OBJECTIVES
- define and identify root;
- distinguish between different types of root systems;
- describe and illustrate different regions of a root apex;
- describe various modifications and functions of roots;
- describe and distinguish between primary structure of dicot and monocot root;
- illustrate and explain the mode of secondary growth in a dicot root;
- describe the deep-seated (endogenous) origin of lateral roots.
|
|
FUNCTIONS OF ROOTS
(i) Anchorage –
Roots anchor the plant firmly to the soil (mechanical function).
(ii) Absorption –
Roots absorb water and mineral salts and conduct them
upwards (physiological function).
(iii) Special functions –
By undergoing modifications in their structure, roots
perform special physiological functions like food storage, assimilation,
absorption of atmospheric moisture, sucking food from host, better gaseous
exchange and mechanical functions like floating (buoyancy), stronger
anchorage and climbing.
|
|
ROOT SYSTEM
The root system is the descending (growing downwards) portion of the plant axis. When a seed germinates, radicle is the first organ to come out. It elongates to form primary or the tap root. It gives off lateral branches (secondary and tertiary roots) and thus forms the root system.
It branches through large and deep areas in the soil and anchors the plant very firmly. It also plays another vital role of absorbing water and mineral salts from the soil and transporting them upwards.
CHARACTERISTICS OF ROOTS
- Non green due to absence of chlorophyll;
- Not divided into nodes and internodes;
- Absence of leaves and buds;
- Positively geotropic (grow towards gravity);
- Positively hydrotropic (grow towards water);
- Negatively phototropic (grow away from light).
TYPES OF ROOT SYSTEMS
Root systems are mainly of two types:
(i) Tap root system —
It is the root system that develops from the radicle and
continues as the primary root (tap root) which gives off lateral roots. They provide very strong anchorage as they are able to reach very deep into the soil. It is the main root system of dicots e.g. gram, china rose, neem
(ii) Fibrous root system —
In this root system, the primary root is short lived.
A cluster of slender, fiber-like roots arises from the base of the radicle and
plumule which constitute the fibrous root system. They do not branch
profusely, are shallow and spread horizontally, hence cannot provide strong anchorage. Fibrous root system is the main root system of monocots, e.g. maize, grasses, wheat.
TYPES OF ROOTS
(i) Tap root –
It is the primary and the main root that develops from the radicle,
bears many branches and remains underground. It is usually found in dicots
e.g. sunflower, mustard, carrot, mango (Fig. a).
(ii) Adventitious root –
These are roots that develop from any part of the plant
except the radicle. They may be aerial or underground
(Fig. b). They may grow from node (money plant, bamboo), stem cutting
(rose), tree branch ( banyan)or stem base (fibrous roots in monocots).
REGIONS OF ROOT
Apical region of roots of any root system shows the same zones or regions as can
be seen in Fig.a. A longitudinal section of root apex (Fig.b) shows the
following structure
1. Root cap region —
It is a thimble-
like structure produced by meristematic
(rapidly dividing) zone and protects the tender apex (apical meristem) from
harsh soil particles. As the root grows further down in soil, root cap wears out but it is constantly renewed . In aquatic plants (Pistia and water hyacinth) root cap is like a loose thimble called root pocket.
2. Region of meristematic cells —
is a small region of actively dividing cells called the apical meristem. It consists of :
(i) Dermatogen (outermost layer whose cells mature into epiblema and root cap);
(ii) Periblem (inner to dermatogen whose cells mature into cortex) and
(iii) Plerome (central region whose cells mature into stele). In monocots, cap is formed by independent group of cells known as Calyptrogen.
3. Region of elongation —
Lies next to the meristematic region, the cells elongate and enlarge to make the root grow in length.
4. Region of maturation —
Lies next to the region of elongation. The cells mature
and differentiate into various tissues constituting
(i) Root hair or piliferous region having unicellular hairs which absorb water and mineral salts from soil and
(ii) Permanent region which lies behind the root hair zone and is without hairs. It produces lateral roots, anchors the plant in soil and conducts water and minerals upwards.
|
