The root system is of two types:
1. Tap root system
2. Adventitious root system
Root developing from any part of the plant other than the radicle is called adventitious root. In some plants, like GRASS, MONSTERA and BANYAN TREE, many roots arise from parts of the plant (from a stem, leaf or other plant parts) other than the radicle. These types of roots are called adventitious roots. Thus the adventitious root system is comprised of the adventitious roots of a plant along with their branches. Most aerial roots and stilt roots are adventitious.
The adventitious roots of plants like Parthenocissus tricuspidata (commonly known as Japanese creeper, Boston ivy) and Hedera helix (also called Common Ivy, English Ivy) appear along the stem. These rootlets used to cling to walls and tree trunks and aid the plants in climbing.
Adventitious roots are produced all along the rhizome (also referred as creeping rootstalks or rootstocks.), a stem of a plant that is generally seen underground, and often sends out roots and shoots from the nodes, mainly on the lower surface.
Specialized stems of Stolons, Corms, Tubers may have adventitious roots.
The Adventitious Roots System can be MODIFIED in the following ways:
1. Food-Storage Roots
In certain plants roots are enlarged and store large quantities of starch and other carbohydrates. The adventitious roots store food and become fleshy and swollen while assuming the following shapes.
(a) Tuberous Roots:
These plants are without any definite shape. Example: Sweet Potato
(b) Fasciculated Root (tuberous root):
In these types of plants the tuberous roots occur in clusters at the base of the stem Example: Asparagus and Dahlia.
(c) Nodulose Roots:
In these types of plants the roots become swollen near the tips. Examples: Mango ginger and Turmeric
2. Roots are modified for additional support
(d) Stilt roots:
In these types of plants the adventitious roots arise from the first few nodes of the stem. To provide support to the plant these roots penetrate obliquely down (sloping at an angle) into the soil.
Examples: Maize, sugarcane and Pandanus.
(e) Prop roots:
Prop roots give mechanical support to the aerial branches. As we can see, banyan trees with many large prop roots that have developed from the branches give mechanical support to the aerial branches.
The lateral branches grow vertically downwards into the soil. Slowly, the roots become thick and stout to act as pillars for the tree.
3. Roots are modified for other vital functions
(f) Epiphytic roots:
These adventitious roots grow as epiphytes to develop special kinds of aerial roots which hang freely in the air upon the branches of other trees and sometimes upon some other object such as a building or a telegraph wire.
These aerial roots possess a special sponge like tissue called velamen. Since these plants do not have direct contact with the soil, velamen helps the plant in absorbing the atmospheric moisture and nutrients from the air, rain and debris and stores them for the plant.
We may see epiphytic roots in some orchids, ferns, cacti, mosses and algae.
(g) Photosynthetic or assimilatory roots:
In certain plants the adventitious roots become green and carry on photosynthesis (Photosynthesis is the process by which green plants turn carbon dioxide and water into food using energy from sunlight).
Photosynthetic roots of Epiphytic orchids and Tinospora are among various kinds of aerial roots produced by plants.
The green aerial roots of Epiphytic which grow above the ground surface, using other plants or objects for support carry out photosynthesis. Similarly during rainy season in Tinospora (Tinospora cordifolia) which is known by the common name Guduchi, roots arise as green hanging threads from the nodes of the stem. They assimilate co2 in the presence of sunlight.
(h) Parasitic roots or Haustoria roots
These plants cannot make their own food and obtain its food from the host, as they have no chlorophyll, necessary for photosynthesis. Parasitic plant derives its sustenance from another plant and develops along the stem in contact with the host.
In these plants the adventitious roots are given out from the nodes and penetrate into the host tissue via peg-like projections called haustoria and enter into its conducting tissue. From these conducting tissues (water-conducting and food-conducting tissues) of the host they acquire the required food materials
Examples: Cuscuta, Pinedrops, Dodders, Broomrapes etc.