospheric nitrogen and converts it into ammonia and supplies to the host plant. In return, the bacterium receives food and shelter from the host plant. This type of mutual relationship between two different organisms is called symbiosis. The root nodules also contain a pink colored pigment called leg-hemoglobin.

Example: Arachis and Dolichos

 Types of root systems

  

Parasitic roots: The plants which depend on other plants are called as parasitic plants. Here the plant which gives shelter and food material to the parasite is called as the host plant. Parasitic plants produce special adventitious roots called as parasitic roots or haustoria which penetrate into the roots or shoots of the host plant to absorb food and other required nutrients. Based on the site of penetration the parasites can be named as stem parasites (when the parasite depends on the stem of the host plant) or root parasites (when the parasite depend on the root of the host plant).

When the parasite depends on the host only for water and minerals, it is called as partial parasite or semi parasite or incomplete parasite. On the other hand when the parasite absorbs both water, food and minerals from the host, it is called complete or total parasite.

Partial stem parasites:

• These are chlorophyllous leafy parasites.
• These absorb water and mineral salts from the host and prepare their own food material.
• Loranthus is a partial stem parasite which grows on trees like Neem, mango etc.
• Viscum is another partial stem parasite which grows on many forest trees.
• Haustoria of Partial stem parasites enter only into xylem of collateral vascular bundles.

Complete stem parasites:

• These are non-chlorophyllous parasites.
• These both water and food materials from the host.
• Cuscuta is a total stem parasite found on Duranta. Cuscuta is also called as dodder.
• Haustoria enter both Xylem and phloem of collateral vascular bundles

Partial root parasites:

• These are chlorophyllous parasites.
• These absorb water and mineral salts from the host plants and prepare their own food material.
• Striga is a partial root parasite found on the roots of grass, Sorhum, Saccharum and zea etc.
• Santalum is a partial root parasite on the roots of Dalbergia and Lantana. Santalum is the largest parasite and it is a parasite only in the early stages of the life.
• Haustoria of Partial root parasites enter only into xylem of radial vascular bundles.

Complete root parasites:

• These parasites are non-chlorophylous.
• These both water and food materials from the host.
• Orobanche is a total root parasite found on Nicotiana. Orobanche is also called as broom rape.
• Balanophora is another example of complete root parasite.
• Haustoria enter both Xylem and phloem of radial vascular bundles
• Rafflesia is a complete root parasite found in Sumatra and Java. It bears the biggest flower in the plant kingdom.
• Santalum album is an incomplete root parasite at the seedling stage. It later becomes autotrophic.

 Types of root systems

  

OTHER ROOT MODIFICATIONS

Prop roots

1. These are pillar like roots. And hence the name pillar roots.
2. These roots arise from the branches.
3. They remain hanging in the air for some time.
4. Roots finally reach the ground positively geotropically and grow in thickness.
5. Roots support the spreading branches which are weak. Thus, prop roots offer mechanical support to the plant.

Example: Ficus benghalensis

Stilt roots

1. They are also known as brace roots
2. These roots develop from the basal nodes of the stem.
3. Stilt roots grow obliquely and positively geotropically.
4. These give additional support to the plant.
5. Stilt roots are present in monocots (Saccharum); psammophytes (plants growing in sandy localities) and mangroves (Rhizophora).
6. In Pandanus, stilt roots are formed from the base of the stem. These show multiple root caps.
7. In Zea mays, stilt roots arise from the basal nodes.  

Buttress roots

1. They are also known as Ballast roots or plank roots or Root buttresses.
2. In large trees some lateral roots grow aerially near the base of the stem and run parallel to soil surface.
3. As these roots are laterally compressed and look like planks, they are also called plank roots.
4. These roots give additional support to the stem. So, these are called buttress roots.

Example: Terminalia, Bombax, Ceiba

Climbing roots

1. Some plants with weak stems climb by means of aerial sensitive adventitious roots called as climbing roots.
2. With the help of climbing roots the plants cling to the support.
3. These roots secrete a sticky material which dries up and attaches the root to the support. So these are also called clinging roots.
4. In Piper betle, a bunch of short climbing, roots arises at each node.
5. In Pothos, the climbing rots arise all over the stem
6. In Vanilla, single tendrillar climbing root develops at each node.
7. Plants with climbing roots are called root climbers.
8. Root climbers when cut off from soil, look like epiphytes, but in reality they are not so. Hence these plants are described as false epiphytes or pseudo epiphytes.

Example: Monstera

Balancing Roots or Floating roots

1. These are formed in free floating plants.
2. Here a bunch of roots develops at the nodes
3. These roots are useful in balancing the plants, thus preventing them from toppling down even in rough weather.
4. As these balance the plants these are called balancing roots.
5. Balancing roots of Eichornia contain polyarch vascular bundles.

Example: Pistia, Eichornia

Contractile roots

1. Some underground stems like corms show these types of roots.
2. These roots are adventitious and have the capacity to contract.
3. As they contract, the corm is puffed down. Thus the level of the corm is maintained.
4. These roots pull the corm, and hence are also known as pull roots

Example: Amorphophallus

 Types of root systems

Reproductive roots

1. Roots of some plants bear buds called radical buds or Adventitious buds. These radical buds develop into new aerial plants
2. Thus, radical buds are useful in vegetative propagation.

Example: Murraya, Millingtonia

Mycorrhiza or Fungal roots:

1. This is an association between a fungus and a root of higher plants. Such a root is called mycorrhiza.
2. Mycorrhizae are common in saprophytic flowering plants which grow on humus. So, these are also called saprophytic roots.
3. These plants absorb nutrients from the humus through the fungus.

Example: Monotropa (Indian pipe plant)    

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