Brin g Your T ex t to L ab !!!
Vascular Plant Anatomy:
Flowering Plants
Objectives:
1. To observe what the basic structure of vascular plants is, and
how and where this form originates.
2. To begin to understand how form and function are connected
and how plants work.
I. INTRODUCTION TO FORM AND FUNCTION
A. General
Anyone who has grown plants realizes the importance of providing
proper amounts of water, nutrients, and light. Unless you’re the proverbial
"green thumb", it may appear that plants are decidedly temperamental about
getting these necessities in the proper amounts. How do you suppose that
plants that grow unattended meet these needs in an unpredictable natural
environment? Only cursory observation is required to reveal the specialized
features of plants that enable them to satisfy these needs and survive the
inclement environmental periods. Such features are adaptations acquired
through evolution.
Much of the organization of the plant is related to adaptations for
obtaining necessary levels of water, nutrients and light. Examples of these
adaptations can be found from the molecular and cellular levels, through
organs, tissues, and the entire plant organization. In fact, understanding
many adaptations requires knowledge of how these levels are integrated in
the plant. For example, the necessity of the plant to carry out a minimum
level of photosynthesis is related to the organization of specialized
photosynthesizing organelles in specialized cells, and often in organs
specialized to this task, like leaves.
Brin g Your T ex t to L ab !!!
Many other cell types in the leaf are specialized to support the
principal cells that carry out photosynthesis. In the most basic sense, one
can even argue that roots are organized to absorb water and nutrients that
support the photosynthetic process, and that both stems and roots provide
the physical support for maintaining the principal photosynthetic organs in a
favorable light environment. Most stem and root tissue can be best
understood when viewed from this perspective.
Secondary functions of plant organs, however, are quite important
and account for many variations in forms at all levels. These functions
include: protection from desiccation, carbohydrate storage, protection
from herbivores, protection from pathogen invasion, and even protection
from fire result in variations of the basic cell, tissue, and organ forms.
These adaptations have developed according to the environmental
pressures that have molded individual species in preceding generations.
The combination of adaptations causes each species to be unique and
distinctive, a concept that also could be appreciated from the taxonomy
and field botany laboratory. In this and the next lab, we will examine these
adaptations and their related functions.
The roots, stems, and leaves, as other organs, consist of groups of
tissues that carry out the processes essential to life. A tissue is a group
of cells with some similarity in structure, function or position. Cells that
carry out a particular function usually have very similar characteristics
adapted to that particular function. When they are first formed, plant
cells are relatively unspecialized. They acquire their unique forms as they
age, a process called differentiation.
Growth in plants is a nearly constant process terminated only by death
(indeterminate growth compared to determinate growth seen in animals).
Within plants are tissues that "specialize" in growth. That is, tissues whose
cells remain undifferentiated and retain the ability to divide. These tissues,
called meristems, occur at strategic positions where growth is required to
support the plant's continuing quest for light, water, and other essentials
from the environment. As these cells divide they give rise to cells that
“specialize” through differentiation to form the other tissues in the plant.
As you work though the labs, examine each organ or tissue: 1) with the
view of locating these basic tissue types, 2) examine the variations of cell
types within these tissues, and their relative positions to one another, 3)
remind yourself of the functions, or if the function is not clear, speculate
about the function as you examine the forms, and 4) draw everything you see
(preferably) in color.
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B. Tissue Types. Although there would appear to be a vast array of tissue
types in the plant, there are only three basic types, with almost unlimited
variation on them:
1. Dermal: Dermal tissue is adapted almost exclusively for
protection against desiccation or external agents such as pathogens.
2. Vascular: Vascular tissue is adapted primarily for physical support
and translocation or movement of materials within the plant.
3. Ground: Ground tissue is the most varied in function and form,
but is generally associated with functions of synthesis and
accumulation of organic compounds.
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II. Methods
Exercise #1 PLANT MORPHOLOGY
As you begin your investigation of the structure and function of
plants, you need an understanding of the general shape and form of the
whole plant. In this exercise you will study a bean (or other plant),
identifying basic features of the three vegetative organs: roots, stems, and
leaves. In following exercises you will investigate the cellular structure of
these organs, mostly in cross sections.
Examine the plant and identify the following:
SHOOTS:
a. Nodes are regions of the stem from which leaves,
buds and branches arise and which contain areas of
meristematic tissues (areas of cell division).
b. Internodes are regions of the stem between the
nodes.
c. Terminal buds are located at the tips of stems and
branches. They enclose the short apical meristem, which
gives rise to leaves, buds, and all primary tissue of the
stem. Only stems produce buds.
d. Axillary or lateral buds are located in the leaf axils
at the nodes; they may give rise to lateral branches.
Leaves consist of flattened blades attached at the node
of a stem by a stalk, or petiole.
ROOTS:
a. Primary and secondary roots. The primary root is the
first root produced by the plant embryo and may
become a long taproot. Secondary roots arise from
meristematic tissue deep within the primary root.
Root tips consist of a root apical
meristem that gives rise to a root cap and to all
the primary tissues of the root proper. A short
distance from the root tip is a zone of root hairs
(specialized epidermal cells), the principal site
of water and mineral absorption.
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Exercise #2 PRIMARY MERISTEMS
Plants produce new cells throughout their lifetime as a result
of cell divisions in meristems. Tissues produced from apical
meristems are called primary tissues, and this growth is called
primary growth. Primary growth occurs along the plant axis at the
shoot and root tips. Away from the zone of active cell division, new
cells elongate and differentiate for specific functions resulting in, for
example, parenchyma and epidermis.
Examine a prepared slide of a longitudinal section through an
apical bud of Elodea ( or Anachris). Use low power for an overview
and then increase magnification. Follow the cells down from the apical
meristem and note changes. (Draw what you see.)
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Exercise #3 ROOT ANATOMY
There are slides of roots in cross sections and longitudinal
sections available in lab. Look at each under the microscope and
draw what you see. Make sure you can identify epidermis, cortex,
endodermis, stele (vascular cylinder), pericycle, xylem and phloem.
Where do the branch roots arise?
Exercise #4 STEMS
***At the beginning of lab take a sample of the plant provided as
directed by your TA, and put the stem in Toluidine stain.***
Now, make thin cross sections and longitudinal sections of the
stem through an internode and a node. Place the sections in a drop
of water on a clean glass slide.
a. The dermal tissue system consisting mainly of epidermis and
possibly cuticle.
b. The ground tissue system, background tissue that fills the
spaces between the epidermis and the vascular tissue. The
cortex (between the epidermis and vascular bundles) and the
central pith function as storage and support.
c. The vascular system composed of discrete vascular bundles, a
continuous system of xylem and phloem providing transport and
support.
As always make accurate drawings of what you see. The dicot root you
looked at earlier had the vascular tissue together in the center of the
root. The dicot stem has vascular tissue in discrete bundles arranged
in a ring. There is a transition zone that occurs at ground level. What
do you think the vascular tissue looks like there?
The vascular tissue branches off into the petiole and blade of the
leaf. Try making a section of a p e t i o l e .
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Leaves are the organs especially adapted for photosynthesis. The thin
blade portion provides a very large surface area for the absorption of light
and the uptake of carbon dioxide through the stomata. The leaf is basically a
layer of parenchyma cells (the mesophyll) between two layers of epidermis.
The loose arrangement of parenchyma cells within the leaf allows for a great
surface area for the rapid exchange of gases. In this lab study you will
examine the structure of the leaf in cross section as well as looking at the
surface of leaves and the cells that are contained therein.
There are also fixed slides of Coleus stems available to view and
draw.
Exercise #5 Epidermal Peel of a Leaf
Make an epidermal peel from the lower surface of the leaf material
provided. Place it carefully in a drop of water on a clean microscope slide.
Notice how some cells are very different than others in the epidermis.
These pairs of cells are the guard cells of the stomata (which is some plants
may have between 30,000 and 50,000 guard cells/square inch). Describe
them with both a picture and in your own words. Remember that the opening
or pore between the guard cells is the stomata.
What is the function of the stomata? If possible, make a peel from
a leaf that has been kept in the dark. Any differences? Why? Note any
internal structures in the guard cells. What might their function be? Make
a peel from the upper surface of the leaf. Note the distribution of
stomata. Are there as many stomata in the upper surface as in the lower?
Why or Why not? What is going on here? How do you think water lily
stomata are distributed?
Exercise #6 Leaf Anatomy
Look at cross sections of a lilac (Syringa ) leaf under medium power.
Note the cuticle, epidermis (upper and lower), mesophyll (palisade and
spongy), air spaces, and stomates. How are form and function related in the
leaves that you have examined?
And as always, draw what you see.
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