Modifications of Roots:
When roots have to perform some special function, in addition to or instead of their normal functions, they make some structural changes which are described as modifications of root.
(A) Modifications of Tap Root
(I) For Food Storage:
The tap root becomes swollen and fleshy due to the stored food.
The secondary roots remain thin. Hypocotyl (embryonic region between cotyledons and radicle) may also join the tap root in storing food.
Stem is reduced and discoid in the beginning and bears radical leaves.
The swollen tap root acquires some typical shape and is accordingly classified into following three types:
i) Fusiform Root:
The storage root which is swollen in the middle part and tapers towards base and apex is called fusiform root.
e.g. Radish. (Raphanus sativus)
ii) Conical Root:
The storage_root which is broad ·at the base and gradually tapers towards apex is called conical root.
e.g. Carrot (Daucus carota)
iii) Napiform Root:
The base of root which is much swollen, almost spherical and abruptly tapers into narrow apex, is called napiform root.
e.g. Beet ( Beta vulgaris)
(II) For Respiration:- Pneumatophores or Respiratory roots :
Plants growing in saline swamps, marshy places and salt lakes are called halophytes.
Many halophytes develop special kinds of roots called respiratory roots or pneumatophores.
Roots of these plants do not get air for respiration as the soil is water-logged. As a result absorption of minerals is affected.
Such plants produce special roots from the underground roots of the plant near the soil, which grow vertically upwards, i.e. negatively geotropic and come out of the soil in the form of conical spikes.
They occur in a large number around the tree trunk and are provided with pores called lenticels.
The lenticels help in gaseous exchange required for respiration e.g., Rhizophora, Avicennia, Sonneratia, Heritiera (vern. sundri), etc.
(B) Modifications of Adventitious Roots
(I) For Food Storage:
1. Simple tuberous roots:
These roots become swollen and do not assume a definite shape.
They are always borne singly.
These roots arise from the nodes of stem and enter in the soil. e.g., sweet potato or shakarkand (lpomoea batatas).
2 Fasciculated tuberous roots:
A cluster of adventitious roots of some plants become thick and fleshy due to storage of food.
These are known as fasciculated tuberous roots. There are many tuberous roots at the base of the stem. E.g. Dahlia and Asparagus.
(II) For Mechanical Support
1. Prop roots:
These roots arise from horizontal branches of trees like banyan tree (Ficus benghalensis) and grow vertically downwards till they penetrate the soil.
Secondary growth occurs in these roots, so that they become thick and act like pillars to provide mechanical support to the heavy branches.
A banyan tree growing in the Indian Botanical Garden, Howrah (Kolkata) has nearly 1700 such prop roots.
The crown of the tree has a large circumference. The tree is about 200 years old.
2. Stilt roots:
These roots normally arise from a few lower nodes of a weak stem in some monocots, shrubs and small trees.
They grow obliquely downwards and penetrate the soil and provide mechanical support to the plant.
In plants like maize, sugarcane, bajara, jowar these roots grow in whorls.
After penetrating the soil, they provide support to stem. In screw pine (kewada) or Pandanus these roots arise only from the lower surface of the obliquely growing stem to provide support.
These roots bear much folded multiple root cap.
3. Climbing roots:
Climbing plants like piper produce roots from their nodes, by means of which they attach themselves to some support and climb over it. e.g., money plant (Pathos), kali mirch (Piper nigrum), pan (Piper betel).
(III) For Special Functions - Roots
1. Epiphytic roots:
Some small plants like orchids grow perched on the horizontal branches of big trees in forest to get sunlight.
Such plants have green leaves and can photosynthesize.
These plants are called epiphytes.
Epiphytes develop special aerial, hanging roots called epiphytic roots. These roots are spongy due the presence of a special tissue called velamen, which is situated outside the cortex.
The cells of velamen tissue are hygroscopic, have porous walls and with the help of this velamen tissue these roots absorb moisture from the atmosphere. e.g., Vanda, Dendrobium, etc.
These roots fulfill the need of water in epiphytes as they do not have normal roots penetrating the soil to absorb water.
These roots are also called assimilatory roots, as they are greenish white in color, have chloroplast, and can photosynthesize up to certain extent.
2. Sucking roots or Haustoria:
These are highly specialized and microscopic roots, developed by parasites to absorb nourishment from the host. In partial parasites like Viscum album they penetrate only xylem elements of the host to absorb water and minerals.
In total parasites like Dodder or Cuscuta, (Amarwel), they establish a connection with vascular strand of host and suck food directly from phloem and water and minerals from xylem. Such roots are called parasitic roots, sucking roots or haustoria.
