RANGKUMAN BAB 38

38.1
Meristems are vital tissues in plants, responsible for both primary and secondary growth. Apical
meristems, located at the tips of roots and shoots, drive primary growth, leading to the
elongation of the plant body. These meristems give rise to three types of primary meristems:
protoderm, procambium, and ground meristem, which form the epidermis, primary vascular
tissues, and ground tissue, respectively.

In addition to apical meristems, some plants have lateral meristems, which contribute to
secondary growth, increasing the girth of the plant. In woody plants, two lateral meristems are
present: the cork cambium and the vascular cambium. The cork cambium produces the cork
cells of the outer bark, while the vascular cambium forms secondary vascular tissue, including
secondary xylem (wood) and secondary phloem.

The plant body consists of a root system and a shoot system. The root system absorbs water
and ions from the soil, while the shoot system, composed of stems and leaves, serves as a
framework for photosynthesis. Leaves, the primary sites of photosynthesis, vary in size, number,
and structure, influenced by environmental factors.

Plant tissues are categorized into three basic types: ground tissue, dermal tissue, and vascular
tissue. Ground tissue, primarily composed of parenchyma cells, functions in storage,
photosynthesis, and secretion. Dermal tissue, consisting of the epidermis in primary growth,
forms the outer protective covering of the plant. In secondary growth, the bark, considered a
part of the dermal tissue system, serves as the outer protective layer.

Vascular tissue, which includes xylem and phloem, plays a crucial role in conducting water,
dissolved minerals, and carbohydrates throughout the plant. Xylem, responsible for water and
mineral transport, consists of vessel members and tracheids, while phloem, which transports
carbohydrates and other substances, comprises sieve-tube members.

In summary, meristems are essential for plant growth and development, giving rise to various
tissues and organs that support the plant's structure and functions. Primary and secondary
growth, driven by apical and lateral meristems, contribute to the establishment of the plant body
plan, enabling the plant to adapt and thrive in diverse environments.

38.2
Plants have three basic tissues: dermal, ground, and vascular. The dermal tissue, or epidermis, is
covered with a cuticle and contains specialized cells such as guard cells and trichomes. Guard
cells, which contain chloroplasts, form stomata that allow for gas exchange. Trichomes, which
vary in form, help keep the leaf surface cool and reduce evaporation. Root hairs, tubular
extensions of epidermal cells, increase the root's surface area and efficiency of absorption.

Ground tissue is composed of parenchyma, collenchyma, and sclerenchyma cells. Parenchyma

cells, the most common type, have large vacuoles, thin walls, and an average of 14 sides. They
are capable of dividing and usually remain alive after maturing. Collenchyma cells, which have
thickened walls, provide support for plant organs. Sclerenchyma cells have tough, thick walls and
usually lack living protoplasts when mature. They serve to strengthen the tissues in which they
occur.

Vascular tissue is composed of xylem and phloem. Xylem, the principal water-conducting tissue,
contains vessels and tracheids. Vessels, which are found almost exclusively in angiosperms,
conduct water more efficiently than tracheids. Xylem also provides support for the plant body.
Phloem, the principal food-conducting tissue, transports organic materials from one part of the
plant to another. If a plant is girdled, the plant will eventually die from starvation of the roots.

The formation of trichomes and stomata has been extensively studied in Arabidopsis. The
development of trichomes requires four genes, while eight genes are necessary for extension
growth. Mutations in these genes can result in distorted trichomes and root hairs. The patterning
of stomatal distribution has also been studied through the use of mutants, providing information
on the timing of stomatal initiation and intercellular communication.

In summary, plants have three basic tissues, each with multiple cell types. Dermal tissue covers
the plant body, ground tissue provides support and storage, and vascular tissue conducts water,
nutrients, and organic materials. The formation and distribution of specialized cells and tissues is
regulated by genetic and environmental factors.

38.3

Roots have a simpler organization and development compared to stems. They are divided into
four zones: the root cap, the zone of cell division, the zone of elongation, and the zone of
maturation. The root cap, which has no equivalent in stems, is composed of two types of cells:
the inner columella cells and the outer lateral root cap cells. Its functions include protecting the
delicate tissues behind it, providing a medium for beneficial nitrogen-fixing bacteria, and
perceiving gravity. The columella cells contain amyloplasts that collect on the sides of cells
facing the pull of gravity, causing the root to bend in that direction.

The zone of cell division contains the apical meristem, which is shaped like an inverted, concave
dome of cells. The daughter cells of the apical meristem soon subdivide into the three primary
tissue systems: protoderm, procambium, and ground meristem. The patterning of these cells
begins in this zone, but it is not until the cells reach the zone of maturation that the anatomical
and morphological expression of this patterning is fully revealed.

In the zone of elongation, the cells produced by the primary meristems become several times
longer than wide, and their width also increases slightly. The small vacuoles present merge and
grow until they occupy 90% or more of the volume of each cell. No further increase in cell size
occurs above the zone of elongation.

The cells that have elongated in the zone of elongation become differentiated into specific cell
types in the zone of maturation. The cells of the root surface cylinder mature into epidermal
cells, which have a very thin cuticle. Many of the epidermal cells each develop a root hair, which
greatly increases the surface area and therefore the absorptive capacity of the root.

Some plants have modified roots that carry out specific functions. Aerial roots, such as those of
epiphytic orchids, extend out into the air and may be green and photosynthetic. Adventitious root
formation in ivy depends on the developmental stage of the shoot. Pneumatophores, which are
spongy outgrowths from the roots of some plants that grow in swamps, facilitate the oxygen
supply to the roots beneath. Contractile roots, such as those from the bulbs of lilies, contract by
spiraling to pull the plant deeper into the soil each year. Parasitic roots, such as those of dodder,
produce peglike roots called haustoria that penetrate the host plants around which they are
twined and parasitize them. Food storage roots, such as those of sweet potatoes, produce at
intervals many extra parenchyma cells that store large quantities of carbohydrates. Water
storage roots, such as those of some members of the pumpkin family, may produce roots
weighing 50 or more kilograms. Buttress roots, such as those of certain species of fig, provide
considerable stability.

38.4

Stems are the main axis of shoots, responsible for nutrient transport and support. They have an
external form with a node at the leaf attachment area and an internode in between. Leaves can
be arranged spirally, opposite, or in whorls, with a leaf blade and sometimes a petiole.
Herbaceous stems are usually green and photosynthetic, while woody stems have protective
features like winter bud scales and stipules.

Internally, stems consist of an apical meristem that produces primary tissues. Three meristems
develop from the apical meristem: protoderm (epidermis), ground meristem (parenchyma), and
procambium (xylem and phloem). A vascular cambium develops between the primary xylem and
phloem in dicots, allowing for secondary growth. In monocots, vascular bundles are scattered
and do not form a continuous ring, preventing secondary growth.

Woody dicots develop a cork cambium in the outer cortex, producing cork tissues and
phelloderm cells. Cork tissues cut off water and food to the epidermis, which dies and sloughs
off. Lenticels, unsuberized cells beneath stomata, allow gas exchange to continue.

Modified stems serve special purposes, such as natural vegetative propagation. Bulbs, like
onions and lilies, are large buds with fleshy leaves and adventitious roots. Corms, such as
crocuses and gladioluses, consist mostly of stem tissue with papery, nonfunctional leaves and
adventitious roots. Rhizomes, found in perennial grasses and irises, are horizontal stems with
axillary buds and adventitious roots.

Runners and stolons, like strawberries and Irish potatoes, are horizontal stems that grow along
the ground. Tubers, like Irish potatoes, are swollen stem tips that store carbohydrates. Tendrils,
found in grapes and Boston ivy, are modified stems that aid in climbing. Cladophylls, seen in
cacti and prickly pear, are flattened, photosynthetic stems resembling leaves.

38.5

Leaves are crucial for basic plant functions, especially as the primary sites of photosynthesis on
land. They are determinate structures, usually with flattened blades and a stalk (petiole),
although monocots like grasses lack a petiole and tend to sheathe the stem. Leaves can be
categorized into two groups based on their veins: microphylls have one vein and are small,
associated with the phylum Lycophyta, while megaphylls have several veins and leave a gap in
the stem's vascular cylinder.

Leaf blades come in various forms, from oval to deeply lobed to having separate leaflets. Simple
leaves have an undivided blade, while compound leaves are divided into leaflets. Leaf blades may
be alternately arranged or in opposite pairs, and less often, in whorls.

The entire surface of a leaf is covered by a transparent epidermis, with stomata for gas exchange
and regulation of water movement. The mesophyll, the tissue between the upper and lower
epidermis, contains chloroplasts and is interspersed with veins of various sizes. In most dicot
leaves, there are two distinct types of mesophyll: palisade mesophyll closest to the upper
epidermis and spongy mesophyll beneath it. Monocot leaves, however, have undifferentiated
mesophyll and lack palisade and spongy layers.

Leaves have also undergone remarkable adaptations as plants colonized various environments.
These modifications include floral leaves (bracts), spines, reproductive leaves, window leaves,
shade leaves, and insectivorous leaves. Some plants, like the Venus flytrap, have leaves that
capture and digest insects to supplement their nutrient intake. These adaptations highlight the
incredible diversity and resilience of plant life.

QUESTIONS
. What are the three major tissue systems in plants? What are their functions?
a. Dermal Tissue
b. Ground Tissue
c. Vascular Tissue
• Ground tissue supports the plant and stores food and water.
• Epidermis forms an outer protective covering for the plant.
• Vascular tissue conducts water, carbohydrates, and dissolved minerals to different parts of the
plant.

. What is the function of xylem? How do primary and secondary xylem differ in origin? What
are the two types of conducting cells within xylem?
Function: Transports soluble mineral nutrients and water molecules from the roots to the aerial
parts of the plant. Xylem conducts water and minerals from the roots to shoots and leaves. The
main difference between primary xylem and secondary xylem is that primary xylem is formed by
the primary growth of the procambium whereas secondary xylem is formed by the secondary
growth of the vascular cambium. The main function of the xylem tissue in plants is to conduct
water and minerals from root to the leaf.
The conducting cells of the xylem – tracheids and vessel elements (also called vessel members)
– have the remarkable feature that they perform their main physiological role when they are
dead.

. What is the function of phloem? How do the two types of conducting cells in phloem
differ?
Function: Transports food and other nutrients including sugar and amino acids from leaves to
storage organs and growing parts of the plant.

Phloem, a tissue in plants responsible for transporting sugars and other organic nutrients,
contains two primary types of conducting cells: **sieve tube elements** and **companion
cells**. Here's how they differ:
### 1. **Sieve Tube Elements**
- **Structure**: Sieve tube elements are elongated cells that are aligned end-to-end to form a
continuous tube. The end walls between these cells, called sieve plates, have large pores to
allow for the flow of sap.
- **Function**: Their main function is to transport the products of photosynthesis, primarily
sucrose, from the leaves to other parts of the plant.
- **Cytoplasm**: They have a thin layer of cytoplasm but lack a nucleus, ribosomes, and other
organelles. This reduction in cellular contents minimizes obstruction to the flow of sap.
- **Life Span**: Sieve tube elements are alive at maturity but rely on companion cells to stay
functional.

### 2. **Companion Cells**

- **Structure**: Companion cells are smaller and located adjacent to sieve tube elements. They
are connected to sieve tube elements via plasmodesmata, which are microscopic channels that
allow for the transfer of materials between the two cell types.
- **Function**: Companion cells play a crucial role in maintaining the function of sieve tube
elements. They supply the necessary metabolic support, including ATP and proteins, and help
load and unload the sugars into and out of the sieve tubes.
- **Cytoplasm**: Unlike sieve tube elements, companion cells have a dense cytoplasm and
contain a nucleus, ribosomes, and other organelles necessary for cellular function.
- **Life Span**: Companion cells are also alive at maturity and have a longer life span
compared to sieve tube elements.

In summary, sieve tube elements are specialized for transport and have minimal cellular content,
while companion cells are metabolically active and essential for maintaining the function of the
sieve tube elements.

. Comparison of Monocot and Dicot Roots

Monocot and dicot roots differ significantly in the arrangement of tissues, reflecting their distinct
evolutionary paths. Here’s a comparison:

Monocot Roots

Vascular Bundle Arrangement: In monocot roots, the vascular bundles (xylem and phloem) are
arranged in a circular pattern around the central pith. Typically, there are more than six vascular
bundles.
Pith: Monocot roots have a prominent pith at the center, which consists of parenchymatous cells.
Xylem: The xylem vessels are typically large and scattered in a ring, with the metaxylem located
towards the center and the protoxylem towards the periphery.
Phloem: The phloem tissues are located between the xylem vessels in a circular arrangement.
Cortex: The cortex is relatively wide, consisting of several layers of parenchyma cells, and it
contains air spaces.
Pericycle: The pericycle, located just outside the vascular bundles, is where lateral roots
originate.
Endodermis: The endodermis, the innermost layer of the cortex, is distinct, with Casparian strips
that regulate the flow of water and nutrients.
Dicot Roots

Vascular Bundle Arrangement: In dicot roots, the xylem is typically arranged in a star-shaped
pattern, with the phloem located between the arms of the xylem. There are usually 2-6 xylem
bundles.
Pith: Dicot roots either have a very small or absent pith.
Xylem: The xylem is arranged in a central star-like structure, with the number of arms (rays)
varying among species.
Phloem: Phloem is found in between the arms of the xylem star, creating an alternating pattern of
xylem and phloem.
Cortex: The cortex is also composed of parenchyma cells, but it is generally narrower than in
monocots and lacks air spaces.
Pericycle: The pericycle is a single layer of cells just inside the endodermis, and it also gives rise
to lateral roots.
Endodermis: Similar to monocots, dicot roots have an endodermis with Casparian strips, but it
tends to be more prominent.

. Formation of Lateral Branches of Roots

Lateral roots, also known as branch roots, are formed from the pericycle, which is a layer of cells
just inside the endodermis of the root.

Initiation: The process of lateral root formation begins with the pericycle cells, which become
mitotically active. This activity is often triggered by hormonal signals, particularly auxin.
Development: The dividing pericycle cells form a small protrusion that pushes outward through
the surrounding tissues of the root, including the endodermis, cortex, and epidermis. As these
new root cells continue to divide, they form a new root meristem, which eventually develops into
a lateral root.
Emergence: The developing lateral root grows and emerges from the parent root, eventually
establishing itself in the soil as an independent root structure capable of absorbing water and
nutrients.
This process ensures that lateral roots arise from deep within the root structure, maintaining a
connection to the vascular tissue of the parent root and ensuring the efficient transport of water,
nutrients, and signals.

. Types of Cells Produced by Vascular Cambium Divisions

The vascular cambium is a lateral meristem that plays a crucial role in the secondary growth of
plants, primarily in dicots and gymnosperms. When the vascular cambium divides, it produces
different types of cells depending on the direction of the division:

Outward Division

Phloem Cells: When the vascular cambium divides outwardly (towards the outer side of the stem
or root), it produces secondary phloem cells. Secondary phloem is responsible for transporting
organic nutrients, mainly sugars, from the leaves to other parts of the plant.
Inward Division

Xylem Cells: When the vascular cambium divides inwardly (towards the inner side of the stem or
root), it produces secondary xylem cells. Secondary xylem, commonly known as wood, is
responsible for transporting water and minerals from the roots to the rest of the plant and also
contributes to the structural support of the plant.
Lateral Division

More Cambial Cells: When the vascular cambium divides laterally (parallel to the surface of the
stem or root), it produces additional cambial cells. These new cambial cells contribute to the
expansion of the cambium ring, allowing it to continue producing secondary xylem and phloem
as the plant increases in girth.

. Why Don’t Monocots Have Secondary Growth?

Monocots do not undergo secondary growth due to several key differences in their structure and
development compared to dicots:

1. Lack of Vascular Cambium

In monocots, the vascular bundles are scattered throughout the stem, rather than arranged in a
ring as in dicots. Because of this scattered arrangement, monocots do not develop a vascular
cambium, the lateral meristem responsible for secondary growth (increase in thickness). Without
a vascular cambium, monocots cannot produce the secondary xylem and phloem required for
secondary growth.
2. Primary Growth Focus

Monocots are primarily herbaceous and are adapted to environments where rapid vertical growth
(primary growth) is more beneficial than thickening (secondary growth). They achieve structural
support through other mechanisms, such as fibrous roots and strengthened vascular bundles
surrounded by sclerenchyma (supporting tissues).
3. Evolutionary Adaptation

Monocots have evolved different strategies for support and resource transport that do not rely
on the formation of secondary tissues. These adaptations allow monocots to thrive without the
need for a thickened,

. Differences Between Simple and Compound Leaves

Simple Leaves:

Structure: A simple leaf has a single, undivided blade that is attached to the stem by a petiole
(leaf stalk). The blade may be lobed or toothed, but the division does not reach the midrib or
petiole.
Examples: Oak, maple, and mango are examples of plants with simple leaves.
Compound Leaves:

Structure: A compound leaf has a blade that is divided into multiple leaflets. These leaflets are
attached to a central rachis (an extension of the petiole) or directly to the petiole, depending on
the type of compound leaf.
Types: Compound leaves can be further classified into two types:
Pinnately Compound: Leaflets are arranged along a central rachis, resembling a feather.
Examples include rose and neem.
Palmately Compound: All the leaflets arise from a common point at the end of the petiole,
resembling the palm of a hand. Examples include horse chestnut and clover.
Three Common Types of Leaf Growth Patterns
Alternate (Spiral) Phyllotaxy:
Description: In alternate phyllotaxy, a single leaf grows at each node, and the leaves are arranged
alternately along the stem in a spiral pattern. This arrangement allows for maximum exposure to
sunlight.
Examples: Sunflower, oak, and ivy exhibit alternate phyllotaxy.
Opposite Phyllotaxy:
Description: In opposite phyllotaxy, two leaves grow at each node, directly opposite each other
on the stem. This arrangement provides a balanced distribution of leaves.
Subtypes:
Decussate: Successive pairs of leaves are arranged at right angles to each other (e.g., mint).
Superposed: Successive pairs of leaves are aligned directly above each other (e.g., guava).
Examples: Maple, guava, and periwinkle have opposite phyllotaxy.
Whorled Phyllotaxy:
Description: In whorled phyllotaxy, three or more leaves grow at each node, encircling the stem.
This pattern is less common but can be advantageous in certain environments where light and
space are limited.
Examples: Nerium (oleander) and Alstonia have whorled phyllotaxy.