27.1: Introduction
Plant Cells
Plant cells can be classified into three major categories based on their cell wall structure. Most plant cells are parenchyma cells (par- meaning equal), with an evenly-thickened primary cell wall. Some areas of a plant, particularly in young shoots, will require more flexibility, allowing them to bend without breaking. In these regions, you will often find collenchyma cells (coll- meaning glue), cells with strange, unevenly-thickened primary walls. Both of these cell types are alive at maturity. A third cell type, sclerenchyma cells (scler- meaning hard), develops a rigid secondary wall that is composed of lignin. This secondary wall provides structural support, but it also cuts off the exchange of water and other molecules permitted by the plasma membrane. Ultimately, this results in the death of the cell at maturity and loss of internal cell components.
Plant Tissues: Parenchyma, Collenchyma, Sclerenchyma
Plant cells performing a similar function can be assembled into tissues. This lab will introduce you to the three primary cell types found in plants, the tissues where you can find these cell types, and specialized cells that they can differentiate into. Specialized cells covered in this lab will be either parenchyma, collenchyma, or sclerenchyma (referred to as the “cell type”) and will have evolved for a specific function, such as conducting water. At the end of the lab, you will complete a table with the cell types, specialized cells, and functions of the major tissues within a leaf.
Plant Organs: Roots, Stems, Leaves
Plants are nature’s great water filters. They absorb water from the soil through their roots (if they have roots), use this water to maintain homeostasis, and whatever is left evaporates from open stomata across the epidermis of the plant. Each water molecule that leaves the plant is electrically charged and, due to these charges, tugs on the molecule behind it. The vast majority of water absorbed by most plants will exit via this process, creating a continuous vacuum of water through the plant, from the soil to the atmosphere. This evaporation of water from plant tissues is called transpiration . This process is highly dependent on the current environment, but to provide some perspective, an acre of corn transpires about 3,500 gallons of water per day.
Why do plants transpire if they are losing so much water everyday? Why not close their stomata to prevent water from escaping? There are many reasons why plants have evolved to transpire instead of retaining water.
- Transpiration drives the flow of water and dissolved nutrients through the plant. If plants stop releasing water through the stomata, they will stop pulling in the nutrients dissolved in that water essential for plant function.
- Transpiration provides evaporative cooling. As water leaves the plant tissues into the atmosphere, it takes energy with it in the form of heat. Much like when we sweat, this allows the plant to cool and maintain homeostasis. This is particularly valuable in hot environments.
- Leaving stomata open is required for photosynthesis (except in certain plants, more on this in Lab 10: Photosynthesis). Carbon dioxide enters the plant through the stomata and oxygen is released as a waste product. If the stomata are closed, the plant cannot form sugars.
When water evaporates from plant tissues, it is called transpiration . Ninety percent of water that evaporates from terrestrial surfaces occurs via transpiration--plants are the world’s greatest water filters! Water is absorbed by (most) plants through specialized organs called roots. The earliest plants, the bryophytes, don’t have roots. Instead, these plants rely on the absorption of water across the entire plant body and dispersal of this water by osmosis. For this lab, we will focus on the later groups of plants--the tracheophytes --that have specialized tissues for water absorption and transportation throughout the plant.
Roots
An essential part of a plant’s survival is obtaining access to water. Early plants did this by having small, creeping forms that grew in areas that would stay moist, never far from any surface. These plants did not have roots or the ability to transport water with xylem tissue. Instead, they absorbed and lost water across their tissues. As you can imagine, this would be a limiting state for plants and, soon after they moved onto land, plants evolved true roots with vascular tissue. Lignified tissues in roots would provide increased strength and stability for burrowing into the substrate (likely not yet a true soil) and access to water stored underground. Vascular tissue throughout the plant would allow water absorbed through the roots to be transported to other areas of the plant, meaning that tissues could elevate out of the water, getting increased access to sunlight.
In this lab, you will learn the general developmental pathway for tissues in the root, as well as the different anatomical organization of two groups of flowering plants: monocots and eudicots . Monocots, like corn and other grasses, germinate from seed with a single first leaf (called a cotyledon). Eudicots germinate with two leaves. Though this seems like a trivial distinction, these groups differ in many areas of growth and development.
Plants that have adapted to different environments might develop different root systems in response to the stressors in that environment. Observe the different root systems available in lab and try to classify them as one or more of the following:
Types of Root Systems
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Netted or Taproot System
In soils where water is readily available for most of the year, plants might develop a netted root system where many similar diameter roots capture as much water and nutrients as possible to outcompete their neighbors. In climates where there are droughts or freezes, plants might develop a taproot system , where a larger central root can burrow deeper into the soil profile, accessing water reserves that other plants cannot.
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Storage Roots
A larger diameter root can also store water and/or sugars for long periods. This type of root is called a storage root . A large central root, such as in the middle left on the following page, could be both a taproot and a storage root.
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Adventitious Roots
Adventitious roots emerge from stem tissue. This can happen when there is an underground stem, such as in the system at the top of the diagram on the following page, or to serve as a prop root , as in the center of the diagram.
Stems
The shoot of a plant is responsible for two major functions: photosynthesis and reproduction . In most plants, the leaves carry out photosynthesis, while the stems provide stability to elevate those leaves above potential competition. In annual plants, whose entire life cycle from germinate to death is completed in a single year, the epidermis on the stem is often photosynthetic, as well.
Tissues in the shoot are derived from the shoot apical meristem (SAM). Just like in the root, the SAM produces three primary meristems, which produce the primary tissues:
- Protoderm → Epidermis
- Ground meristem → Cortex and pith (simply ground tissue in monocots)
- Procambium → Primary xylem and primary phloem
These primary tissues will then either differentiate into specialized cells or, as is the case in many eudicots, become meristematic and produce secondary tissues.
Shoots and Leaves
Shoots are composed of nodes. Each node has a leaf and an axillary bud , which emerges from the leaf axil . The leaf is located just below the bud, which will become either a branch or a flower. The identity of each organ is determined by its location in the node.
In the diagram below, label the bolded features, as well as the photosynthetic part of the leaf (the blade ) and the stem of the leaf (the petiole ). Label the bolded features in the diagram below.
Scars and Features of a Woody Shoot
When a leaf or bud falls off, it leaves a scar behind. Leaf scars are crescent shaped and have small, circular bundle scars within them where vascular bundles traversed the plant tissues. A leaf scar will be located below a bud, branch, or bud scar .
Plants that grow in temperate regions generally have a growing season. Each year, the newly emerging growth is within the terminal bud which is protected by terminal bud scales . When these scales fall off as the new growth emerges, they leave a region of terminal bud scale scars behind. Regions between terminal bud scale scars represent one year of growth.
On woody shoots, a layer of bark has replaced the epidermis (more on this in lab 8). To continue to exchange gases with the exterior environment, the bark layer develops small tears called lenticels . These can be small circular or elongated scars.
Label the bolded features in the diagram below. Draw in leaf scars where they are missing!
Leaf Arrangement
As shoots develop, the leaves are arranged in a particular order which can differ from plant to plant. In some plant species, one leaf emerges from the branch at a time (left) so leaves appear to alternate from side to side. This type of leaf arrangement is called alternate . In other plants, two leaves are formed on either side of the stem at the same time. This type of leaf arrangement is called opposite (middle). If three or more leaves are produced at the same region on the stem, the leaf arrangement is whorled (right).
Leaf Anatomy
Leaves are specialized organs for performing photosynthesis. A leaf is often a relatively large, flat surface used to optimize sunlight capture. However, surfaces are areas that water can evaporate from, so a large amount of surface area exposed to sunlight results in increased transpiration. The anatomy of a leaf has everything to do with achieving the balance between photosynthesis and transpiration 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.
Organ Modifications
Over the course of each different plant’s evolutionary history, environmental pressures can lead to modifications of these features in predictable ways that we can then classify. For example, plants that frequently encounter drought will experience selection for the ability to access water when there is no water in the environment. The plants that evolve water-storage will have better survival in this environment and are more likely to successfully reproduce. However, animals in this dry environment might then seek out their water-filled tissues. Plants that either hide or protect these water reserves are more likely to survive and experience less herbivory. Over time, as the environment continues to select for these defenses, they improve. And thus, over long periods of evolutionary time, one lineage of plants evolves into cacti, while another related lineage evolves in a different environment to be carnations.
There are many selective pressures on plants. Climate has a strong selective effect on the anatomy and morphology of plants, particularly water availability and access to sunlight. Herbivory is another strong selective pressure, as plants cannot run away from their predators. Instead, they must evolve other deterrents while balancing the energy costs required for these adaptations with energy devoted to reproduction. A third strong selective pressure is nutrient availability, which is often determined by the soil environment. For example, serpentine soils have low levels of nitrogen and calcium. Plants in serpentine habitats often evolve the ability to trap and digest insects to absorb the missing nutrients. These “carnivorous” plants are not heterotrophic, because they do not use the trapped insects as an energy source.
Roots
Root tissue is derived from the root apical meristem. In some plants, environmental stressors will select for plants whose root tissues perform functions other than water absorption and anchorage. These organ modifications have specific names, depending on what function they serve.
- Storage roots : In most roots, surface area is maximized for water absorption. In a storage root, the volume becomes more important. Cells in the cortex are enlarged and contain leucoplasts.
- Pneumatophores : Gases diffuse 10,000x more slowly in water than in air. In plants that grow in saturated soils, such as mangroves, roots cells need to access oxygen to perform cellular respiration at a rate that they cannot accomplish through water. Pneumatophores are roots that emerge above the surface of the saturated zone to “breathe” (pneumo- means lung) and exchange gases with the environment.
- Adventitious roots : Unlike most roots, adventitious roots emerge from stem tissue. A root apical meristem is derived from tissues in the stem, then root tissues are formed from the RAM, as normal. Adventitious roots can be produced from nodes on horizontal or climbing stems or in response to environmental stressors.
- Prop roots : Prop roots are adventitious roots with the specific function of providing stability to a plant. This might happen in unstable soils, on climbing plants, or in plants that have a shallow root system.
Stems
Stem tissue is derived from the shoot apical meristem. In many plants, there is a central stem that lateral stems emerge from. You can distinguish stems from roots by the presence of nodes. You can distinguish lateral stems from leaves by location within the node: stems emerge above the leaf.
Just like with roots, stems can be adapted to a particular function in response to environmental stressors.
- Cladode : A cladode is a stem that has increased surface area to perform photosynthesis. This is usually because the leaves have been modified to some other purpose and are no longer performing photosynthesis. In essence, the stem is imitating a leaf.
- Succulent : In the case of succulence, it is the volume of the stem that increases. Stem tissues develop large, specialized cells called hydrenchyma to store extra water. The plant can access this water for metabolism in periods of drought.
- Tuber : Some stems are modified for storage of starches instead of water. A tuber is an underground stem that can be identified by its nodes (often referred to as “eyes” on a potato tuber). Each node is capable of producing a new shoot.
- Corm : A corm is also modified for storage of starches. A corm is swollen tissue at the base of the shoot with linear nodes travelling across it. From these nodes, papery leaves emerge.
- Rhizome : Some plants produce horizontal stems that are used for asexual reproduction. A rhizome is a horizontal stem that is underground. Roots emerging from the nodes of rhizomes are adventitious.
- Stolon : Similar to a rhizome, a stolon is a horizontal stem used for asexual reproduction. Unlike a rhizome, stolons are formed above the soil surface.
- Thorn : A thorn is a lateral branch that has been modified to protect the plant, usually from herbivory. Thorns have a subtending leaf or leaf scar.
- Stem tendril : A tendril is a lateral branch that has been modified for climbing. A stem tendril will have a subtending leaf or leaf scar.
Leaves
Leaf tissue is derived from the shoot apical meristem. You can distinguish leaves from stem tissue by location within the node: leaves emerge below the axillary bud, lateral stem, or flower. Under normal conditions, the primary function of a leaf is photosynthesis. However, environmental stressors can select for the following modifications:
- Succulent : Much like in stems, leaves can also be modified for water storage in environments where there is drought. You can distinguish a succulent leaf from a succulent stem because the stem will have nodes (the leaf will not).
- Bulb : Leaves can also be modified to store starch. A bulb, like an onion, is composed of fleshy leaves that surround a short, central stem.
- Spine : A leaf modified to be sharp for protection is called a spine. You can distinguish a spine from a thorn by the location within the node.
- Leaf tendril : Sometimes leaves will be modified for climbing. You can distinguish a leaf tendril from a stem tendril by the location within the node.
- Trap : In environments where nutrients are low, some plants have evolved to capture insects for access to nitrogen, phosphorus, and calcium. There are a few ways plants can achieve this. One is to modify a leaf into some sort of trap.
- Phyllode : Another instance of leaf imitation is when the petiole of the leaf becomes flat and leaf-like to perform photosynthesis. Similar to a cladode, this is usually in response to a modified leaf. The pitcher plant on the right shows a phyllode, leaf tendril, and trap all in one leaf.
- Stipular spines : Another sharp armament against herbivory can be a replacement of a stipule with a spine. These can be distinguished from other sharp modifications, as they emerge in pairs at the base of a leaf.
Other Plant Adaptations
In addition to the major organs, plants can have adaptations to specific tissues, cells, or molecules produced by the plant.
- Prickles : Similar to thorns and spines, prickles are a protective adaptation. In this case, it is a modification of the epidermal (and sometimes cortex) tissue. Prickles can emerge anywhere on the shoot, unlike spines and thorns, which are restricted to nodes.
- Trichomes : Trichomes are epidermal cells that have been modified as hairs. While they can still serve a protective function, they tend to be much smaller and less rigid than prickles.
- Raphides : Some plants have molecular, internal sharp armaments. In plants like Tradescantia, calcium oxalate is crystallized into needle-like structures called raphides.
Chop up the leaf or stem of a Tradescantia and make a squash mount to see raphides within the plant tissues.
- Latex : About 10% of flowering plants have evolved latex production, a sticky substance exuded when plants are damaged. This latex can prevent infection of wounds. Many groups of plants have independently evolved the production of latex gum up the mouthparts of herbivorous insects. In some plants, the latex also includes toxic compounds to aid in defense. For example, in milkweeds (Asclepias) this latex contains neurotoxins.