and primary xylem These primary vascular tissues maybe
arrange like a signet ring The primary phloem usually occupies
4.2 INTERNAL MORPHOLOGY
the outer part and the xylem, inner in location with vascular
The division and increase in number and size of cells in the cambium usually located between the two
acc meristem or promeristem results in the elongation of the
In some, the primary vascular tissues maybe formed like
young shoot with ma Increase in number and size,
separate rings each group is called vascular bundle with a
differentiation occurs, that is ce morphologically and
fascicular cambium located in between. The center-most
physiologically from the meristematic type where from. Thus, a
portion is the pith composed mostly of large parenchymatous
short distance below the apical menstem we can fine of
cells with numerous intercellular spaces
primary meristematic tissues, namely the protoderm, grown
monstein and procambium. Old Woody Dicot Stem
Young Dicot Stems (Herbaceous and Woody) In perrenial dicots (shrubs and trees) after the primary tissues
are fully formed, the vascular bundle and cork cambium
Primary growth brings about the elongation of stems and
become active and give rise to new cells. This increase in
establishes the basic pattern of cells
diameter or thickness of stem is called secondary growth. A
In young dicot stems (both herbaceous and woody), the
In a dicot stem, the fascicular cambium becomes meristematic
primary plant body composed of three regions, namely:
and forms a strip of interfascicular cambium on either side and
1. Epidermal region - usually a single superficial layer of cells forms a cambiurn ring. This ring gives rise to now cells on both
that covers all other primary tissues: The outer wall of cells sides. Those which are cut off on outer side modify into
exposed to air is usually covered with a waxy substance called secondary phloem. The new cells cut off on the inner side of
cutin the cambium forming the secondary xylem or wood. The
cambium is more active on the inner side and so the main bulk
Epidermis may also be differentiated into epidermal cells, of the dicot stem is secondary xylem after secondary growth.
quard cells and epidermal hairs or trichomes Primary xylem remains more or less intact in or around the
2. Cortex region this complex region is derived from the center of the stem.
ground meristem This is found inward next to the epidermis. It Annual Rings
consists of large thin-walled parenchyma which is the principal
tissue so with collenchyma, sclerenchyma and some secretory In areas of climatic variations, the activity of the cambium is
cells. not the same throughout the year.
3 Vascular cylinder region- this is the innermost tissue In spring, the cambium is more active and forms a large
region of the stem This is composed of the primary vascular number of vessels with wider cavities. The wood formed
tissue. This primary vascular tissue consist of primary phloem during the spring is called springwood. During inactive period,
the vessels that are produced are smaller in size and this Monocot Stem
wood is called summer wood. These two types of wood form a
Most monocot stems lack secondary growth. They only
concentric ring known as the annual ring or growth rings.
possess primary permanent tissues. A typical monocot stem
Successive annual rings are formed year after year and by
would show the following structures:
counting the total number of annual rings the age of the tree
can approximately be determined. 1. Epidermis - this is the outermost layer of the stem, which is
single-celled. Cuticle maybe present or absent. Stomata may
Heartwood and Sapwood
also be found here.
In old trees, the central region of secondary wood is filled with
2. Cortex - this consists of large thin-walled parenchyma cells
tannin and other substances and this is called the heartwood,
not differentiated specifically into cortex, endodermis, etc. as in
It is dark in color because of the presence of tannins, oils,
dicot. Just below the epidermis a few layers of collenchyma /
gums, resins, etc. This region functions for mechanical
sclerenchyma cells forming the hypodermis maybe found.
support. The outer region of secondary wood is lighter in color
and is known as the sapwood. This is used to transport water 3. Vascular cylinder - the arrangement of vascular bundles
and minerals from root to the leaves. are scattered. They are found in large numbers in the ground
tissue, more in number towards the periphery than in the
The Periderm
center. The vascular bundles are closed because they have
This is the protective tissue that replaces the epidermis. Radial one xylem and phloem with the cambium layer absent.
expansion resulting from secondary growth eventually ruptures
the epidermis of stems and roots. This ruptured epidermis is
replaced by periderm which is a suberized layer that protects
underlying tissues.
Periderm consists of 3 tissues, the phellogen (cork cambium),
phellen (cork) and phelloderm (secondary cortex).
The cork cambium gives rise to the cork. Cork cells are dead,
suberized and thick-walled. They are impervious to water.
The bark consists of all dead tissues outside the cork
cambium, it is composed of epidermis, lenticels, cork and
sometimes hypodermis. The bark has small pores for gas
exchange called lenticels.
.Types of roots and root systems
End of the root that absorbs minerals and water is the and root TAPROOT
cap
The primary root, or radicle, is the first organ to appear when a
some cells in the outer layer develop root hairs. The cortex seed germinates. It grows downward into the soil, anchoring
and vascular cylinder are separated by the endodermis. the seedling. In gymnosperms and dicotyledons (angiosperms
with two seed leaves), the radicle becomes a taproot. It grows
lower epidermis-covers underside of leaf.
downward, and secondary roots grow laterally from it to form a
upper epidermis-covers top of leaf. taproot system. In some plants, such as carrots and turnips,
the taproot also serves as food storage.
Stomata pore to allow gas exchange.
FIBROUS
guard cells control open/close of stomata
Grasses and other monocotyledons (angiosperms with a
Vein (Fibrovascular bundle) - contains xylem + phloem single seed leaf) have a fibrous root system, characterized by
Mesophyll- middle of leaf a mass of roots of about equal diameter. This network of roots
does not arise as branches of the primary root but consists of
Cuticle-Waxy covering, prevent water loss (Sweet potato, many branching roots that emerge from the base of the stem.
potato, radish)
ADVENTITIOUS
Roots grow from any part of the plant other than the radicle.
They may develop from base of stems, nodes, internodes and
in some cases from leaves.They are especially numerous on
underground stems, such as rhizomes, corms, and tubers, and
make it possible to vegetatively propagate many plants from
stem or leaf cuttings. Certain adventitious roots, also known as
aerial roots, either pass for some distance through the air
before reaching the soil or remain hanging in the air. Some of
these, such as those seen in corn (maize), screw pine, and
banyan, eventually assist in supporting the plant in the soil.
Four root functions include: anchoring the plant in place,
taking up water and nutrients, transporting water and nutrients
to other parts of the plant, and storing extra food.
SPECIAL FUNCTIONS OF ROOTS
1. Food storage – enlarged roots with large quantities of
starch and carbohydrates.
2. Water storage – fascicled roots – enlarged fleshy and
succulent roots.
3. Reproduction / propagation – formation of adventitious
root buds.
4. Gas exchange – presence of pneumatophores
especially in mangrove.
5. Support – a.) buttress roots – big roots that look like
they arise from the base of a trunk.
b.) prop or stilt roots – aerial roots arising from
the branches.
c.) brace roots – aerial roots arising from the
main stem which then penetrates the
ground.
d.) clinging roots – aerial roots that clings to
some kind of support.
6. Protection – presence of spines or thorns.
7. Photosynthesis – green aerial roots just like the
velumen roots or orchids.
