The Basement of the Plant
A house however small or big is supported by a basement below the ground which is invisible. The same way a plant or a tree is supported by a root system underground. As the building cannot stand high without a strong basement, a plant cannot stand upright without a strong root system.
Root is the underground axis of the plant body that anchors the plant in the soil. The whole underground part of a plant, in general, makes up the root system. The root system is generally positively geotropic and negatively phototropic. It has no nodes, internodes, leaves and normal buds. It is generally non chlorophyllous and brown colored. Lateral roots develop endogenously.
Types of Root System:
1. Taproot System: It develops from a radical which is long living and grows vertically and produces lateral roots obliquely. It is commonly found in monocots.
2. Adventitious Root System: Radical is short lived and degenerates. Fibrous roots develop from parts other than radical. It is commonly found in monocots.
Generally the roots perform the following functions:
1. They provide anchorage to the plant.
2. They absorb water and minerals from the soil.
3. They also conduct water and minerals to the shoot system.
But roots may perform additional functions according to the needs of the environment.
Definition: the permanent changes that occur in the structure of a root to perform new functions suitable for the environment are called root modifications.
Types of Root Modifications:
1. Storage roots / Tuberous roots
2. Vela men roots or Epiphytic roots
3. Photosynthetic or Assimilatory roots
4. Respiratory roots or Pneumatophores
5. Nodular roots
6. Parasitic roots or Haustoria
1. Storage Roots:
The roots that store food and swell up are called storage roots or tuberous roots. Generally biennial dicot plants store food during their first year of vegetative growth. These are called root crops. Ex: Daucus carota (carrot), Raphanus sativus (radish) and Beta vulgais (beet root). In monocots like Asparagus, the group of adventitious roots store food. These groups of fibrous roots that store food are called fasciculate roots.
2. Vela men Roots or Epiphytic Roots:
The plants that live on the branches of big trees to get enough sunlight are called epiphytes. The epiphytic roots contain two types of roots.
a. Clinging Roots: They help in fixing the plant to the host tree.
b. Velamen Roots: They are long, thick, branced and freely hand in the air. They contain a hygroscopic tissue called velamen tissue. The velamen tissue helps in absorbing moisture from the atmosphere. Ex: Vanda.
3. Photosynthetic Roots or Assimilatory Roots:
In some epiphtes the stem and leaves are absent. The roots in these plants are aerial, flat and green colored and carry out photosynthesis. Ex: Taeniophyllum, Tinospora, Trapa
4. Respiratory Roots or Pneumatophores:
Mangrove plants living in water logged conditions suffer from lack of oxygen. These plants develop aerial roots which posies minute pores for absorption of oxygen from air. These pores are called pneumathodes and the roots bearing them are called pneumatophores or respiratory roots or Breathing roots. Ex: Rhizophora and Avicennia.
5. Nodular Roots:
The roots of legume plants show symbiosis with Rhizobioum bacteria which produce nodules on the roots. Such roots with nodules are called nodular roots. The Rhizobium bacteria fix atmospheric nitrogen into ammonia and supply it to the host. In turn the bacteria receive food and shelter. Ex: Arachis, Dolichus.
6. Parasitic Roots or Haustoria:
The plants which depend on other plants completely or incompletely for their food are called parasitic plants. They have special roots which help in absorptionof water, minerals and food from the host. These roots are called haustoria or parasitic roots. The parasites may be root parasites or stem parasites. They may be complete parasites, i.e they absorb food, minerals and water from the host, or incomplete parasites which absorb only minerals and water from the host.
Many more modifications like prop roots in Banyan (Ficus bhengalensis), stilt roots (Zea mays), plank roots (Buttress roots-Terminalia), serve the function of additional mechanical support to the plant. The climibing roots of pothos and betel vine help the plant to climb up the support.