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30.2A: Functions of Stems - Biology

30.2A: Functions of Stems - Biology



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A stem connects the roots to the leaves, provides support, stores food, and holds the leaves, flowers, and buds.

Learning Objectives

  • Summarize the main function and basic structure of stems

Key Points

  • Most stems are found above ground, but some of them grow underground.
  • Stems can be either unbranched or highly branched; they may be herbaceous or woody.
  • Stems connect the roots to the leaves, helping to transport water, minerals, and sugars to different parts of the plant.
  • Plant stems always have nodes (points of attachments for leaves, roots, and flowers) and internodes (regions between nodes).
  • The petiole is the stalk that extends from the stem to the base of the leaf.
  • An axillary bud gives rise to a branch or a flower; it is usually found in the axil: the junction of the stem and petiole.

Key Terms

  • node: points of attachment for leaves, aerial roots, and flowers
  • internode: a section of stem between two stem nodes
  • petiole: stalk that extends from the stem to the base of the leaf
  • axillary bud: embryonic shoot that lies at the junction of the stem and petiole that gives rise to a branch or flower

Stems

Stems are a part of the shoot system of a plant. They may range in length from a few millimeters to hundreds of meters. They also vary in diameter, depending on the plant type. Stems are usually above ground, although the stems of some plants, such as the potato, also grow underground. Stems may be herbaceous (soft) or woody in nature. Their main function is to provide support to the plant, holding leaves, flowers, and buds; in some cases, stems also store food for the plant. A stem may be unbranched, like that of a palm tree, or it may be highly branched, like that of a magnolia tree. The stem of the plant connects the roots to the leaves, helping to transport absorbed water and minerals to different parts of the plant. The stem also helps to transport the products of photosynthesis (i.e., sugars) from the leaves to the rest of the plant.

Plant stems, whether above or below ground, are characterized by the presence of nodes and internodes. Nodes are points of attachment for leaves, aerial roots, and flowers. The stem region between two nodes is called an internode. The stalk that extends from the stem to the base of the leaf is the petiole. An axillary bud is usually found in the axil (the area between the base of a leaf and the stem) where it can give rise to a branch or a flower. The apex (tip) of the shoot contains the apical meristem within the apical bud.


Modifications of Stem (Explained with Diagram)

Some of the most important types of modifications of stem are as follows:

I. Underground modifications of Stem II. Subaerial modifications of Stem III. Aerial modifications of Stem.

I. Underground modifications of Stem:

Many plants produce underground stems for perennation and food storage. They produce aerial shoots annually.

Although they resemble roots superficially but can be distinguishable from roots by the presence of following features:

(i) Presence of nodes and internodes

(ii) Presence of scale leaves, buds and adventitious roots at the nodes.

(iii) Internal structure resembles that of aerial stem and not of root.

Some underground modified stems are as:

It is fleshy, non-green underground stem. It has distinct nodes and internodes. The nodes bear dry scale leaves with axillary buds. Terminal buds also present. Adventitious roots arise from the lower side. The rhizome that grow obliquely is called root stock rhizome (e..g., Alocasia, Dryopteris, Banana etc.) and when grow horizontally is called straggling rhizome [e.g. Ginger, termeric, Cannaetc.).

It is a highly condensed discoid stem its upper surface a terminal bud and many fleshy scale leaves are present. A cluster of adventitious roots arise from the base of the bulb. The bulbs may be tunicated or scaly. A tunicated bulb is covered by a sheath of dry membranous scale leaves called tunic, e.g., onion and garlic. In case of garlic, the bulb consists of an aggregate of bulblets or cloves, each covered by its individual tunic. A scaly bulb is without any tunic, e.g., Lily.

It is a condensed form of rhizome growing in vertical direction. It is more or less spherical with a flattered base. The corm has distinct circular nodes and in ternodes. The n ides bear scale leaves and axillary buds. Adventitious roots arise either from its base or all over the body. Examples- colocasia, corcus, Amorphophallus.

Stem tuber is a swollen tip of an underground lateral Stem (Stolon). It is covered by a corry Skin with a number small depressions called eyes’. Each eye represents a node, bearing one or more buds subtended by a leaf scar (= scale leaf). A big scar at one end (heel end) of a potato marks its attachment to the stolon. Adventitious roots are usually absent e.g., Potato.

II. Subaerial modifications of Stem:

In subaerial modifications, the stem is partly aerial and partly underground. Short aerial branches and adventitious roots develop at the nodes. Detachment of entire branch or a node can develop into a new plant. The plants are commonly known as creepers and their subaerial stem modifications meant for vegetative propagation.

The weak sub-aerial stems are modified into following four types:

It is a creeping stem with long internodes, running horizontally on the soil surface. The nodes bear axillary buds, scale leaves and adventitious roots. Runner arises from an axillary bud. A mother plant often produces a number of runners in all direction. Runners break off and grow into individual plants, and thus help in vegetative propagation. Examples – C-Cynodon (Lawn grass), Oxalis (Wood sorrel), Centella (Or. Thalkudi) etc. An underground runner is called sobole, e.g. Agropyron.

It arises from the basal, underground part of the main stem. It grows horizontally for a distance under the soil and then emerges obliquely upwards. It develops a leafy shoot and adventitious roots before separating from the mother plant. The common sucker-bearing plants are Chrysanthemum (Or. Sebati), Musa (banana), Mentha (Or podina), strawberry, pineapple etc.

It is a weak lateral branch that arises from the base of main stem. After growing aerially for some time it bends downwards to touch the ground, where its terminal bud gives rise to a new shoot and adventitious roots. The common stolon bearing plants are Jasmine, Colocasia, and Vellisneria etc.

It is a short runner with one internode long. It originates from leaf axil, grows as a short horizontal branch. It produces a rosette of leaves above and adventitious roots below. Offsets art generally found in aquatic plants like Pistia (water lettuce), Eichomia (water hyacinth), Houseleek etc.

III. Aerial modifications of Stem:

In certain plants, the aerial stem or buds get modified to perform special functions like climbing, protection, food storage, vegetative propagation etc.

The various aerial modifications are as follows:

Stem or its branches get modified into green thread like leafless structures called tendrils which are meant for climbing. These may be branched or un-branched. A scale leaf is always present at the point of branching of the tendril.

Stem tendrils are of four types:

(a) Axillary tendrils-e.g., Passiflora

(b) Extra-axillary tendrils-e.g., Cucurbita, Luffa

(c) Leaf-opposed or Apical bud tendrils, e.g., Grapevine (Vitis)

(d) Floral bud or Inflorescence tendrils – e.g., Antigonon, Cardiospermum (Balloon vine

These are straight, pointed, hard or woody structures sometimes they bear leaves, flowers or even may be branched. In Citrus, Duranta and Aegel thorns are modified axillary buds in Carrissa (Or. Khirkoli) terminal bud gets modified into thorn. Thorns are used as organs of defence or climbing (e.g. Bougainvillea) and check transpiration.

These are fleshy, green flattened or cylindrical branches of unlimited growth. The leaves are modified into spines or scales to check transpiration. They take part in photosynthesis and store water. These are seen in xerophytic plants like Opuntia, Euphorbia, Casuarina, Cocoloba etc.

4. Cladodes or Cladophylls

These are green cylinderical or flattened leaf-like branch of limited growth. In Asparagus, the cladodes are one internode long and in Ruscus the cladodes are two internode long. They help in photosynthesis.

These are modified vegetative or floral buds with stored food and meant for vegetative propagation. In Dioscorea, bulbils are condensed axillary buds while in Agava and lily the floral buds develop into bulbil. They detach to become new plants.

6. Thalamus (= Receptacle or torus):

It is a condensed stem axis that bears words of floral organs -calyx, corolla, androecium and gynoecium. In Gynandropsis, Cleomeand Silene the thalamus exhibits clear nodes and internodes.


Background

Before looking into each of the sub-structures and their respective functions within the brain stem, let’s first look at the brain stem’s relation to the nervous system.

The Nervous System, Neurons, and Brain

The nervous system is a major system spanning the entire body that plays a key role in survival and regulation. It is responsible for relaying sensory information from the body to the brain, where the brain then sends appropriate responses back to the body. These responses can vary- from motor to physiological to storage.

The nervous system is made up of individual nerve cells (or neurons), which recognize signals from the body and its environment. The neurons pass along these signals to their respective destinations in the brain almost instantly via electrical signaling. When one nerve passes the signal to the next nerve, a synapse occurs. This is where electrical signals become chemical in the spaces between two neurons, before becoming electrical again at the next neuron.

Neurons make up the entirety of the nervous system, which is broken into two physical sub-systems: the central nervous system and the peripheral nervous system. The central nervous system includes the brain and spinal cord, while the peripheral nervous system includes all other neurons throughout the body. The brain itself is made up of four regions: the cerebrum, cerebellum, diencephalon, and brain stem. While each region has distinct differences and roles in relation to the rest of the body, there are many interconnected pathways and neural connections that can pass through multiple structures.

The Brain Stem

Brain stem and additional structures labeled. The brain stem consists of the midbrain, pons, and medulla oblongata.

The brain stem as a whole is not a single structure. Instead, it contains three main structures- the midbrain, pons, and medulla oblongata. Each of these regions contain prominent sub-structures and roles that are centralized to each region, as well as overlapping between regions. These regions are also responsible for containing the origins of several cranial nerves.

The brain stem- with all its sub-structures- has many important functions in the autonomic nervous system (which will be described in detail in the next section). Specifically, the brain stem plays key roles in the cardiovascular, respiratory, and digestive systems, as well as in other involuntary functions throughout the body.

The brain stem is extremely vital for survival, where losing the neural connections is highly deadly. Medically, brain stem death is an “irreversible loss” in regaining consciousness and the ability to breath. During brain stem death, the brain stem fails to function, but there can still be projections present in the cortex. Once the cortical and brain stem projections are both lost however, the organism undergoes “biological death”. True death occurs when cardiopulmonary activity ceases as well. Ventilators can be used to extend the heart beating and oxygen circulation following brain stem death, but there is no true cure.

The Autonomic Nervous System

There are two major functional nervous systems in the body: the somatic nervous system and the autonomic nervous system. The somatic nervous system is responsible for regulating and carrying out voluntary responses throughout the body. Specifically, these are the responses that the conscious is aware of (such as lifting your arm to pick up a drink or kicking your legs to perform a dance routine). As a result, the somatic nervous system typically targets skeletal muscles.

However, voluntary movement is not the only action occurring in the body. The body also undergoes many involuntary movements, which are motions that are not conscious. Movements such as this include heartbeat- as controlled by cardiac muscle- and digestion- as controlled by smooth muscle. This category also includes glandular functions. All of these actions fall under the control of the autonomic nervous system.

The autonomic nervous system is further broken down into two sub-categories: the sympathetic and parasympathetic nervous systems. As part of the autonomic nervous system, these two sub-systems also control parts of the body for involuntary movement. The sympathetic system (nicknamed the “fight or flight response”) allows the body to prepare itself for stressful situations. This can include increased heart rate, increased glucose release into the blood, and inhibited digestion. On the contrary, the parasympathetic system (nicknamed the “rest and digest response”) allows the body to increase and store energy. This can be accomplished by slowing down heart rate and increasing digestion.

The brain stem plays a large role in controlling the autonomic nervous system, including both the sympathetic and parasympathetic nervous systems.


Stems are usually categorized as &minus

Underground Stem

The stem that grows inside the soil is known as underground stem. E.g. Potato.

Such type of stems store food for contingency period.

Subaerial Stem

The stem, which partial remains inside the soil and partial above (i.e. in the air), is known as subaerial stem. E.g. Cynodon

Aerial Stem

The stem, which entirely remains in the air (i.e. out-side of soil or water), is known as aerial stem. E.g. passiflora, grapes, etc.


Clonal hematopoiesis: mechanisms driving dominance of stem cell clones

The discovery of clonal hematopoiesis (CH) in older individuals has changed the way hematologists and stem cell biologists view aging. Somatic mutations accumulate in stem cells over time. While most mutations have no impact, some result in subtle functional differences that ultimately manifest in distinct stem cell behaviors. With a large pool of stem cells and many decades to compete, some of these differences confer advantages under specific contexts. Approximately 20 genes are recurrently found as mutated in CH, indicating they confer some advantage. The impact of these mutations has begun to be analyzed at a molecular level by modeling in cell lines and in mice. Mutations in epigenetic regulators such as DNMT3A and TET2 confer an advantage by enhancing self-renewal of stem and progenitor cells and inhibiting their differentiation. Mutations in other genes involved in the DNA damage response may simply enhance cell survival. Here, we review proposed mechanisms that lead to CH, specifically in the context of stem cell biology, based on our current understanding of the function of some of the CH-associated genes.


159 Stems

By the end of this section, you will be able to do the following:

  • Describe the main function and basic structure of stems
  • Compare and contrast the roles of dermal tissue, vascular tissue, and ground tissue
  • Distinguish between primary growth and secondary growth in stems
  • Summarize the origin of annual rings
  • List and describe examples of modified stems

Stems are a part of the shoot system of a plant. They may range in length from a few millimeters to hundreds of meters, and also vary in diameter, depending on the plant type. Stems are usually above ground, although the stems of some plants, such as the potato, also grow underground. Stems may be herbaceous (soft) or woody in nature. Their main function is to provide support to the plant, holding leaves, flowers and buds in some cases, stems also store food for the plant. A stem may be unbranched, like that of a palm tree, or it may be highly branched, like that of a magnolia tree. The stem of the plant connects the roots to the leaves, helping to transport absorbed water and minerals to different parts of the plant. It also helps to transport the products of photosynthesis, namely sugars, from the leaves to the rest of the plant.

Plant stems, whether above or below ground, are characterized by the presence of nodes and internodes ((Figure)). Nodes are points of attachment for leaves, aerial roots, and flowers. The stem region between two nodes is called an internode . The stalk that extends from the stem to the base of the leaf is the petiole. An axillary bud is usually found in the axil—the area between the base of a leaf and the stem—where it can give rise to a branch or a flower. The apex (tip) of the shoot contains the apical meristem within the apical bud .


Stem Anatomy

The stem and other plant organs arise from the ground tissue, and are primarily made up of simple tissues formed from three types of cells: parenchyma, collenchyma, and sclerenchyma cells.

Parenchyma cells are the most common plant cells ((Figure)). They are found in the stem, the root, the inside of the leaf, and the pulp of the fruit. Parenchyma cells are responsible for metabolic functions, such as photosynthesis, and they help repair and heal wounds. Some parenchyma cells also store starch.


Collenchyma cells are elongated cells with unevenly thickened walls ((Figure)). They provide structural support, mainly to the stem and leaves. These cells are alive at maturity and are usually found below the epidermis. The “strings” of a celery stalk are an example of collenchyma cells.


Sclerenchyma cells also provide support to the plant, but unlike collenchyma cells, many of them are dead at maturity. There are two types of sclerenchyma cells: fibers and sclereids. Both types have secondary cell walls that are thickened with deposits of lignin, an organic compound that is a key component of wood. Fibers are long, slender cells sclereids are smaller-sized. Sclereids give pears their gritty texture. Humans use sclerenchyma fibers to make linen and rope ((Figure)).


Which layers of the stem are made of parenchyma cells?

Like the rest of the plant, the stem has three tissue systems: dermal, vascular, and ground tissue. Each is distinguished by characteristic cell types that perform specific tasks necessary for the plant’s growth and survival.

Dermal Tissue

The dermal tissue of the stem consists primarily of epidermis , a single layer of cells covering and protecting the underlying tissue. Woody plants have a tough, waterproof outer layer of cork cells commonly known as bark , which further protects the plant from damage. Epidermal cells are the most numerous and least differentiated of the cells in the epidermis. The epidermis of a leaf also contains openings known as stomata, through which the exchange of gases takes place ((Figure)). Two cells, known as guard cells , surround each leaf stoma, controlling its opening and closing and thus regulating the uptake of carbon dioxide and the release of oxygen and water vapor. Trichomes are hair-like structures on the epidermal surface. They help to reduce transpiration (the loss of water by aboveground plant parts), increase solar reflectance, and store compounds that defend the leaves against predation by herbivores.


Vascular Tissue

The xylem and phloem that make up the vascular tissue of the stem are arranged in distinct strands called vascular bundles, which run up and down the length of the stem. When the stem is viewed in cross section, the vascular bundles of dicot stems are arranged in a ring. In plants with stems that live for more than one year, the individual bundles grow together and produce the characteristic growth rings. In monocot stems, the vascular bundles are randomly scattered throughout the ground tissue ((Figure)).


Xylem tissue has three types of cells: xylem parenchyma, tracheids, and vessel elements. The latter two types conduct water and are dead at maturity. Tracheids are xylem cells with thick secondary cell walls that are lignified. Water moves from one tracheid to another through regions on the side walls known as pits, where secondary walls are absent. Vessel elements are xylem cells with thinner walls they are shorter than tracheids. Each vessel element is connected to the next by means of a perforation plate at the end walls of the element. Water moves through the perforation plates to travel up the plant.

Phloem tissue is composed of sieve-tube cells, companion cells, phloem parenchyma, and phloem fibers. A series of sieve-tube cells (also called sieve-tube elements) are arranged end to end to make up a long sieve tube, which transports organic substances such as sugars and amino acids. The sugars flow from one sieve-tube cell to the next through perforated sieve plates, which are found at the end junctions between two cells. Although still alive at maturity, the nucleus and other cell components of the sieve-tube cells have disintegrated. Companion cells are found alongside the sieve-tube cells, providing them with metabolic support. The companion cells contain more ribosomes and mitochondria than the sieve-tube cells, which lack some cellular organelles.

Ground Tissue

Ground tissue is mostly made up of parenchyma cells, but may also contain collenchyma and sclerenchyma cells that help support the stem. The ground tissue towards the interior of the vascular tissue in a stem or root is known as pith , while the layer of tissue between the vascular tissue and the epidermis is known as the cortex .

Growth in Stems

Growth in plants occurs as the stems and roots lengthen. Some plants, especially those that are woody, also increase in thickness during their life span. The increase in length of the shoot and the root is referred to as primary growth , and is the result of cell division in the shoot apical meristem. Secondary growth is characterized by an increase in thickness or girth of the plant, and is caused by cell division in the lateral meristem. (Figure) shows the areas of primary and secondary growth in a plant. Herbaceous plants mostly undergo primary growth, with hardly any secondary growth or increase in thickness. Secondary growth or “wood” is noticeable in woody plants it occurs in some dicots, but occurs very rarely in monocots.


Some plant parts, such as stems and roots, continue to grow throughout a plant’s life: a phenomenon called indeterminate growth. Other plant parts, such as leaves and flowers, exhibit determinate growth, which ceases when a plant part reaches a particular size.

Primary Growth

Most primary growth occurs at the apices, or tips, of stems and roots. Primary growth is a result of rapidly dividing cells in the apical meristems at the shoot tip and root tip. Subsequent cell elongation also contributes to primary growth. The growth of shoots and roots during primary growth enables plants to continuously seek water (roots) or sunlight (shoots).

The influence of the apical bud on overall plant growth is known as apical dominance, which diminishes the growth of axillary buds that form along the sides of branches and stems. Most coniferous trees exhibit strong apical dominance, thus producing the typical conical Christmas tree shape. If the apical bud is removed, then the axillary buds will start forming lateral branches. Gardeners make use of this fact when they prune plants by cutting off the tops of branches, thus encouraging the axillary buds to grow out, giving the plant a bushy shape.

Watch this BBC Nature video showing how time-lapse photography captures plant growth at high speed.

Secondary Growth

The increase in stem thickness that results from secondary growth is due to the activity of the lateral meristems, which are lacking in herbaceous plants. Lateral meristems include the vascular cambium and, in woody plants, the cork cambium (see (Figure)). The vascular cambium is located just outside the primary xylem and to the interior of the primary phloem. The cells of the vascular cambium divide and form secondary xylem (tracheids and vessel elements) to the inside, and secondary phloem (sieve elements and companion cells) to the outside. The thickening of the stem that occurs in secondary growth is due to the formation of secondary phloem and secondary xylem by the vascular cambium, plus the action of cork cambium, which forms the tough outermost layer of the stem. The cells of the secondary xylem contain lignin, which provides hardiness and strength.

In woody plants, cork cambium is the outermost lateral meristem. It produces cork cells (bark) containing a waxy substance known as suberin that can repel water. The bark protects the plant against physical damage and helps reduce water loss. The cork cambium also produces a layer of cells known as phelloderm, which grows inward from the cambium. The cork cambium, cork cells, and phelloderm are collectively termed the periderm . The periderm substitutes for the epidermis in mature plants. In some plants, the periderm has many openings, known as lenticels , which allow the interior cells to exchange gases with the outside atmosphere ((Figure)). This supplies oxygen to the living and metabolically active cells of the cortex, xylem, and phloem.


Annual Rings

The activity of the vascular cambium gives rise to annual growth rings. During the spring growing season, cells of the secondary xylem have a large internal diameter and their primary cell walls are not extensively thickened. This is known as early wood, or spring wood. During the fall season, the secondary xylem develops thickened cell walls, forming late wood, or autumn wood, which is denser than early wood. This alternation of early and late wood is due largely to a seasonal decrease in the number of vessel elements and a seasonal increase in the number of tracheids. It results in the formation of an annual ring, which can be seen as a circular ring in the cross section of the stem ((Figure)). An examination of the number of annual rings and their nature (such as their size and cell wall thickness) can reveal the age of the tree and the prevailing climatic conditions during each season.


Stem Modifications

Some plant species have modified stems that are especially suited to a particular habitat and environment ((Figure)). A rhizome is a modified stem that grows horizontally underground and has nodes and internodes. Vertical shoots may arise from the buds on the rhizome of some plants, such as ginger and ferns. Corms are similar to rhizomes, except they are more rounded and fleshy (such as in gladiolus). Corms contain stored food that enables some plants to survive the winter. Stolons are stems that run almost parallel to the ground, or just below the surface, and can give rise to new plants at the nodes. Runners are a type of stolon that runs above the ground and produces new clone plants at nodes at varying intervals: strawberries are an example. Tubers are modified stems that may store starch, as seen in the potato (Solanum sp.). Tubers arise as swollen ends of stolons, and contain many adventitious or unusual buds (familiar to us as the “eyes” on potatoes). A bulb , which functions as an underground storage unit, is a modification of a stem that has the appearance of enlarged fleshy leaves emerging from the stem or surrounding the base of the stem, as seen in the iris.


Watch botanist Wendy Hodgson, of Desert Botanical Garden in Phoenix, Arizona, explain how agave plants were cultivated for food hundreds of years ago in the Arizona desert in this video: Finding the Roots of an Ancient Crop.

Some aerial modifications of stems are tendrils and thorns ((Figure)). Tendrils are slender, twining strands that enable a plant (like a vine or pumpkin) to seek support by climbing on other surfaces. Thorns are modified branches appearing as sharp outgrowths that protect the plant common examples include roses, Osage orange, and devil’s walking stick.


Section Summary

The stem of a plant bears the leaves, flowers, and fruits. Stems are characterized by the presence of nodes (the points of attachment for leaves or branches) and internodes (regions between nodes).

Plant organs are made up of simple and complex tissues. The stem has three tissue systems: dermal, vascular, and ground tissue. Dermal tissue is the outer covering of the plant. It contains epidermal cells, stomata, guard cells, and trichomes. Vascular tissue is made up of xylem and phloem tissues and conducts water, minerals, and photosynthetic products. Ground tissue is responsible for photosynthesis and support and is composed of parenchyma, collenchyma, and sclerenchyma cells.

Primary growth occurs at the tips of roots and shoots, causing an increase in length. Woody plants may also exhibit secondary growth, or increase in thickness. In woody plants, especially trees, annual rings may form as growth slows at the end of each season. Some plant species have modified stems that help to store food, propagate new plants, or discourage predators. Rhizomes, corms, stolons, runners, tubers, bulbs, tendrils, and thorns are examples of modified stems.

Visual Connection Questions

(Figure) Which layers of the stem are made of parenchyma cells?

(Figure) A and B. The cortex, pith, and epidermis are made of parenchyma cells.

Review Questions

Stem regions at which leaves are attached are called ________.

Which of the following cell types forms most of the inside of a plant?

  1. meristem cells
  2. collenchyma cells
  3. sclerenchyma cells
  4. parenchyma cells

Tracheids, vessel elements, sieve-tube cells, and companion cells are components of ________.

The primary growth of a plant is due to the action of the ________.

Which of the following is an example of secondary growth?

  1. increase in length
  2. increase in thickness or girth
  3. increase in root hairs
  4. increase in leaf number

Secondary growth in stems is usually seen in ________.

  1. monocots
  2. dicots
  3. both monocots and dicots
  4. neither monocots nor dicots

Critical Thinking Questions

Describe the roles played by stomata and guard cells. What would happen to a plant if these cells did not function correctly?

Stomata allow gases to enter and exit the plant. Guard cells regulate the opening and closing of stomata. If these cells did not function correctly, a plant could not get the carbon dioxide needed for photosynthesis, nor could it release the oxygen produced by photosynthesis.

Compare the structure and function of xylem to that of phloem.

Xylem is made up tracheids and vessel elements, which are cells that transport water and dissolved minerals and that are dead at maturity. Phloem is made up of sieve-tube cells and companion cells, which transport carbohydrates and are alive at maturity.

Explain the role of the cork cambium in woody plants.

In woody plants, the cork cambium is the outermost lateral meristem it produces new cells towards the interior, which enables the plant to increase in girth. The cork cambium also produces cork cells towards the exterior, which protect the plant from physical damage while reducing water loss.

What is the function of lenticels?

In woody stems, lenticels allow internal cells to exchange gases with the outside atmosphere.

Besides the age of a tree, what additional information can annual rings reveal?

Annual rings can also indicate the climate conditions that prevailed during each growing season.

Give two examples of modified stems and explain how each example benefits the plant.

Answers will vary. Rhizomes, stolons, and runners can give rise to new plants. Corms, tubers, and bulbs can also produce new plants and can store food. Tendrils help a plant to climb, while thorns discourage herbivores.

Glossary