Growth in plants occur by the activity
of the apical meristems present in the apices or tips of stem, branches and
roots. Apical meristems divide and form new cells. In due course such tissues
become permanent and lays down the basic or fundamental body of plants. Thus
the tissues which derive their origin from the apical meristem are called
primary permanent tissues and the plant body made of primary tissues is called
primary body. Thus, the apical meristem
is responsible for the formation of the primary growth of the plants.
Thus, apical meristems cause linear
growth.
In monocotyledons and pteridophytesthis
primary structure remain as such throughout the life of the plants. It is
structurally and functionally both self-sufficient.
In Dicotyledons and Gymnosperms the
primary growth is not able to bear the load of plant – branches, flowers,
fruits. Therefore, there is required additional strength to the stem and
branches.
Thus, growth in thickness is called
secondary growth. The increase in thickness due to the addition of secondary
tissues cut off by the vascular cambium and the cork cambium in the stellar and
extra-stelar regions respectively, is called secondary growth.
Secondary growth is common in
gymnosperms and dicotyledons, but is normally absent in monocotyledonous
plants. However, in some exceptional genera of monocotyledons such as Dracaena
and Yucca, anomalous secondary growth can be observed.
Secondary
Growth in Dicot Stems
Secondary growth in dicot stems is
initiated in the intrastelar region with the activities of cambium (fascicular
or interfascicular cambium). In the extrastelar region cork cambium gives rise
to the periderm.
Intrastelar Secondary
Growth:
The secondary growth occurring in the
stellar region is called intrastelar secondary growth. It includes the
following:-
1.
Formation
of Cambial Ring:
In young dicotyledonous
stem limited number of vascular bundles are arranged in form of a ring. Each
vascular bundle is conjoint, collateral, open and endarch. They are called as
open because a single layer of cambium cells called the intra fascicular cambium (fascicle
– bundle) is present between the xylem and phloem.
In between the vascular
bundles there are medullary rays, the cells of which become mertistematic and
form a cambium strip called the interfascicular
cambium, i.e., the cambium in between the two vascular bundles.
The fascicular and
inter fascicular cambium join laterally to form a complete ring of cambium.
Activity
of Cambium:-The vascular cambium consists of
two types of initiating cells – the fusiform
initials and ray initials. The
fusiform initials are elongated and spindle shaped. They produce the elements
of secondary xylem and secondary phloem. The ray initials are small and
isodiametric. They produce phloem rays to the outside and the xylem rays to the
inside. These rays are called the vascular rays.
Those cells which are produced outside the
cambial ring differentiate into secondary phloem and those produced to the
inner side of the cambial ring differentiate into secondary xylem.
The cambium cells
divide continuously in this manner producing secondary tissues on both sides of
it. In this way, new cells are added to the xylem and phloem, and the vascular
tissues increase in size.
Normally the cambium
produces more amount of secondary xylem to the innerside and less amount of
secondary phloem to the outerside.
The cell formed from
the ray initials of cambium in the region between the vascular bundles become
the secondary medullary rays. They extend from pith to the secondary xylem and
phloem. The portion of the ray present in the xylem region is called xylem ray
or wood ray and the portion of the ray in the phloem is called phloem ray.
These rays help in radial conduction of water, salts and food materials.
Extra
Stelar Secondary Growth:
Due to the formation of
secondary vascular tissues in the stellar region, an outward pressure is
exerted on the epidermis. Due to this, epidermisgets stretched and ultimately
tends to rupture exposing the living cells.
At this stage, a new
protective layer called the periderm is produced in the cortical region.
Periderm is formed by
the activity of a secondary lateral meristem called phellogen or cork cambium.
Secondary growth in
cortex begins with the appearance of a meristematic layer either sub-epidermal
or epidermal (e.g., Teak, Azadirachta) or in the cortex (e.g., Aristolochia).
In contrast to the
vascular cambium, the phellogen is relatively simple in structure and composed
of one type of cells. They are rectangular and have vacuolated protoplasts and
may contain tannins and chloroplasts.
The cells of phellogen
divide vertically and cut off many cells toward the outside and toward the
inside. The cells formed towards the innerside develop into secondary cortex or phelloderm and those
cells formed towards the outer side develop into phellem or cork. Usually more amount of cork is produced than the
secondary cortex. The phellogen (cork cambium), phellem (cork) and phelloderm
(secondary cortex) together constitute periderm.
Phellem arises towards
the outerside of the phellogen. They are polygonal and uniform in shape. The
cells are closely arranged without intercellular spaces and with thin cellulose
cell walls. The cells later become dead by losing their protoplasts and their
walls become thicker due to the deposition of suberin. The cells are impervious
to water and gases. They give protection to inner parts of the organ.
Commercial Cork:
The phellem of
Quercussuber (oak tree) is the source of commercial cork. In this plant, the
phellogen arises in the epidermis, which forms extended masses of cork tissues.
At the age of twenty years, when the tree is about 40 cm in circumference, this
outer layer, known as virgin cork.this cork is stripped off for the quick
formation of commercial cork.
The exposed tissue
dries out to about 1/8 inch in depth. A new phellogen is established beneath
the dry layer and rapidly produces a massive cork of a better quality than the
first. After 9 or 10 years the new cork layer of formed with sufficient
thickness to be commercially valuable and is in turn removed.
The stripping of the
cork take place at intervals of about nine years until the tree is 150 or more
years old. The commercial cork cells have thin walls and cells are filled with
air. Due to suberin, it is impervious to water and resistant to oil. Because of
air filled lacuna, the cork is light in weight, and has thermal insulator
qualities. The important properties of the commercial cork are its
imperviousness, its lightness, toughness and elasticity.
Phelloderm:
The phellogen cuts off
the phelloderm cells towards inner side. The phelloderm cells are living cells
with cellulose walls. The cells contain vacuolated cytoplasm and shows a
conspicuous nucleus. In most plants, they resemble cortical cells but they are
arranged in radial rows because they arise from the tangentially dividing
phellogen.
In some species, they
act as photosynthetic tissue and aid in starch storage.
Bark:
All the tissues outside
the vascular cambium of the stem is called as bark. Thus, it includes the
secondary phloem and periderm.
As the periderm
develops, it becomes separated by a non-living layer of cork cells from the
living tissues. The tissue layers thus separated become dead.
When the cork cambium
arises from the inner layers of the cortex, the bark is thick; e.g., Thuja.
If it is formed from the outer layers, the bark is thin; e.g., Psidium guajava.
When the cork cambium
is organized in the form of a complete ring, the bark that is produced also
develops in the form of a ring, which can be stripped easily. This is known as
ring bark. E.g., Betula, Clematis. Whereas in Psidium, Eucalyptus,
the cork cambium originates in strips, the bark is in form of overlapping
strips. The bark is removed as strips or scales. Such bark is known as scale
bark.
Bark in Cinchona
(yields quinoine) and Cinnamomum (source of Dalchini) are commercially
important.
Lenticels:
Due to secondary
growth, the periderm develops in place of the epidermis. Since the cork tissue
is composed of closely arranged, dead, suberin coated cells, gaseous exchange between
the internal tissues and the external atmosphere is obstructed.
So, to carry out the
gaseous exchange, small openings composed of mature cells develop. These
openings are called as lenticels. They are located opposite to stomata and
carry out their function in the secondary body of the plants.
The lenticels originate
beneath the stomata, either just before, or simultaneously with the initiation
of the first layer of the periderm. As the lenticel formation begins, the
parenchyma cells found near the sub-stomatal cavity lose their chlorophyll and
divide irregularly in different planes giving rise to a mass of colourless,
rounded, thin walled, loose cells called complementary cells.
As the complementary
cells increase in number, pressure is caused against the epidermis and it
ruptures. The thin walled loose complementary cells alternate with masses of
more dense and compact cells called the closing cells. These cells together
form a layer called closing layer.
Complementary cells are
thin-walled, rounded and loose with sufficiently developed intercellular spaces
among them. Their cell walls are not suberized. Due to the presence of profuse
intercellular spaces, the lenticels perform the function of exchange of gases
between the atmosphere and internal tissues of the plant.
The secondary xylem in
the stems of perennial plants commonly consists of concentric layers each one
of which represents a seasonal increment. In transverse section of the axis,
these layers appear as rings, and called annual rings or growth rings.
They are commonly
termed as annual rings because in woody plants of temperate regions and in
those of tropical regions where there is an annual alternation of growing and
dormant period, each layer represents the growth of one year.
The cambium exhibits
its activity as periodical or seasonal due to climatic variation. During spring
season, the plant has to translocate more water and mineral because they
develop new buds, leaves and flowers. Therefore, the cambium becomes more
active in this season and forms xylem vessel with wider cavities. The xylem
formed during spring season is called early
wood or spring wood.
On the other hand,
during winter the rate of assimilation is decreased and there is less need of
vessels for sap transport, the cambium is less active and gives rise to narrow
vessels, tracheids and wood fibres. The xylem formed during winter is called late wood or autumn wood.
Thus spring wood with
wider vessels and autum wood with narrow vessels formed during one year
together make an annual ring or growth ring. Thus, the periods of active growth
alternate with the periods of slow growth.
Generally, the late
wood is more denser and harder than the early wood.
By counting the total
number of annual rings, the age of the plant can approximately be determined.
Thus, determination of age of a tree by counting the annual rings is known as dendrochronology.
Porous
wood and Non Porous wood:
Gymnosperms (conifer
and cycads) lack vessels and their wood is made of only tracheids. Therefore,
their wood is called non-porous and soft
wood. On the other hand angiosperm wood is made of tracheids and vessles
both. Therefore, their wood is called porous
and hard wood. Hard wood or soft wood have no relation with physical
hardness of wood.
In porous woods, when
large vessels of unequal diameter are arranged more or less in a ring, the wood
is called ring porous wood. E.g.,
Castanea ring porous vessels conduct more water. On the other hand, when
vessels of equal dimensions are found uniformly distributed, the wood is called
diffuse porous wood, e.g., Acer,
Betula.
The outer region of the
old trees consisting of recently formed xylem elements is sapwood or alburnum. This is of light colour and contains some
living cells in association with vessels and fibres. This part of the stem
performs the physiological activities, such as conduction of water and
minerals, storage of food, etc.
The central region of
the old trees, which was formed earlier is whose cells are inactive,
non-fucntional without any living cells is called as heart wood or duramen. The secondary xylem in this region is filled
up with tannins, resins, gums and other substances which make it hard and
durable and it is dark in colour. Their vessels are plugged with tyloses.
The function of
heartwood is no longer conduction, it gives only mechanical support to the
stem.
The sapwood changes
into heartwood very gradually. During the transformation a number of changes
occur – all living cells lose protoplasts, water content of cell walls are
reduced, food material are withdrawn from the living cells, tyloses are formed.
From economic point of
view, heartwood is more useful than sapwood. Heartwood, as timber is more
durable than sapwood, because the reduction of food materials available for
pathogens by the absence of protoplasm and starch.
The haemotoxylin is
obtained from the heartwood of Haematoxylon campechianum.
Because of the absence
of resin, gums and colouring substances, sapwood is preferred for pulpwood, and
for wood to be impregnated with preservatives.
Tyloses:
In many plants, axial
and ray parenchyma cells located next to the vessels form ballon-like outgrowth
through pit cavities into the lumen of the vessels. These outgrowths are called
as tyloses.
The parenchyma cells,
adjoining the half-bordered pits of vessels, penetrate into the vessel in the
form of short protuberances. These protuberances gradually enlarge to form
ballon-like structuresThe nucleus and
part of the cytoplasm of the parenchyma cell commonly migrate into the tyloses.
The tyloses are filled with starch, resins, gums and other substances.
Usually, they are
sufficiently large and the lumen of the vessel is almost blocked. They add to
the durability of the wood. Tyloses prevent rapid entrance of water, air and
fungus by blocking the lumen of the vessel. In many plants the development of
tyloses takes place by means of wounding.
