Roots are very important parts of a plant. They mainly provide anchorage and help in absorption and transport of nutrients from the soil.
There are mainly two types of root systems. They are:
Tap root system
Adventitious root system
However, in some plants, roots perform additional functions such as storage of food and support to weak stems or tall trees. In some plants they also help in exchange of gases. In order to perform these additional functions, the tap roots and adventitious roots undergo some changes in their morphology and structure.
For Storage of Food: Tap roots of some plants become fleshy due to storage of reserve food. The main root stores extra food whereas secondary roots are thin and are involved in absorption of minerals from the soil. The swollen roots occur in different shapes. Based on their shapes, tap roots may be classified as:
Conical - The main root is conical in shape with a broad base and tapers gradually at the apex. Example is carrots.
Napiform - This tap root is spherical in shape and tapers sharply at the tip. Examples are turnip and beetroot.
Fusiform - The main root is swollen in the middle and tapers at both the ends. Example is radish.
Tuberous - This root has no definite shape. It gets swollen and fleshy. Example is a 4 o’clock plant.
For Nitrogen Fixation: Nodular roots (Tuberculated roots) - Members of the family Leguminosae (e.g pea, groundnut, beans, gram) have swellings called root nodules on their main root as well as on their branches. These nodules store nitrogen fixing bacteria called Rhizobium which fix atmospheric nitrogen into nitrates and supply them to the plants. The nodules are red in colour due to presence of a pigment known as leg- haemoglobin. The plant in turn provides shelter and nutrients to the bacteria.
For Storage of Food: adventitious roots also become swollen due to storage of food. These are of the following types.
Tuberous - These roots are swollen but have no definite shape. They arise from the nodes of the prostrate stem. Example is sweet potato.
Fasciculated - The swollen roots develop in clusters at the base of the stem and have definite shapes. Examples are dahlia and asparagus.
Nodulose - In these only the tips of the roots become swollen due to accumulation of food. Examples are mango- ginger and arrowroot.
Moniliform (Beaded roots) - The roots are swollen at regular intervals giving a beaded appearance. Examples are grasses and bitter gourd.
Annulated - The roots appear as if they are formed of numerous discs placed one above the other. Example is Ipecac.
Photosynthetic or Assimilatory Roots - These roots develop chlorophyll when exposed to sun and can prepare food for plants by photosynthesis. Example is Tinospora (orchid).
Epiphytic or Hygroscopic Roots - In some plants like the orchids which grow as epiphytes upon trunks of trees, the stem and leaves are absent and only aerial, ribbon like roots are present which contain sponge-like tissue called velamen. These roots hang freely in air and are hygroscopic in nature, that is they absorb moisture from air with the help of velamen.
Saprophytic Roots (Mycorrhizal roots) - These roots are associated with fungal hyphae. Mycorrhiza refers to a symbiotic (mutually beneficial) relationship between the roots of a plant and fungi. The plant and the fungus are mutually beneficial to each other, as the fungus provides water and minerals to the plant, and the plant provides prepared food to the fungus. Some associations are endomycorrhizae in which the fungal hyphae grow inside the plant’s roots, and some are ectomycorrhizae where the hyphae grow on the surface of the roots. Examples are Monotropa and Sarcodes.
Pneumatophores (Respiratory roots) - Plants such as mangrove which grows in marshy soils, develop some aerial roots. They grow vertically up into the air. Such roots are called breathing roots or pneumatophores. They appear as conical spikes. Each such root has numerous pores called lenticels at the tips through which gases diffuse in and out. Example is Rhizophora (mangrove plants) found in Sunderbans of West Bengal.
Reproductive Roots - Adventitious roots of some plants help in vegetative propagation by developing buds that give rise to new shoots. Example is sweet potato.
Haustoria (Sucking roots/ Parasitic roots) - Parasitic plants develop microscopic sucking roots called haustoria that penetrate into the tissues of host plants to derive nutrition. The parasitic plants cannot prepare their own food as they lack chlorophyll. They have to depend on host plants for nutrients. Examples are Dodder (Cuscuta) and Amarbel.
For Mechanical Support
Prop Roots - In plants such as bamboo and banyan, there are roots hanging downwards from the heavy branches. They provide extra support to the stem of the tree. They penetrate the soil giving support to the horizontally spreading branches of the tree. The roots are long, woody and appear as pillars or columns. That is why they are also known as columnar roots.
These roots also have lenticels for breathing.
The great banyan tree in Kolkata is more than 250 years old and has 1600.
Stilt Roots- In some plants like maize and sugarcane extra supporting roots arise from the lower nodes of the main stem. These roots grow downwards in a slanting manner and enter the soil. They help to keep the plant upright by providing extra anchorage. These are called stilt roots. They have multiple root caps. Examples are sugarcane, maize and screw pine.
Climbing Roots - These aerial roots are found in weak stemmed plants such as climbers. They arise from the nodes and internodes. The weak stems of the climbers twine around the support with the help of these roots. Examples are betel, black pepper and money plant. In Vanilla single tendril-like roots arise at the nodes. These are called tendrillar roots.
Clinging Roots - These are short, branched, adventitious roots found in epiphytes. They arise at the nodes and fix the plant on the bark of the tree. Examples are orchid and Piper betel.
Buttress Roots - These are large, wide plank like aerial roots developing at the base of the stem of a shallowly rooted tree. Generally, they are found in tropical forests that are nutrient deficient and not very deep. They prevent the tree from falling over (hence the name buttress) and also absorb more nutrients.
Floating Roots - Aquatic plants develop spongy, air filled roots at the nodes. These roots contain air filled tissue called aerenchyma. The roots are called floating roots as they store water and help in the floating of plants. They also help in the exchange of gases. Example is Jussiaea.
Contractile Roots - These are also called pull roots. They are modified adventitious roots found at the base of underground stems. They contract and swell to keep the aerial shoots at proper depths in the soil. Examples are Canna and Allium.
Root Thorns - In Pothos and many palms, the adventitious roots become hard, pointed and thorn like. These are called root thorns.