Plant Development I: Tissue differentiation and function

Learning Objectives

  1. Recognize relationships between plant embryonic structures and mature plant morphology
  2. Describe the organization and functions of plant organs (roots, stems, and leaves), and relate morphology to function
  3. Describe the features and functions of plant tissues, identify and describe cell types associated with each tissue, and relate cell and tissue morphology with function
  4. Differentiate between monocot and eudicot body plan characteristics, including organization of the vascular and ground tissues in stems and roots

Plant Embryogenesis

The information below is adapted from OpenStax Biology 32.2

Prior to fertilization, the plant egg cell contains a gradient of a plant hormone called auxin, where one side of the egg cell has a high concentration of auxin, and the other side of the egg has a lower concentration. Auxin behaves like a cytoplasmic determinant, setting up the apical/basal axis (similar to the anterior/posterior axis in animals) in the very first cell division. Following fertilization of the ovule by sperm, the plant zygote divides asymmetrically which segregates the auxin as follows:

  • The apical (top) cell contains the higher concentration of auxin; this cell which will go on to become the plant embryo
  • The basal (bottom) cell contains little auxin; this cell will go onto develop into a structure called the suspensor, which functions like an umbilical cord to provide nutrients from from maternal to embryonic tissue.

Through multiple rounds of cell division followed by differentiation, the apical cell ultimately gives rise to structures 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 hypocotyl (“below-cotyl”) will develop into the stem as the plant matures.
  • The radicle will develop into roots as the plant matures.

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)

Plant Body Organization

The information below was adapted from OpenStax Biology 30.1

Like animals, plants are multicellular eukaryotes whose bodies are composed of organs, tissues, and cells with highly specialized functions. Tissues are groups of similar cells that work together on a specific task. Organs are structures made up of two or more tissues organized to carry out a particular function, and groups of organs with related functions make up the different organ systems. Seeded plants (angiosperms and gymnosperms) have two organ systems:

  • The root system, which supports the plants and absorbs water and minerals, is usually underground.
  • The shoot system, consisting of stems, leaves, and the reproductive parts of the plant. The shoot system generally grows above ground, where it absorbs the light needed for photosynthesis.

The organ systems of a typical angiosperm (vascular flowering 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.

Seeded plants have three organs: roots, stems, and leaves, and three tissue types: ground tissue, vascular tissue, and dermal tissue. Each organ include all three tissue types. Each tissue is made up of different cell types, and the structure of each cell type influences the function of the tissue. We will go through each of the organs, tissues, and cell types in greater detail below.

The relationships between plant organs, tissues, and cell types are illustrated below.

Plant body organization

We’ll look at each of these levels of plant organization in turn.

Roots

The information below was adapted from OpenStax Biology 30.3

The roots of seeded plants typically occur below ground, and have three major functions:

  • anchoring the plant to the soil
  • absorbing water and minerals and transporting them to the shoot system
  • storing the products of photosynthesis

Roots of seeded plants can typically be categorized as either:

  • Tap root systems, which have a main root that grows down vertically and many smaller lateral roots arising from the tap root. Tap roots penetrate deep into the soil and are advantageous for plants growing in dry soils. Tap roots are common in dicots such as dandelions.
  • Fibrous root systems, which are located closer to the surface and have a dense network of roots. Fibrous root systems can help prevent soil erosion. Fibrous roots are common in monocots such as grasses.
Examples of plant tap roots alone and along a weathered river bank
(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)

Though they are typically underground, roots can vary widely in structure based on evolutionary adaptations for specific purposes:

  • Bulbous roots, like onions, store starch.
  • Prop roots are are a type of above-ground root that provide additional support to anchor the plant.
  • Pneumatophores are a type of above-ground root that facilitate gas exchange in plants that live in stagnant, oxygen-poor water like swamps.
  • Some tap roots, such as carrots, turnips, and beets, are adapted for sugar/starch storage.
  • Epiphytic roots, present in plants like Spanish moss, collect water and nutrients from air and dust, enabling a plant to grow on another plant
  • Haustoria are root-like structures of parasitic plants, like mistletoe, that penetrate into the tissue of a host plant, allowing the parasite to siphon off water and nutrients

The Shoot System: Stems

The information below was adapted from OpenStax Biology 30.2

Stems are a part of the shoot system of a plant. They are usually above ground, and their main functions are to:

  • provide structural support to the plant, holding leaves, flowers and buds
  • 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

Plant stem structures include:

  • nodes, which are points of attachment for leaves and flowers
  • internodes, are the regions of stem between two nodes
  • the apical bud, which is located at the tip of the shoot and contains the apical meristem, the site of new growth above ground
  • axillary buds, which may present where a leaf meets a stem, and are sites where branches or flowers may be produced
Plant parts diagram
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

Stems may range in length and diameter, depending on the plant type. Stems are usually above ground, although the stems of some plants, such as the potato, also grow underground. Variations on stem structures include:

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

The Shoot System: Leaves

The information below was adapted from OpenStax Biology 30.4

Leaves are the main sites for photosynthesis, the process used by plants to synthesize food. Leaves of most plants are typically green, due to the abundance of green chlorophyll in the leaf cells. However, some leaves may have different colors caused by other plant pigments that can mask the green chlorophyll.

A “typical” eudicot leaf structure includes:

  • a petiole, which is a structure that attaches the leaf to the stem (the leaves of some plants lack a petiole and attach directly to the stem)
  • veins, which are bundles of vascular tissue that run through the leaf; leaf veins carry water and nutrients, and also provide structural support to the leaf
Part of a leaf
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

The thickness, shape, and size of leaves are evolutionarily adapted to specific environments:

  • 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; these two attributes both 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: Dermal, Vascular, and Ground Tissues

The information below adapted from OpenStax Biology 30.1

Plant tissues fall into two general categories: meristematic tissue, and permanent (or non-meristematic) tissue. Meristematic tissue is functionally equivalent to stem cells in animals: undifferentiated cells that continue to divide and generate new cells and new tissues. (One key difference between animal stem cells and plant meristems is that animal stem cells contribute to replacing aging or injured tissues, while plant meristems contribute to new growth over the life 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 cells of permanent tissue; 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.
  • Vascular tissue transports water, minerals, and sugars to different parts of the plant. Vascular tissue is made of two specialized conducting tissues: xylem, which transports water and also provides structural support, and phloem, which transports sugars from sites 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)
  • Ground tissue carries out different functions based on the cell type and location in the plant, including photosynthesis, structural support for the stem and the vascular tissue, and storage for water and sugars.
Breakout of a plant, it's organs, and tissues
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 greater details of the cell types within plant tissues, this video provides an overview of plant organ structure and tissue function:

 

Plant Cell Types

Now that we’ve discussed plant organs and plant tissues, we will delve deeper into the cell types that comprise each tissue. Each plant tissue is composed of specialize cell types which carry out vastly different functions, which we will discuss in depth below.

But before we go into the differences in the cell types, it’s important to first note that all plant cells have primary cell walls, which are flexible and can expand as the cell grows. Some plant cells 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. We’ll describe each of these different types of cells 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

Dermal tissue is the outer layer of tissue surrounding the entire plant, which covers and protects the plant, and controls gas exchange and water absorption. Dermal tissue can be comprised of:

  • Epidermal cells: The epidermis is usually comprised of a single layer of epidermal cells which provide protection and have other specialized adaptations in different plant organs. Epidermal cells of the stems and leaves are coated by a waxy cuticle that prevents water loss from evaporation; the epidermal cells in the roots function in nearly the opposite way of stem epidermal cells; root epidermal cells aid in water and mineral absorption, and lack a cuticle to allow for water absorption.
  • Guard cells: 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.
  • Root hairs: Root hairs are microscopy extensions of root epidermal cells which increase the surface area of the root, greatly contributing to the absorption of water and minerals.
  • Trichomes: Stems and leaves of some plants also have trichomes, spiky or 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). And also 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 plants 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.

Vascular tissue in plants is made of two specialized conducting tissues: xylem, which conducts water, and phloem, which conducts sugars and other organic compounds. The xylem and phloem are always next 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. A single vascular bundle always contains both xylem and phloem tissues.

Xylem tissue transports water and nutrients from the roots to different parts of the plant. Xylem is composed of vessel elements and tracheids, both of which are tubular, elongated cells that conduct water:

  • Tracheids are found in all types of vascular plants.
  • Vessel elements are found only in angiosperms and a few other specific plants.
  • 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.
  • Tracheids and vessel elements have secondary cell walls hardened with lignin, and they provide structural support to the plant.
  • Tracheids and vessel elements are both dead at functional maturity, meaning that they are actually not living 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 (also called sieve tube elements) conduct sugars and other organic compounds throughout the plant body 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.
  • Companion cells lie adjacent to and share their cytoplasm with the sieve cells, providing metabolic support and regulation.
Light micrograph of squash cross section
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 is all the other tissue in a plant that isn’t dermal tissue or vascular tissue. Ground tissue cells include parenchyma, which carry out photosynthesis in the leaves and performs sugar storage in the roots; collenchyma, which which supports the stems and leaves in areas of active growth; and schlerenchyma, which supports the stem and leaves in areas where growth has ceased:

    • Parenchyma are the most abundant and versatile cell type in plants (if you stumble upon a new type of plant cell that you’ve never heard of, it’s a good bet that it’s a subtype of parenchyma cell). Their primary cell walls are thin and flexible, and most lack a secondary cell wall.
      • Most of the tissue in leaves is comprised of parenchyma cells, which are the sites of photosynthesis; and parenchyma cells in the leaves contain large quantities of chloroplasts for photosynthesis.
      • 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 cells comprise a specialized cortex tissue in roots 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 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.
      • Parenchyma can also be associated with phloem cells in vascular tissue as parenchyma rays.
      • Even though they are characterized as ground tissue, parenchyma cells are totipotent, meaning they can divide and differentiate into all cell types of the plant; parenchyma are the cells that are capable of producing roots from a cut stem.
    • Collenchyma have thicker primary cells walls than parenchyma, and also lack a secondary cell wall.
    • They are long and thin cells that retain the ability to stretch and elongate.
    • Their ability to elongate helps them provide structural support in growing regions of the shoot system; they are highly abundant in elongating stems.
    • If you have ever eaten celery, you have encountered collenchyma cells; they are the “stringy” bits found in celery.
    • Sclerenchyma cells have secondary cell walls composed of lignin, a tough substance that is the primary component of wood.
      • Schelrenchyma cells cannot stretch due to their secondary cell wall, and they provide important structural support in mature stems after growth has ceased.
      • Schlerenchyma cells are dead at functional maturity.
      • Sclerenchyma are found in apple cores and they give pears their gritty texture; we use sclerenchyma fibers to make linen and rope.
    A cross section of a leaf showing the phloem, xylem, sclerenchyma and collenchyma, and mesophyll.
    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 Dicots vs Monocots

    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:

    • Stems: In dicot stems, vascular bundles are arranged in a ring toward the stem periphery (this arrangement is essential for a phenomenon called secondary growth, which we will discuss in the next reading). 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
    • Roots: In dicot roots, the xylem and phloem of the stele (vascular bundle) 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 near the start of the reading).
    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
    • Leaves: 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 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

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