Plant Development I: Tissue differentiation and function

Learning Objectives

  • Describe features, functions, and composition of plant organs, tissues, and cell types
  • Relate morphology (roots, shoots, leaves, tissue systems, cell types) to function
  • Differentiate monocot and eudicot body plan characteristics
  • Recognize relationships between embryonic structures and mature plant morphology

Plant body organization

Like animals, plants are multicellular eukaryotes whose bodies are composed of organs, tissues, and cells with highly specialized functions. The relationships between plant organs, tissues, and cell types are illustrated below.

The stems and leaves together make up the shoot system. Each organ (roots, stems, and leaves) include all three tissue types (ground, vascular, and dermal). Different cell types comprise each tissue type, and the structure of each cell type influences the function of the tissue it comprises. We will go through each of the organs, tissues, and cell types in greater detail below.

Plant Organ Systems

The text below was adapted from OpenStax Biology 30.1

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 supports the plants and absorbs water and minerals, is usually underground. The organ systems of a typical plant are illustrated below.

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. Image credit: OpenStax Biology.

We’ll look at each of these levels of plant organization in turn, and conclude with a discussion of how embryogenesis leads to development of a mature plant:

The Root System

The text below was adapted from OpenStax Biology 30.3

The roots of seed plants have three major functions: anchoring the plant to the soil, absorbing water and minerals and transporting them upwards, and storing the products of photosynthesis. Some roots are modified to absorb moisture and exchange gases. Most roots are underground. Some plants, however, also have adventitious roots, which emerge above the ground from the shoot.

Root systems are mainly of two types (shown below):

  • Tap root systems have a main root that grows down vertically, and from which many smaller lateral roots arise. Tap roots penetrate deep into the soil and are advantageous for plants growing in dry soils. Tap roots are typical of dicots such as dandelions.
  • Fibrous root systems are located closer to the surface and have a dense network of roots. Fibrous root systems can help prevent soil erosion. Fibrous roots are typical of monocots such as grasses.

(a) Tap root systems have a main root that grows down, while (b) fibrous root systems consist of many small roots. Image credit: OpenStax Biology, modification of work by Austen Squarepants/Flickr)

Root structures are evolutionarily adapted for specific purposes:

  • Bulbous roots store starch.
  • Aerial roots and prop roots are two forms of above-ground roots that provide additional support to anchor the plant.
  • Some tap roots, such as carrots, turnips, and beets, are adapted for food storage.
  • Epiphytic roots enable a plant to grow on another plant

The shoot system: stems and leaves

The text below was adapted from OpenStax Biology 30.2

Stems are a part of the shoot system of a plant. Their main function is to provide support to the plant, holding leaves, flowers and buds. Of course they also connect the roots to the leaves, transporting absorbed water and minerals from the roots to the rest of the plant, and transporting sugars from the leaves (the site of photosynthesis) to desired locations throughout the 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 can be of several different varieties:

  • Herbaceous stems are soft and typically green
  • Woody stems are hard and wooded
  • Unbranched stems have a single stem
  • Branched stems have divisions and side stems

Plant stems, whether above or below ground, are characterized by the presence of nodes and internodes (shown below). 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.

Leaves are attached to the plant stem at areas called nodes. An internode is the stem region between two nodes. The petiole is the stalk connecting the leaf to the stem. The leaves just above the nodes arose from axillary buds. By Kelvinsong – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=27509689

The text below was adapted from OpenStax Biology 30.4

Leaves are the main sites for photosynthesis: the process by which plants synthesize food. Most leaves are usually green, due to the presence of chlorophyll in the leaf cells. However, some leaves may have different colors, caused by other plant pigments that mask the green chlorophyll.

A typical eudicot leaf structure is shown below. Each leaf typically has a leaf blade called the lamina, which is also the widest part of the leaf. Typical leaves are attached to the plant stem by a petiole, though there are also leaves that attach directly to the plant stem. The edge of the leaf is called the margin. The vascular tissue (xylem and phloem) run through veins in the leaf, which also provide structural support. The center vein is called the midrib.

Illustration shows the parts of a leaf. The petiole is the stem of the leaf. The midrib is a vessel that extends from the petiole to the leaf tip. Veins branch from the midrib. The lamina is the wide, flat part of the leaf. The margin is the edge of the leaf. Image credit: OpenStax Biology

Leaves may be simple or compound (see below).

  • In a simple leaf, the blade is either completely undivided or it has lobes, but the separation does not reach the midrib (leaf center).
  • In a compound leaf, the leaf blade is completely divided, forming leaflets.

Leaves may be simple or compound. In simple leaves, the lamina is continuous. The (a) banana plant (Musa sp.) has simple leaves. In compound leaves, the lamina is separated into leaflets. Compound leaves may be palmate or pinnate. In (b) palmately compound leaves, such as those of the horse chestnut (Aesculus hippocastanum), the leaflets branch from the petiole. In (c) pinnately compound leaves, the leaflets branch from the midrib, as on a scrub hickory (Carya floridana). The (d) honey locust has double compound leaves, in which leaflets branch from the veins. (Image credit: OpenStax Biology; credit a: modification of work by “BazzaDaRambler”/Flickr; credit b: modification of work by Roberto Verzo; credit c: modification of work by Eric Dion; credit d: modification of work by Valerie Lykes)

The thickness, shape, and size of leaves are adapted to specific environments. Each variation helps a plant species maximize its chances of survival in a particular habitat. Coniferous plant species that thrive in cold environments, like spruce, fir, and pine, have leaves that are reduced in size and needle-like in appearance. These needle-like leaves have sunken stomata (pits that allow gas exchange) and a smaller surface area: two attributes that aid in reducing water loss. In hot climates, plants such as cacti have leaves that are reduced to spines, which in combination with their succulent stems, help to conserve water. Many aquatic plants have leaves with wide lamina that can float on the surface of the water, and a thick waxy cuticle (waxy covering) on the leaf surface that repels water.

Plant tissues

Content below adapted from OpenStax Biology 30.1

Plant tissue systems fall into one of two general types: meristematic tissue, and permanent (or non-meristematic) tissue. Cells of the meristematic tissue are found in meristems, which are plant regions of continuous cell division and growth (analogous to stem cells in animals). Meristematic tissue cells are either undifferentiated or incompletely differentiated, and they continue to divide and contribute to the growth of the plant. In contrast, permanent tissue consists of plant cells that are no longer actively dividing.

Meristems produce cells that quickly differentiate, or specialize, and become permanent tissue. Such cells take on specific roles and lose their ability to divide further. They differentiate into three main tissue types: dermal, vascular, and ground tissue. Each plant organ (roots, stems, leaves) contains all three tissue types:

  • Dermal tissue covers and protects the plant, and controls gas exchange and water absorption (in roots). Dermal tissue of the stems and leaves is covered by a waxy cuticle that prevents evaporative water loss. Stomata are specialized pores that allow gas exchange through holes in the cuticle. Unlike the stem and leaves, the root epidermis is not covered by a waxy cuticle which would prevent absorption of water. Root hairs, which are extensions of root epidermal cells, increase the surface area of the root, greatly contributing to the absorption of water and minerals. Trichomes, or small hairlike or spikey outgrowths of epidermal tissue, may be present on the stem and leaves, and aid in defense against herbivores.
  • Ground tissue carries out different functions based on the cell type and location in the plant, and includes parenchyma (photosynthesis in the leaves, and storage in the roots), collenchyma (shoot support in areas of active growth), and schlerenchyma (shoot support in areas where growth has ceased)is the site of photosynthesis, provides a supporting matrix for the vascular tissue, provides structural support for the stem, and helps to store water and sugars.
  • Vascular tissue transports water, minerals, and sugars to different parts of the plant. Vascular tissue is made of two specialized conducting tissues: xylem and phloem. Xylem tissue transports water and nutrients from the roots to different parts of the plant, and also plays a role in structural support in the stem. Phloem tissue transports organic compounds from the site of photosynthesis to other parts of the plant. The xylem and phloem always lie adjacent to each other in a vascular bundle (we’ll explore why later).

Each plant organ contains all three tissue types. Koning, Ross E. 1994. Plant Basics. Plant Physiology Information Website. http://plantphys.info/plant_physiology/plantbasics1.shtml. (6-21-2017). Reprinted with permission.

Before we get into the details of plant tissues, this video provides an overview of plant organ structure and tissue function:

Plant Cell Types

All plants have primary cell walls, which are flexible and can expand as the cell grows and elongates. Some plants also have a secondary cell wall, typically composed of lignin (the substance that is the primary component of wood). Secondary cell walls are inflexible and play an important role in plant structural support.

Each plant tissue type is comprised of specialize cell types which carry out vastly different functions:

  • Vascular tissue
    • Tracheids
    • Vessel elements
    • Sieve tube cells
    • Companion cells
  • Dermal tissue
    • Epidermal cells
    • Stomata or more accurately, guard cells
    • Trichomes
  • Ground tissue
    • Parenchyma
    • Collenchyma
    • Sclerenchyma

We’ll describe each of these in turn, and consider how tissues carry out similar or different functions in different organs based on the presence of specific cell types.

Cells in dermal tissue

The outer layer of tissue surrounding the entire plant is called the epidermis, usually comprised of a single layer of epidermal cells which provide protection and have other specialized adaptations in different plant organs.

In the root, the epidermis aids in absorption of water and minerals. Root hairs, which are extensions of root epidermal cells, increase the surface area of the root, greatly contributing to the absorption of water and minerals. Roots also contain specialized dermal cells called endodermis, which is found only in the roots and and serves as a checkpoint for materials entering the root’s vascular system from the environment. A waxy substance called suberin is present on the walls of the endodermal cells. This waxy region, known as the Casparian strip, forces water and solutes to cross the plasma membranes of endodermal cells instead of slipping between the cells.

In the stem and leaves, epidermal cells are coated in a waxy substance called a cuticle which prevents water loss through evaporation. To permit gas exchange for photosynthesis and respiration, the epidermis of the leaf and stem also contains openings known as stomata (singular: stoma). 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. Stems and leaves may also have trichomes, hair-like structures on the epidermal surface, that 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.

Visualized at 500x with a scanning electron microscope, several stomata are clearly visible on (a) the surface of this sumac (Rhus glabra) leaf. At 5,000x magnification, the guard cells of (b) a single stoma from lyre-leaved sand cress (Arabidopsis lyrata) have the appearance of lips that surround the opening. In this (c) light micrograph cross-section of an A. lyrata leaf, the guard cell pair is visible along with the large, sub-stomatal air space in the leaf. (credit: OpenStax Biology, modification of work by Robert R. Wise; part c scale-bar data from Matt Russell)

Trichomes give leaves a fuzzy appearance as in this (a) sundew (Drosera sp.). Leaf trichomes include (b) branched trichomes on the leaf of Arabidopsis lyrata and (c) multibranched trichomes on a mature Quercus marilandica leaf. (credit: OpenStax Biology, a: John Freeland; credit b, c: modification of work by Robert R. Wise; scale-bar data from Matt Russell)

Cells in vascular tissue

Just like in animals, vascular tissue transports substances throughout the plant body. But instead of a circulatory system which circulates by a pump (the heart), vascular tissue in plants does not circulate substances in a loop, but instead transports from one extreme end of the plant to the other (eg, water from roots to shoots). Vascular tissue in plants is made of two specialized conducting tissues: xylem, which conducts water, and phloem, which conducts sugars and other organic compounds. A single vascular bundle always contains both xylem and phloem tissues. Unlike the animal circulatory system, where the vascular system is composed of tubes that are lined by a layer of cells, the vascular system in animals is made of cells – the substance (water or sugars) actually moves through individual cells to get from one end of the plant to the other.

Xylem tissue transports water and nutrients from the roots to different parts of the plant, and includes vessel elements and tracheids, both of which are tubular, elongated cells that conduct water. Tracheids are found in all types of vascular plants, but only angiosperms and a few other specific plants have vessel elements. Tracheids and vessel elements are arranged end-to-end, with perforations called pits between adjacent cells to allow free flow of water from one cell to the next. They have secondary cell walls hardened with lignin, and (similar to scherenchyma) provide structural support to the plant.  Tracheids and vessel elements are both dead at functional maturity, meaning that they are actually dead when they carry out their job of transporting water throughout the plant body.

Phloem tissue, which transports organic compounds from the site of photosynthesis to other parts of the plant, consists of sieve cells and companion cells. Sieve cells conduct sugars and other organic compounds, and are arranged end-to-end with pores called sieve plates between them to allow movement between cells. They are alive at functional maturity, but lack a nucleus, ribosomes, or other cellular structures. Sieve cells are thus supported by companion cells, which lie adjacent to the sieve cells and provide metabolic support and regulation.

The xylem and phloem always lie adjacent to each other. In stems, the xylem and the phloem form a structure called a vascular bundle; in roots, this is termed the vascular stele or vascular cylinder.

This light micrograph shows a cross section of a squash (Curcurbita maxima) stem. Each teardrop-shaped vascular bundle consists of large xylem vessels toward the inside and smaller phloem cells toward the outside. Xylem cells, which transport water and nutrients from the roots to the rest of the plant, are dead at functional maturity. Phloem cells, which transport sugars and other organic compounds from photosynthetic tissue to the rest of the plant, are living. The vascular bundles are encased in ground tissue and surrounded by dermal tissue. (credit: OpenStax Biology, modification of work by “(biophotos)”/Flickr; scale-bar data from Matt Russell)

Cells in ground tissue

Ground tissue includes parenchyma, (photosynthesis in the leaves, and storage in the roots), collenchyma (shoot support in areas of active growth), and schlerenchyma (shoot support in areas where growth has ceased).

Parenchyma are the most abundant and versatile cell type in plants. They have primary cell walls which are thin and flexible, and most lack a secondary cell wall. Parenchyma cells are totipotent, meaning they can divide and differentiate into all cell types of the plant, and are the cells responsible for rooting a cut stem. Most of the tissue in leaves is comprised of parenchyma cells, which are the sites of photosynthesis. Leaves typically contains two types of parenchyma cells: the palisade parenchyma and spongy parenchyma. The palisade parenchyma (also called the palisade mesophyll) has column-shaped, tightly packed cells. Below the palisade parenchyma are the cells of the spongy parenchyma (or spongy mesophyll), which are loosely arranged with air spaces that all gaseous exchange between the leaf and the outside atmosphere. Both of these types of parenchyma cells contain large quantities of chloroplasts for phytosynthesis. In roots, parenchyma are sites of sugar or starch storage, and are called pith (in the root center) or cortex (in the root periphery). Parenchyma can also be associated with phloem cells in vascular tissue as parenchyma rays.

Collenchyma, like parenchyma, lack secondary cell walls but have thicker primary cells walls than parenchyma. They are long and thin cells that retain the ability to stretch and elongate; this feature helps them provide structural support in growing regions of the shoot system. They are highly abundant in elongating stems. The “stringy” bits of celery are primarily collenchyma cells.

Schlerenchyma cells have secondary cell walls composed of lignin, a tough substance that is the primary component of wood. Schelrenchyma cells therefore cannot stretch, and they provide important structural support in mature stems after growth has ceased. Interestingly, schlerenchyma cells are dead at functional maturity. There are two types of sclerenchyma cells: fibers and sclereids. Fibers are long, slender cells; sclereids are smaller-sized. Sclereids give pears their gritty texture, and are also part of apple cores. We use sclerenchyma fibers to make linen and rope.

A cross section of a leaf showing the phloem, xylem, sclerenchyma and collenchyma, and mesophyll. By Kelvinsong – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=25593329

Tissue arrangements in different plant organs

Each plant organ contains all three tissue types, with different arrangements in each organ. There are also some differences in how these tissues are arranged between monocots and dicots, as illustrated below:

In dicot roots, the xylem and phloem of the stele are arranged alternately in an X shape, whereas in monocot roots, the vascular tissue is arranged in a ring around the pith. In addition, monocots tend to have fibrous roots while eudicots tend to have a tap root (both illustrated above).

In (left) typical dicots, the vascular tissue forms an X shape in the center of the root. In (right) typical monocots, the phloem cells and the larger xylem cells form a characteristic ring around the central pith. The cross section of a dicot root has an X-shaped structure at its center. The X is made up of many xylem cells. Phloem cells fill the space between the X. A ring of cells called the pericycle surrounds the xylem and phloem. The outer edge of the pericycle is called the endodermis. A thick layer of cortex tissue surrounds the pericycle. The cortex is enclosed in a layer of cells called the epidermis. The monocot root is similar to a dicot root, but the center of the root is filled with pith. The phloem cells form a ring around the pith. Round clusters of xylem cells are embedded in the phloem, symmetrically arranged around the central pith. The outer pericycle, endodermis, cortex and epidermis are the same in the dicot root. Image credit: OpenStax Biology

In dicot stems, vascular bundles are arranged in a ring toward the stem periphery. In monocot stems, the vascular bundles are randomly scattered throughout the ground tissue.

In (a) dicot stems, 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. Sclerenchyma fibers cap the vascular bundles. In the center of the stem is ground tissue.  In (b) monocot stems, vascular bundles composed of xylem and phloem tissues are scattered throughout the ground tissue. The bundles are smaller than in the dicot stem, and distinct layers of xylem, phloem and sclerenchyma cannot be discerned. Image credit: OpenStax Biology

 

Leaves include two different types of photosynthetic parenchyma cells (palisade and spongy). Like all plant organs, they also contain vascular tissue (not shown). Monocots tend to have parallel veins of vascular tissue in leaves, while dicots tend to have branched or net-like veins of vascular tissue in the leaves.

In the (a) leaf drawing, the central mesophyll is sandwiched between an upper and lower epidermis. The mesophyll has two layers: an upper palisade layer comprised of tightly packed, columnar cells, and a lower spongy layer, comprised of loosely packed, irregularly shaped cells. Stomata on the leaf underside allow gas exchange. A waxy cuticle covers all aerial surfaces of land plants to minimize water loss. Image credit: OpenStax Biology

This video provides a nice (albeit dry) summary and synthesis of plant structure and function:

 

Plant embryogenesis

The text below is adapted from OpenStax Biology 32.2

How do each of these tissues arise from a fertilized ovule? As we have previously discussed, the zygote divides asymmetrically into an apical cell which will go on to become the embryo, and a suspensor which functions like an umbilical cord to provide nutrients from from maternal to embryonic tissue. Prior to fertilization, there is a gradient of a plant hormone called auxin across the ovule, with higher concentrations of auxin in the region that will become the apical cell. The asymmetric cell division segregates auxin into the apical cell, establishing the apical/basal axis (analogous to the anterior/posterior axis in animals). Thus early plant development, much like early development in many animal species, begins with segregation of cytoplasmic determinants in the very first cell division.

Through multiple rounds of cell division followed by differentiation, the apical cell ultimately gives rise to the cotyledons, the hypocotyl, and the radicle. The cotyledons, or embryonic leaves, will become the first leaves of the plants upon germination. Monocots tend to have a single cotyledon, while dicots tend to have two cotyledons (in fact, the number of cotyledons present is what gives them the prefix “mono-” or “di-“). The part of the plant that grows above the cotyledons is called the epicotyl (“above-cotyl”). The hypocotyl (“below-cotyl”) will become the future stem, and the radicle, or embryonic root, will give rise to future roots.

The images below shows the general structures and processes involved in seed germination:

Public Domain, https://commons.wikimedia.org/w/index.php?curid=661229

s, seed coats; r, radicle; h, hypocotyl; c, cotyledon; e, epicotyl. Image credit: Image from page 233 of “Principles of modern biology” (1964)

This diagram summarizes the differences between monocots and dicots:

This diagram is showing the differences between monocotyledonous flowers or dicotyledonous flowers. Monocots have a single cotyledon and long and narrow leaves with parallel veins. Their vascular bundles are scattered. Their petals or flower parts are in multiples of three. Dicots have two cotyledons and broad leaves with network of veins. Their vascular bundles are in a ring. Their petals or flower parts are in multiples of four or five. By Flowerpower207 – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=26233760