6. ANATOMY OF FLOWERING PLANTS
THE
TISSUES
- A
tissue is a group of cells having common origin and function.
- Based
on the capability of cell division, plant tissues are 2 groups: Meristematic
and Permanent.
MERISTEMATIC TISSUES (MERISTEMS)
These
are the tissues where active cell division and growth occurs. Based on the
position, meristems are 3 types:
o
Apical meristems: They
occur at the tips of roots and shoots and produce primary tissues. It is 2
types:
§ Root
apical meristem: It occupies the tip of a root.
§ Shoot
apical meristem: It occupies the distant most region of
the stem axis. Some cells left behind from shoot apical meristem, constitute
the axillary bud. They are present in the axils of leaves and can form a
branch or a flower.
o
Intercalary meristems: They
occur between mature tissues. They occur in grasses and regenerate parts
removed by the grazing herbivores.
Apical and intercalary
meristems are primary meristems because they appear early in a plant life
and contribute to the formation of primary plant body. During that, specific
regions of the apical meristem produce dermal tissues, ground tissues and
vascular tissues.
o
Secondary (lateral) meristems: The
meristems that occur in mature regions of roots and shoots. They are
cylindrical meristems. They are seen in gymnosperms and dicots.
E.g. Fascicular vascular cambium, interfascicular
cambium & cork cambium. These are responsible for producing the
secondary tissues.
PERMANENT (MATURE) TISSUES
- The
cells produced by primary and secondary meristems, become structurally and
functionally specialized and lose the ability to divide. They are called permanent
(mature) cells and constitute the permanent tissues.
- They
are 2 types: Simple and Complex.
1.
Simple Permanent Tissues
- The
tissues having all cells similar in structure & function.
- 3
types: Parenchyma, Collenchyma and Sclerenchyma.
a. Parenchyma
-
- It
is found as a homogeneous layer or in patches.
- It
consists of cells with much thickened corners due to deposition of cellulose, hemicellulose
and pectin.
- Intercellular
spaces are absent.
- Cells
are oval, spherical or polygonal and often contain chloroplasts.
- Functions: They
provide mechanical support to the growing parts such as young stem and petiole
of a leaf. The cells that contain chloroplasts assimilate food.
c. Sclerenchyma
-
- They
are usually dead without protoplasts.
- Based
on the form, structure, origin and development, sclerenchyma is 2 types: fibres
& sclereids.
o
Fibres: These
are thick-walled, elongated and pointed cells, generally occurring in groups.
o
Sclereids: These
are spherical, oval or cylindrical, highly thickened dead cells with very narrow
cavities (lumen). These are found in the fruit walls of nuts; pulp of fruits
like guava, pear and sapota; seed coats of legumes and leaves of tea.
- Function:
It provides mechanical support to organs.
2.
Complex Permanent Tissues
- These
are made of more than one type of cells and they work together as a unit.
- 2
types: Xylem and Phloem.
a. Xylem
- It
functions as a conducting tissue for water and minerals from roots to the stem
and leaves.
- It
also provides mechanical strength to the plant parts.
- It
is composed of 4 kinds of elements: tracheids, vessels, xylem fibres and
xylem parenchyma.
o
o Vessel: It
is a long cylindrical tube-like structure made up of many cells (vessel
members), each with lignified walls and a large central cavity. Protoplasm
absent.
Vessel members are interconnected through perforations
in their common walls. The vessels are a characteristic feature of angiosperms.
Gymnosperms lack vessels.
o Xylem
fibres: They have highly
thickened walls and obliterated central lumens. They are septate or aseptate.
o Xylem
parenchyma: Living and thin-walled cells
with cellulosic cell walls. They store food materials (starch or fat) and other
substances like tannins. Radial conduction of water takes place by the ray parenchymatous
cells.
-
Primary xylem is 2 types:
o Protoxylem: The
first formed primary xylem.
o
Metaxylem: The
later formed primary xylem.
-
In stems, protoxylem lies towards the
centre (pith) and metaxylem lies towards the periphery. This type of primary
xylem is called endarch.
- In
roots, protoxylem lies towards periphery and metaxylem lies towards the centre.
This type of primary xylem is called exarch.
b. Phloem (Bast)
It
transports food materials from leaves to other parts.
In angiosperms, phloem is composed of sieve tube elements, companion cells, phloem parenchyma & phloem fibres. Gymnosperms have albuminous cells and sieve cells. They lack sieve tubes and companion cells.
o
The first formed primary phloem (protophloem) consists
of narrow sieve tubes. The later formed phloem (metaphloem) has bigger
sieve tubes.
Function: Conduction
of food materials from leaves.
o Companion
cells: Specialized
parenchymatous cells closely associated with sieve tube elements. Sieve tube
elements & companion cells are connected by pit fields present between
their common longitudinal walls.
Function: Maintain
the pressure gradient in sieve tubes.
o Phloem
parenchyma: It is made up of
elongated, tapering cylindrical cells which have dense cytoplasm and nucleus.
The cell wall is composed of cellulose and has pits through which plasmodesmatal
connections exist between the cells. Phloem parenchyma is absent in most of the
monocots.
Function: It
stores food material and other substances like resins, latex and mucilage.
o Phloem
fibres (bast fibres): These are made up of sclerenchymatous
cells. Generally absent in primary phloem but are found in secondary phloem.
These are much elongated, unbranched and have pointed, needle like apices. Cell
wall is quite thick. At maturity, these fibres lose protoplasm and become dead.
Phloem fibres of jute, flax and hemp are used commercially.
Function: Mechanical support & protection to soft tissues.
