20 Chapter 20: Plant Structure and Function/Animal Body System Organization

Joshua Reid and Mason Tedeschi

Learning Objectives

By the end of this section, students will be able to:

20.1 Use the idea of “form follows function” to explain how organ systems are organized within the body (their form) to sense and respond to stimuli (a function of life).

20.2 Explain how variations in leaf and stem structures have contributed to the survival of different plant species from different biomes.

Human Body System Organization

Organs are collections of tissues grouped together performing a common function. Organs are present not only in animals but also in plants. An organ system is a group of organs that work together to perform major functions or meet physiological needs of the body. Mammals have many organ systems. For instance, the circulatory system transports blood through the body and to and from the lungs. It includes organs such as the heart and blood vessels. The digestive system consists of several organs, including the stomach, intestines, liver, and pancreas. Organ systems can come together to create an entire organism.

There are eleven distinct organ systems in the human body (Figure 20.1). Assigning organs to organ systems can be imprecise since organs that “belong” to one system can also have functions integral to another system. In fact, most organs contribute to more than one system. In later chapters, we will discuss some of these organ systems in more depth.

Fig 20.1 Each organ system works together to perform a specific function

Reading Question #1

Which body system functions to deliver air to sites where gas exchange occur?

A. Reproductive

B. Urinary

C. Nervous

D. Respiratory

Reading Question #2

Which body system functions to provide communication within the body via hormones and directs long-term change in other organ systems to maintain homeostasis?

A. Reproductive

B. Endocrine

C. Integumentary

D. Digestive

Reading Question #3

Which body system functions break down and absorb food so the body can acquire the nutrients it needs?

A. Reproductive

B. Urinary

C. Digestive

D. Muscular

Plant Structure and Function

Plant tissues form organs (such as leaves, stems, or roots), each of which perform a specific set of functions. Together, organs often work to form organ systems. Vascular plants have two distinct organ systems: a shoot system, and a root system. The shoot system consists of two portions: the vegetative (non-reproductive) parts of the plant, such as the leaves and the stems, and the reproductive parts of the plant, which include flowers and fruits. The shoot system generally grows above ground, where it absorbs the light needed for photosynthesis. The root system, which anchors the plant into the ground, absorbs water and minerals, and serves as a storage site for food is usually underground. (Figure 20.2) shows the organ systems of a typical plant.

A dandelion plant with shoot and root system
Figure 20.2 The shoot system of a plant consists of leaves, stems, flowers, and fruits. The root system anchors the plant while absorbing water and minerals from the soil. The dandelion root system consists of a single, thick root that branches into smaller roots.

Stem Structure and Function

Plant stems, whether above or below ground, are characterized by the presence of nodes and internodes (Figure 20.3). Nodes are points of attachment for leaves. Leaves often consist of a thin region that attaches to the stem (the petiole) and a broader blade (see Leaves). The stem region between two nodes is called an internode. 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 called an axillary shoot (Figure 20.4). The shoot apex at the tip shoot contains the shoot apical meristem surrounded by developing leaves called leaf primordia (see Meristems).

Close-up view of a shoot with nodes, internodes, and leaves labeled
Figure 20.3 Leaves are attached to the plant stem at areas called nodes. An internode is the stem region between two nodes. The smaller leaves just above the nodes are the beginning of axillary shoots, which arose from axillary buds. Image modified from OpenStax (CC-BY)
An arrow marks a rounded point emerging where a leaf petiole attaches to the stem
Figure 20.4 Axillary buds of sweet granadilla (Passiflora ligularis). One of the axillary buds is labeled with an arrow. Image modified from Eiku (CC-BY-SA).

The primary stem refers to the herbaceous (non-woody) stem, which has not undergone secondary growth (the growth that produces bark and wood). Some species (all monocots and some eudicots) remain herbaceous for their entire lives, maintaining the primary stem. Other species of eudicots initially form a primary stem but later become woody, replacing the primary stem with the secondary stem. The anatomy of the stem (internal structure) can be examined through longitudinal sections (cutting the stem lengthwise) or in cross sections (cutting a slice of the stem perpendicular to the length).

All three tissue types are represented in the primary stem. The epidermis is the dermal tissue that surrounds and protects the stem. The epidermis typically consists of one layer of cells. A waxy cuticle on the outside of these cells limits water loss. Epidermal cells are the most numerous and least differentiated of the cells in the epidermis. Pores in the epidermis called stomata (singular: stoma) allow for gas exchange. Each stoma is bordered by a pair of guard cells, which regulate stomatal opening. While stomata are present in stems, they occur at higher densities in leaves. 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 (Figure 20.5).

Close-up view of a reddish green stem with thin white hairs extending from it
Figure 20.5 Trichomes extend from the epidermis of this Datrua inoxia stem. Image by Silk666 (CC-BY-SA).

Ground tissue fills much of the stem, forming the cortex directly within the epidermis and the pith (if present) in the center. The outermost portion of the cortex is usually a few layers of collenchyma cells. The remainder of the cortex and pith consist of parenchyma cells. The arrangement of vascular tissue in the stem depends on the species (see below).

Cross sections reveal three possible arrangements of vascular tissue (steles) in the stem. The first arrangement (solenostele) is present in a few eudicots, such as basswood (Tilia). In the solenostele, the vascular tissue appears as a continuous ring (vascular cylinder; Figure 20.6). The second arrangement (eustele) is present in most eudicots such as sunflower (Helianthus) and buttercup (Ranunculus). In the eustele, vascular tissue is clustered into distinct vascular bundles arranged in a ring, allowing for thicker interfascicular regions in between them (Figure 20.8). In these solenosteles and eusteles, the vascular tissue separates ground tissue into an outer cortex and central pith. The third arrangement (atactostele) is present in most monocots, such as corn (Zea mays) and a few eudicots. In the atactostele, vascular bundles are scattered throughout the stem (Figures 20.7 & 20.9). While vascular bundles near the outside of the stem are packed more densely in this third arrangement, their distribution is somewhat disorderly. There is no distinction between the cortex and pith in the third arrangement.

A cross section of a Hypericum perforatum stem
Figure 20.6 Cross section of a common St. John’s Wort (Hypericum perforatum) stem, illustrating a solenostele. The epidermis borders the entire root. The central pith (greenish-blue, in the center) and peripheral cortex (narrow zone 3–5 cells thick just inside the epidermis) are composed of parenchyma cells. Vascular tissue, forming the vascular cylinder, is composed of xylem (red) and phloem tissue (green, between the xylem and cortex) surrounds the pith. Image by RolfDieterMueller (CC-BY)
Cross section illustrating a eustele with vascular bundles in a ring and atactostele with scattered vascular bundles
Figure 20.7 In eusteles (left), vascular bundles are arranged around the periphery of the ground tissue. The xylem tissue is located toward the interior of the vascular bundle, and phloem is located toward the exterior. Primary phloem fibers cap the vascular bundles. In atactosteles (right), vascular bundles composed of xylem and phloem tissues are scattered throughout the ground tissue.
A cross section of a Helianthus stem
Figure 20.8 Cross section of a sunflower (Helianthus) stem, illustrating a eustele. Vascular bundles are arranged in a ring. The interfascicular regions between the vascular bundles are thick compared to the solenostele, where they were too thin to be visible. The ground tissue is separated into an outer cortex and a central pith. The epidermis borders the entire stem. Image labeled from Berkshire Community College Bioscience Image Library (public domain)
A cross section of a corn stem, showing scattered vascular bundles
Figure 20.9 Cross section of a corn (Zea mays) stem, an example of an atactostele, at 40X magnification. Vascular bundles are scattered but are at a higher density near the outside of the stem. In contrast to solenosteles and eusteles, the ground tissue is not separated into a pith and cortex. The epidermis borders the entire stem. Image labeled from Berkshire Community College Bioscience Image Library (public domain)

Stem Modifications

Stems (or entire shoots) of some species deviate in structure and function from a typical stem. These are called stem modifications (shoot modifications). While leaves are part of the shoot, modifications involving just the leaf are discussed separately.

Rhizomes and stolons are horizontal stems that function in propagation (Figure 20.10). Rhizomes are belowground stems that burrow into the ground just below the soil surface. They have short internodes and usually have small, scale-like leaves that are not photosynthetic. Buds from the axils of the leaves make new branches that will grow to become aboveground shoots. Ginger (Zingiber) and Johnson grass (Sorghum halepense) form rhizomes. Stolons (runners) are aboveground horizontal shoots, which sprout and produce a new plants. Compared to rhizomes, stolons have long internodes. Examples include strawberry (Fragaria), spider plants (Chlorophytum), and Bermuda grass (Cynodon dactylon).

Ginger shoots arise from the ginger rhizome, which is thick, has short internodes, and papery leaves.
Several long, white stolons emerge from a spider plant. A plantlet is at the end of the stolon
Figure 20.10 Examples of horizontal stems. Ginger rhizomes are thick and belowground (top), and stolons of this spider plant are aboveground with long internodes (bottom). Leaves and roots emerge from the end of a stolon, forming a smaller plant. Top image by Sengai Podhuvan (CC-BY-SA), and bottom image by Eptalon (CC-BY-SA).

Tubers, corms, and bulbs are modified for starch storage. Tubers, such as potatoes, are thick, belowground stems found at the tips of rhizomes or stolons (figure 20.11). The “eyes” of potato are actually lateral buds, and the tuber body is comprised of many parenchyma cells that contain amyloplasts with starch. Corms and bulbs are modified shoots. A corm is a short, thick underground storage stem with thin scaly leaves (for example, Gladiolus and Crocus; Figure 20.12). A bulb, such as an onion, differs from a corm in the fact that it stores its nutrients in its fleshy leaves (Figure 20.12). Lilies and tulips also form bulbs.

Potato tubers in the soil. They are oval, expanded structures.
Figure 20.11 Potato tubers are expanded, belowground stems modified for starch storage. Image by pxhere (public domain).
A corm cut longitudinally reveals a thick, white stem
An onion bulb cut longitudinally has a small stem and layers of thick, fleshy leaves.
Figure 20.12 Both corms (left) and bulbs (right) are shoots modified for starch storage. The Crocosmia corm has a thick, expanded stem stores starch, and the leaves that surround it are non-photosynthetic and papery. The onion bulb has a relatively small stem surrounded by thick, fleshy leaves. Left image by JonRichfield (CC-BY-SA), and right image by Amada44 (CC-BY-SA).

Some plants have sharp, generally narrow projections that function in defense against herbivores. Such structures are called thorns when they arise from an entire stem. Hawthorn (Crataegus) and Bougainvillea produce thorns. In contrast, prickles form from the surface tissues (epidermis and cortex) of the stem rather than the whole organ (figure 20.13). Rose (Rosa) and blackberry (Rubus) produce prickles. Spines are similar structures derived from leaves.

A hawthorn stem with sharp, brown thorns arising from the axils above each leaf.
Close-up of a rose stem with red, triangular prickles
Figure 20.13 Thorns (left) and prickles (right). The thorns of the hawthorn are derived from the whole stem. They arise from axillary buds in the axils just above the leaves. The sharp projections on the rose stem are technically classified as prickles because they are derived from only the surface tissues of the stem. Left image by Rasbak (CC-BY-SA), and right image by Ingrid Taylar (CC-BY).

Tendrils are thin, string-like structures that allow the shoot to attach to other surfaces to access light. Tendrils can be derived from stems, leaves, or leaflets, and they are common in vines. Morning glory and sweet potato (Ipomoea), grapes (Vitis), and many members of the cucumber family (Cucurbitaceae) such as cucumbers, pumpkin, and squash have tendrils that arise from stems (Figure 20.14).

A wild cucumber, with small, oval, spiky fruits and scattered tendrils.
Figure 20.14: The thin, pale tendrils of the wild cucumber plant arise from stems and coil around surrounding structures. Image by erwin66as (public domain)

Structure and Function of Leaves

Each leaf typically has a flat, wide portion called the blade (lamina), which is also the widest part of the leaf (Figure 20.15). Some leaves are attached to the plant stem by a stalklike petiole and are called petiolate leaves (Figure 20.16). Although petioles are narrow and often resemble stems, they are considered part of the leaf. A petiolate leaf thus consists of the blade and the petiole. Petioles usually attach at to the margin (edge) of the blade along the base, but in peltate leaves, the petiole is attached underneath the blade (Figure 20.17). Leaves that do not have a petiole and are directly attached to the plant stem are called sessile (apetiolate) leaves (Figure 20.18). In a special type of sessile leaves called perfoliate leaves, the stem passes through the center of the blade (Figure 20.18). Many leaves have a midrib, which travels the length along the center of the leaf. The midrib contains the main vein (primary vein) of the leaf as well as supportive ground tissue (collenchyma or sclerenchyma).

Parts of a typical leaf include the petiole, lamina, midrib, and margin.
Figure 20.15 A typical eudicot leaf. Many leaves consist of a stalk-like petiole and a wide, flat blade (lamina). The midrib extends from the petiole to the leaf tip and contains the main vein. Additional veins branch from the midvein. The margin is the edge of the leaf.
Petiolate leaves of geranium. Each leaf appears slightly crumpled with a dark patch near the margins.
In sessile leaves, the wide lamina attaches directly to the stem.
Figure 20.16 The petiolate leaves of the geranium consist of a petiole and blade (lamina). The wide lamina is attached to the stalk-like petiole. In the sessile leaves of Aster amellus, the blade is attached directly to the stem. Left image by Melissa Ha (CC-BY), and right image by Enrico Blasutto (CC-BY-SA).
Peltate leaves emerge from a pot. The long petioles attach underneath the round leaf blades.
Figure 20.17 In the peltate leaves of nasturtium, petioles attach underneath the leaf blade, similar to an umbrella. Image by Melissa Ha (CC-BY).
The round leaf of miner's lettuce has a stem passing through the center with a small white flower at the stem apex.
Figure 20.18 The stem runs through the center of the blade in the perfoliate leaves of miner’s lettuce. Image by Wikimediaimages/Pixabay (Pixabay license).

Tissue Organization in Leaves

All three tissue types are represented in leaves. The epidermis represents the dermal tissue, the mesophyll that fills the leaf is ground tissue, and the vascular bundles that form the leaf veins represent vascular tissue (Figure 20.19). These three tissues will be discussed using a eudicot leaf that is adapted to a moderate amount of water (mesophytic leaf). Variations in leaf structure are discussed later on this page.

A labeled microscopic slide of a cross section through a eudicot leaf
Figure 20.19 A cross section through a eudicot leaf. The upper epidermis is a single layer of parenchyma cells. There are no stomata present in the upper epidermis of this leaf. Below the epidermis, cells (appearing pink due to staining of the nuclei and chloroplasts) are arranged in columns, forming the palisade mesophyll. Beneath the palisade mesophyll is the spongy mesophyll. The cells are approximately the same size as the palisade mesophyll, but there are large intercellular spaces between them. The lower epidermis is another single layer of parenchyma cells, but several stomata (flanked by guard cells) are visible in this epidermal layer. A large vascular bundle is in the center of the leaf. The xylem (stained pink) is on the top and the phloem is on the bottom. Image by Maria Morrow (CC-BY-NC).


Leaf Adaptations

The broad, flat shape of most leaves increases surface area relative to volume, which helps it capture sunlight; however this also provides more opportunity for water loss. The anatomy of a leaf has everything to do with achieving the balance between photosynthesis and water loss in the environment in which the plant grows. Plants that grow in moist areas can grow large, flat leaves to absorb sunlight like solar panels because sunlight is likely more limiting than water. Plants in dry areas must prevent water loss and adapt a variety of leaf shapes and orientations to accomplish the duel tasks of water retention and sunlight absorption. In general, leaves adapted to dry environments are small and thick with a much lower surface area-to-volume ratio.

In regards to water, there are three main types of plants: mesophytes, hydrophytes, and xerophytes. Mesophytes are typical plants which adapt to moderate amounts of water (“meso” means middle, and “phyte” means plant). Many familiar plants are mesophytes, such as lilac, Ranunculus (buttercup), roses, etc. Hydrophytes grow in water (“hydro” refers to water). Their leaf blades are frequently highly dissected (deeply lobed) to access gases dissolved in water, and their petioles and stems have air canals to supply underwater organs with gases. Hygrophytes (not discussed further) live in constantly wet environment, their leaves adapted to rapidly release water through the stomata. They sometimes even excrete of water drops through the leaf margins (guttation). Xerophytes are adapted to the scarce water (“xero” refers to dryness). Xerophytes are found in deserts and Mediterranean climates (such as in much of California), where summers are hot and dry. The leaves of mesophytes are called mesophytic, hydrophyte leaves are called hydrophytic, and so on. Adaptaions in hydrophytic and xerophytic leaves and discussed below in more detail.

Pine Leaves

Pines evolved during a period in Earth’s history when conditions were becoming increasingly dry, and pine needles have many adaptations to deal with these conditions. Many of these adaptations are similar the xerophytic leaves of some angiosperms (described above) because pines themselves are xerophytes.

The epidermis of the leaf seems to be more than one cell layer thick (figure 20.20). These subsequent layers of epidermis-like tissue under the single, outer layer of true epidermis are called the hypodermis , which offers a thicker barrier and helps prevent water loss. The epidermis itself is coated on the outside by a thick layer of wax called the cuticle. Because waxes are hydrophobic, this also helps prevent water loss through the epidermis. The stomata are typically sunken, occurring within the hypodermis instead of the epidermis. Sunken stomata create a pocket of air that is protected from the airflow across the leaf and can aid in maintaining a higher moisture content (figure 20.21).

A labeled cross section of a pine needle
Figure 20.20 Cross section of a pine leaf (needle). Much like the Nerium leaf, this leaf is coated in a thick cuticle and there is a hypodermis below the epidermis (because this leaf is so round, there is not really a distinct ‘upper’ and ‘lower’). There are no stomatal crypts, but the stomata are sunken, located in the hypodermis. The leaf has a low surface area to volume ratio (more volume, less surface area), which decreases water loss. In the center of the leaf, There is a large region surrounded by an suberized endodermis (much look in a root). There are two vascular bundles within this region, surrounded by transfusion tissue. Image and caption (modifed) by Maria Morrow (CC-BY-NC).
Labeled close up of a pine needle with sunken stomata and a thick cuticle.
Figure 20.21 A closer view of a sunken stoma and the outermost layers of the pine needle. The thick cuticle is visible as a transparent layer coating the small epidermal cells. Each of the epidermal cells has a thick cell wall. The hypodermis is composed of 3-4 layers of small, tightly packed cells that also have thick walls (sclerenchyma). The guard cells of the stoma are located about three layers below the epidermis, and the cuticle can be seen extending down over them. The stoma is open in this image. Below the stoma, there is a gap of air space, then highly invaginated mesophyll cells. Image by Maria Morrow (CC-BY-NC).

Within the mesophyll, there are several canals that appear as large, open circles in the cross section of the leaf. These are resin canals. The cells lining them secrete resin (the sticky stuff that coniferous trees exude, often called pitch), which contains compounds that are toxic to insects and bacteria. When pines evolved, not only was the Earth becoming drier, but insects were evolving and proliferating. These resin canals are not features that help the plant survive dry conditions, but they do help prevent herbivory. In addition to prevention of herbivory, resin can aid in closing wounds and preventing infection at wound sites.

There are two bundles of vascular tissue embedded within a region of cells called transfusion tissue, which functions in transporting materials to and from the mesophyll cells. The transfusion tissue and vascular bundles are surrounded by a distinct layer of cells called the endodermis. This is similar to the tissue of the same name in the root, but the cells are not impregnated with the water-repelling compound suberin.

Finally, the overall shape of the leaf allows for as little water loss as possible by decreasing the relative surface area, taking a rounder shape as opposed to a flatter one. This low surface area-to-volume ratio is characteristic of xerophytes.

Corn Leaves

The model organism for monocots in botany is usually corn (Zea mays). In corn, there are approximately the same number of stomata on both the upper and lower epidermis. The mesophyll is not divided into two distinct types. The vascular bundles all face the same directly (appearing circular in cross section) because they run parallel to each other.

Corn is not necessarily a xerophyte, but it is adapted to deal with high temperatures. One of these adaptations, C4 type photosynthesis is discussed in Photorespiration and Photosynthetic Pathways and results in a cell arrangement called Kranz anatomy. The vascular bundles are surrounded by obviously inflated parenchyma cells that form a structure called a bundle sheath, and these are packed with chloroplasts (Figure 20.22). (Bundle sheaths surround vascular bundles of other types of leaves as well, but the bundle sheath cells are much smaller). Mesophyll cells encircle the bundle sheath cells. In Cphotosynthesis, carbon dioxide is first gathered by the mesophyll cells and temporarily stored as a four-carbon sugar. This four-carbon sugar is transferred to the bundle sheath cells, where it is broken down to release carbon dioxide. It is in the bundle sheath cells where a process called the Calvin cycle, and glucose is ultimately produced. Cphotosynthesis concentrates carbon dioxide inside the bundle sheath cells, reducing the need to frequently open stomata for gas exchange. This helps conserve water.

A vascular bundle in a corn leaf
Figure 20.22 A vascular bundle of a corn (Zea mays) leaf. There are two vascular bundles in this image. The one on the left is difficult to distinguish and most of what you see are the enlarged bundle sheath cells. The larger vascular bundle on the right has less prominent bundle sheath cells, though they still form a distinct border between the vascular tissue and the mesophyll. The xylem tissue is located closer to the upper epidermis. You can locate it by searching for the large, open cells (vessel elements) with red-stained secondary walls. Below the xylem is the phloem tissue, which encompasses a smaller area. The larger cells in the phloem are sieve-tube elements, and the smaller ones are companion cells. Images from Berkshire Community College Bioscience Image Library (public domain).

When moisture is plentiful, the corn leaves are fully expanded and able to maximize photosynthesis. When moisture is limited, the leaves roll inward, limiting both moisture loss and photosynthetic capacity. This is accomplished by the presence of bulliform cells in the upper epidermis (Figure 20.23). These clusters of enlarged cells are swollen with water when there is abundant water available. As the water content in the plant decreases, these cells shrivel, causing the upper epidermis to curl or fold inward at these points. This adaptation to sun exposure can be found in many other grasses, as well (corn is a member of the Poaceae, the grass family).

Cross section of a corn leaf appears rectangular with several circles (vascular bundles) in it.
Figure 20.23 Cross section of Zea mays (corn). The bulliform cells of are the group of tall cells along the upper epidermis, just to the left of the opening (stoma).

Sun and Shade Leaves

The light intensity experienced by a developing leaf influences its structure. Leaves that develop when consistently exposed to direct sunlight (sun leaves) thus differ from leaves exposed to low light intensities (shade leaves) in several ways (Figure 20.24). Relative to shade leaves, sun leaves are smaller and thicker. This reduces surface area relative to volume, conserving water, which would otherwise be easily lost under bright sunlight and resultantly warmer temperatures. In contrast, the broad, thin shape of shade leaves helps capture sufficient light when light intensity is low. The thicker cuticle of sun leaves also limits water loss. They have more palisade parenchyma and more vascular tissue. Sun leaves can maintain a high photosynthetic rate at high light intensities, but shade leaves cannot.

Cross section of a sun leaf is thick with multiple rows of columnar palisade parenchyma and dense chloroplasts (stained red).
Cross section of a shade leaf with one row of columnar palisade parenchyma
Figure 20.24 Cross sections of a sun leaf (left) and shade leaf (right). The palisade parenchyma of the sun leaf consists of several layers, but there is only a single layer in the shade leaf. The chloroplasts (red dots) are also packed more densely in both the palisade and spongy mesophyll cells of the sun leaf compared to the shade leaf. The sun leaf is overall thicker. Images by Melissa Ha (CC-BY).

Leaf Modifications

The structure and function of a leaf can be modified over the course of evolution as a plant adapts to a particular environment (Figure 20.25). When function of the leaf blade is no longer primarily photosynthesis, some other plant part is usually modified to take its place. Storage leaves are thick leaves underground that store starch (as with a bulb; see Stem Modifications). Succulent leaves are also thickened leaves, but they are found above ground, still conduct photosynthesis, and function primarily in water storage (Figure 20.26).

Examples of leaf modifications. A single leaf has been filled in on each of the four examples.
Figure 20.25 A diagram of leaf modifications. In each plant (A-D), one leaf has been shaded with orange. In A, the blade has been modified for a trap, the petiole is flattened into a phyllode. In B, the blade is a compound leaf and the petiole is flattened into a phyllode. In C, some of the leaves are thin, coiling tendrils (emerging below axillary buds). In D, the basal leaves are thick and fleshy with a short stem (a bulb). Diagram by Nikki Harris (CC-BY) with labels and color added by Maria Morrow.
A plant viewed from the top with succulent leaves. The leaves are thicker than normal leaves.
Succulent leaves are much thicker than regular leaves. They are nearly cylindrical and come to a dull point at the end.
Figure 20.26 Succulent leaves are thickened for water storage. Left photo by Simon Burchell (CC-BY-SA), and right image by Melissa Ha (CC-BY).

Spines are sharp projections derived from leaves that function in plant defense. Almost all cacti (in the plant family Cactaceae) have spines (Figure 20.27). Other examples include barberry and some Acacia species, in which large spines house mutualist ants. Spines can also be formed from stipules (stipular spines) or bud scales. Recall from Stem Modifications that thorns and prickles have the same function, but they are derived from whole stems or the outer tissue layers of stems, respectively.

Several spines from a cactus are pale white with orange tips. In the white area, small greenish dots can be distinguished.
Figure 20.27 Cactus spines are not stems, but modified leaves. These spines have a thick cuticle and stomata can be seen as darker green areas within the pale white cuticle. Photo by André Karwath (CC-BY-SA).

Tendrils are another structure that can originate from multiple structures, including stems, leaves, or leaflets. These are narrow, coiling structures that climbing plants attach to nearby structures for support (Figure 20.28).

A wide leaf blade with a tip that extends into a tendril curling around an adjacent stem
Pinnately compound leaves of a pea plant. A few of the leaflets at the end are narrow and coiling, functioning as tendrils.
Figure 20.28 Left: The tips of the leaves of glory lily are modified as tendrils. Right: some of the leaflets in the compound leaves of pea plants are modified as tendrils. Left image by Krishna satya 333 (CC BY-SA), and right image by Melissa Ha (CC-BY).

Showy bracts are brightly colored leaves function in attracting pollinators. From a distance, showy bracts often appear like the petals of a flower. However, the actual flowers are typically small and in a cluster surrounded by the bracts (Figure 20.29).

A plant with regular green leaves and hot pink bracts that appear like flower petals.
Closeup view of dogwood. Four pink bracts appear like petals. The actual flowers are in the center of the four bracts.
Figure 20.29 Showy bracts are brightly colored leaves modified to attract pollinators in Bougainvillea (left) and dogwood (right). Although bracts superficially look like petals, the actual petals are part of tiny flowers at the central point where the bracts meet. Images by Melissa Ha (CC-BY).

Carnivorous plants grow in bogs where the soil is low in nitrogen, and their leaves are adapted to help them to survive this nutrient-poor environment. In these plants, leaves are modified as traps to capture insects. While they still conduct photosynthesis to capture energy and synthesize sugars (and are thus autotrophs), they rely on insect-capturing leaves as a supplementary source of much-needed nitrogen, similar to fertilizer. Several examples of carnivorous plants are the cobra lily (Darlingtonia), various pitcher plants (Nepenthes [Figure 20.30], CephalotusSarracenia), the butterwort (Utricularia), the sundew (Drosera; Figure 20.31), and the best known, the Venus flytrap (Dionaea).

Dionaea (venus fly trap) leaf trap with teeth and flattened phyllode, labeled The Nepenthes leaf trap looks like a pitcher. A narrow tendril is attached to it. The flat phyllode is barely visible.

The teeth of a Venus fly trap (left) and the deep chamber formed by the leaf of a pitcher plant (right).
Figure 20.30 The (a) Venus flytrap has modified leaves that can capture insects. When an unlucky insect touches the trigger hairs inside the leaf, the trap suddenly closes. The opening of the (b) pitcher plant is lined with a slippery wax. Insects crawling on the lip slip and fall into a pool of water in the bottom of the pitcher, where they are digested by bacteria. The plant then absorbs the smaller molecules. Top two photos by Maria Morrow (CC-BY). Bottom photos are modification of work by Peter Shanks; credit b: modification of work by Tim Mansfield.
Drosera (sundew) with glandular trichomes to trap insects
Figure 20.31 The ends of sundew (Drosera sp.) leaves are covered with glandular trichomes. Each trichome has a blob of sticky fluid at the tip. Insects are attracted to the fluid, becoming trapped in it, as the plant slowly digests their bodies for the mineral nutrients that are lacking in their environment. Image by Maria Morrow (CC-BY).

Root Structure and Function

There are two types of root systems. The first is a fibrous root system which has multiple big roots that branch and form a dense mass which does not have a visible primary root (“grass-like”). The other is the tap root system which has one main root that has branching into lateral roots (“carrot-like”).

Root systems are mainly of two types (Figure 20.32). Eudicots have a tap root system, while monocots have a fibrous root system. A tap root system has a main root that grows down vertically, and from which many smaller lateral roots arise. Dandelions are a good example; their tap roots usually break off when trying to pull these weeds, and they can regrow another shoot from the remaining root). A tap root system penetrates deep into the soil. In contrast, a fibrous root system is located closer to the soil surface, and forms a dense network of roots that also helps prevent soil erosion (lawn grasses are a good example, as are wheat, rice, and corn). Some plants have a combination of tap roots and fibrous roots. Plants that grow in dry areas often have deep root systems, whereas plants growing in areas with abundant water are likely to have shallower root systems.

A carrots contains tap roots that have thin lateral roots extending from them. Fibrous root systems can be found beneath the soil.
Figure 20.32 (a) Tap root systems have a main root that grows down, while (b) fibrous root systems consist of many small roots. (credit b: modification of work by “Austen Squarepants”/Flickr)

Along with having different systems, there are primary root that originated from the root of the seedling (Figure 20.33) and secondary (lateral) roots originate from the primary roots, and adventitious roots originate on stems or leaves, rather than from the base of the embryo. Adventitious roots can grow if plant cuttings are placed in water (Figure 20.33).

Wheat and Mung Bean seedlings, showing roots, stem, and cotyledonsJade plant cutting, showing adventitious roots growing from stem
Figure 20.33 a) Mung bean tap root (eudicot) and wheat bean fibrous root (monocot) from sprouts. b) Jade plant cutting, showing adventitious roots growing from stem after being placed in water. Credit: Kammy Algiers (CC-BY).

Reading Question #4

Which plant structure (organ) is responsible for supporting the plant and transporting nutrients and water?

A) Roots

B) Leaves

C) Stem

D) Cuticle

Reading Question #5

Which of the following is not one of the three types of plant tissue found in leaves?

A) Vascular Bundles

B) Mesophyll

C) Spines

D) Epidermis



Adapted from Clark, M.A., Douglas, M., and Choi, J. (2018). Biology 2e. OpenStax. Retrieved from https://openstax.org/books/biology-2e/pages/1-2-themes-and-concepts-of-biology?query=%22organ%20system%22&target=%7B%22type%22%3A%22search%22%2C%22index%22%3A0%7D#fs-id2155753

Adapted from Ha, M., Morrow, M., & Algiers, K. (n.d.). Botany. Retrieved from https://bio.libretexts.org/Bookshelves/Botany/Botany_(Ha_Morrow_and_Algiers)/03%3A_Plant_Structure

Barrickman, N., Bell, K., and Cowan, C. (n.d.) Human Biology. Pressbooks. Retrieved from https://slcc.pressbooks.pub/humanbiology/chapter/chapter-12-organ-systems-of-the-human-body/

Lumen Learning. (2021). Fundamentals of Biology I. https://library.achievingthedream.org/herkimerbiologyfundamentals1/chapter/plant-sensory-systems-and-responses/

5.1: Organs and systems of the human organism. (2019, February 18). Medicine LibreTexts; Libretexts. https://med.libretexts.org/Bookshelves/Anatomy_and_Physiology/Book%3A_Human_Anatomy_and_Physiology_Preparatory_Course_(Liachovitzky)/05%3A_Higher_Levels_of_Complexity-_Organs_and_Systems/5.01%3A_Organs_and_Systems_of_the_Human_Organism


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