Download INTRODUCTION People have sought to know about the origins of

INTRODUCTION
People have sought to know about the origins of themselves and the things
around them for as long as anyone knows. Seeking an understanding of the
evolutionary origins of living things is a natural outgrowth of that yearning.
In the past two hundred years, since the time of de Jussieu, de Candolle, and
Lamarck, biologists have developed a sophisticated set of principles and data
relevant to understanding the evolutionary origin, or better said the
phylogeny (literally the birth of clans), of the major groups of living things.
The vascular plants, including everything from clubmosses to flowering
plants and including ferns, scale trees, cycads, and conifers, are the most
prominent group of plants, and thus the ones most often considered in
studies of plant evolution. In 1790 Goethe, better known as playwright and
philosopher, published the contention that all of the above-ground structures
of the higher plants are either stem or leaf. This hypothesis was the first and
most fundamental of the modern ideas relevant to our understanding of
vascular plant evolution. The idea incorporates at the outset the closely
related concepts of homology and transformation, which are so basic to any
understanding of the change of organismal form in either developmental or
evolutionary time.
Homology has two similar but distinct meanings. The well-known idea that
the appendages of a lobster - not only claws, walking legs, and swimmerets,
but also antennae, jaws, and eyes - are equivalent is actually an expression of
the idea of homology. The explanation runs that lobster appendages are all
fundamentally the same: in different parts of the body, the basic plan is
altered to suit the needs of the organism. The altering of the plan is a
developmental transformation: it happens during the early differentiation of
the lobster body. This sort of homology has been known since Goethe's time
as serial homology. Goethe applied it to explain the form of plants: he saw
all of the appendages of plants (leaves, bracts, sepals, petals, stamens, and
pistils) as serially homologous with leaves, just transformed during
development into the various more specialized structures.
On the other hand, evolutionary homology is equivalence of structural
features explained by inheritance from a shared evolutionary ancestor. Here
is the home of the idea that bird wing, bat wing, horse foreleg, and human
forearm are all equivalent because the common ancestor of all these animals
had a forelimb from which the various specialized forelimbs were derived by
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evolutionary transformation. Features that appear similar, but are not so
because of inheritance from a common ancestor, are examples of
convergence. The discerning of homology from convergence is the most
basic of all theoretical challenges to students of organismal evolution.
In vascular plant morphology, the basic challenge is to perceive homology in
spite of evolutionary transformations in form. Two things become critical:
to define the criteria by which structural features can be judged to be
homologous (equivalent through common ancestry) and to identify the
typical kinds of transformations that render equivalent structures different in
form. The general criteria for homology (that is deciding that two structural
features are homologous) are three: 1) The structural features are equivalent
in position; hence the petals of all flowers are considered homologous
because they invariably lie above the sepals and outside below the stamens.
2) Structural features intermediate between the two under consideration exist
in other organisms; hence a leaf (megaphyll) is homologous with a system of
branching stems because plants with intermediate structural features have
been found in the fossil record. 3) The structural features share a common
developmental pathway: for example, a crocodile’s leg and a dolphin’s
flipper are known to be homologous because both develop in the same way
from embryonic limb buds.
Homology without transformation does not make a complete concept. There
are two basic things to understand about transformation: what kinds of
transformations are there and what are the evolutionary forces that drive
these transformations. In the most general terms, there are three kinds of
transformations: 1) heteromorphy (change in the form of structural features),
2) heterotopy (change in the position of structural features), and 3)
heterochrony (change in timing of developmental events). The inferences
about evolution developed in this manual usually address heteromorphy
more than the other two kinds of transformations, simply because they are
basic to comparative morphology. There are three basic kinds of
heteromorphy (change in form of structural features): enlargement,
reduction, and fusion. The first two relate to the size of features or parts of
features, the third relates to the evolutionary combination of two or more
features into one composite structure.
Fusion is without a doubt the most important kind of transformation
considered in plant morphology. It has played a major role, so goes the
hypothesis, in the origin of the leaf, the seed, the pine cone, and the fruit.
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Composite structural features derived through evolutionary fusion of
separate parts have special features of their own that are powerful in
deciding questions of homology versus convergence. Three kinds of fusion
have been postulated to explain the transformations and homology of
composite structural features in vascular plants: union (fusion of a feature to
itself, for instance the fusion of two edges of a leaf to yield a carpel);
connation (fusion of similar parts, for instance the fusion of petals to yield a
corolla tube); and adnation (fusion of different parts, as in the fusion of
stamens to corolla in mint flowers).
The rest of this manual is an inquiry into the homology of features among
various plant groups, into the transformations that yielded the diversity of
form evident in these groups, and the evolutionary forces that determined
those transformations.
J. W. von Goethe
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The Laboratory in Plant Biology 108:
There are two basic goals for this lab: 1) to give you direct and extended
contact with the plant groups and their characteristics, and 2) to allow you
time to do some of your own thinking about the evolutionary relationships
among vascular plants. The most important thing to remember is, there is no
one right way to classify vascular plants, or anything for that matter, because
all classification is tied to inferences about the course of past events.
There are both dissecting microscopes and compound microscopes in the
lab. Treat the scopes you use with respect: clean lenses with lens paper; wipe
stages clean with damp paper towels; and bring any scope problems to your
teaching fellow's attention.
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I. An Introduction to the Vascular Plant Body: Organs, Tissues, and
Cells
I. Introduction to the plant body - the major plant organs
A. In contrast to animals, whose organs are many and diverse, plants have
only four basic kinds of organs: stems, leaves, roots, and sporangia. We'll
concentrate on the first three of these today.
1. Examine the flowering plant on the lab bench in front of you and
locate the stems, leaves, and roots. (You may gently unearth the plant
from its pot to observe the roots.) Find a leaf and note the point at which
it is attached to the stem. This juncture of leaf and stem is called a node;
between the nodes are the internodes. In the axil of the leaf, look for the
axillary bud, a structure that has the potential to develop into a new
branch. Describe the stem, leaf, and root of this plant, with respect to
shape and location on the plant body. What is the function of each
organ?
2. Now turn your attention to the potted fern on the bench and try to
locate the three organs mentioned above. Where is the stem?
II. Introduction to prepared slides and use of the microscope
A. A word on plant sections. We'll be using prepared slides throughout the
semester - most are cross (c.s.) or longitudinal (l.s.) sections. Transverse is a
synonym of cross section; they mean exactly the same thing. Longitudinal
sections can be of two types, either radial (the plane of the section is right
through the center of the object) or tangential (the plane of the section does
not pass through the center of the object). The same object can look quite
different when cut in these various planes, as you shall soon see. To affix
these terms in your mind, sketch a log split into each of these three kinds of
sections.
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B. Getting familiar with the Olympus CX41 compound. microscopes used
in the Plant Biology 108 lab. Pick up a slide of the Psilotum rhizome cross
section (a rhizome is an underground stem) and place it on the stage of your
microscope. Examine it first under low power (10X). Note that you can see
nearly the entire cross section under low power. Practice moving the slide
around on the mechanical stage of the scope. Examine the slide under
different light intensities. Does the maximum intensity yield the best image?
Now choose one particular region of the slide to focus on. Adjust first the
coarse (inner knob) then fine (outer knob) focus to learn the range of control
that each provides. When you have your chosen region in perfect focus,
position it in the center of the field of view and go to the next higher power
(25X).
Notice that at the higher power, the region you focused on is still near the
center of the field. It is probably in pretty good focus, too. Do the same
thing again and go to 40X. Note how the area in the field of view decreases
with increasing magnification.
Now go back to low power and choose another small feature on the slide
(say, a cell that has some distinctive feature that catches your eye). Position
that feature close to, but not directly at, the center of the field of view. Go to
higher magnification. Can you still find your chosen feature easily? What
does this tell you about how to use your microscope efficiently?
III. Introduction to tissues and cells of the mature stem
A. First a word on biological stains used in preparation of the slides used in
the botany laboratory: Note that the various cells and tissues are stained one
of two basic colors, red or greenish blue. The red color is provided by
safranin - a stain that binds to biological molecules that are negatively
charged. The most prominent among red stained features in your slide are
the tracheary cells of the xylem, located at the center of the section.
Tracheary cells (and a few other cell types as well) stain red because their
cell walls are impregnated with lignin, a varnish-like substance that provides
structural rigidity. Because lignin is negatively charged, it binds safranin
efficiently. Other cells and tissues in your section are stained greenish blue.
These are stained with fast green, a stain that colors many cell and tissue
types.
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B. Now we'll look at the cells and tissues themselves. The mature plant
body can conveniently be divided into three tissue systems: the dermal
system, which forms the outer covering of the plant body, the vascular
system, which are the transport tissues (xylem and phloem) and their
associates, and the ground system, which comprises the remainder of the
plant body. We take a closer look at these below. Terms that may be new to
you are printed in italics. Some are defined in the text, others you will need
to look up.
Psilotum aerial stem cross section
1. Examine a prepared slide of a Psilotum (whisk-fern) aerial stem, cross
section. Note that the three tissue systems are readily identifiable in
the stem cross section.
a. To the exterior of the stem is the dermal tissue, the epidermis.
Locate the cuticle, the thick waxy layer in contact with the
environment. Because Psilotum has no true leaves, most
photosynthesis is carried out in the tissues of the stem. Hence
stomates, pores permitting gas exchange between the environment
and the plant's interior, are common in the epidermis of the aboveground axes; locate these between the ridges in the cross-section.
b. Internal to the epidermis is a broad cortex, mostly of parenchyma
cells. Note that the outermost layers of cortex are irregular and
spongy: why do you suppose this is so? Also notice that many of
them are filled with purple-staining starch grains, the plant's
carbohydrate reserve. Internal to this region is a zone of
sclerenchyma fibers, red-staining cells that are elongate, have thick
secondary walls, and function to provide structural support. Inside
of this is another zone of parenchyma cells. Can you find the
endodermis at the boundary of the cortex and the stele? (If not,
check a slide of the Psilotum rhizome - the underground axis.)
c. In the center of the stem is the vascular tissue of the stele. A small
rod of sclerenchyma occupies the center of the stele (look for the
thick-walled fibers - they're probably stained a duller red then the
surrounding tracheids); surrounding it is the primary xylem, xylem
that was produced by the apical meristem. You should have no
trouble distinguishing the smaller tracheids of the protoxylem from
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the larger ones of the metaxylem (later maturing xylem tissues) in
this section. What can you say about the distribution of protoxylem
and metaxylem in the Psilotum stem? Primary phloem is located
between the arms of the xylem. Try to pick out the sieve cells: they
are small and angular. Surrounding the xylem and phloem is the
pericycle (it won't be distinguishable in this section), a cylinder of
parenchyma cells that gives rise to secondary roots in many groups
of plants; not in Psilotum, though - this wacko has no roots. No
wonder people have been arguing about it for years! (See your
syllabus.)
Before we leave the Psilotum stem, take a look at how different the
same structures look when seen in longitudinal vs. cross section:
examine a prepared slide (l.s.) of the Psilotum sporangium,
concentrating on the stem just below the sporangium (large, globose
structure) itself. Compare the appearance of the cells and tissues in
long section with those in the cross section just examined.
As you work with prepared slides in the Plant Biology 108 laboratory,
bear in mind that the two-dimensional section you are examining is
but one "view" of a three-dimensional object. One of your challenges
will be to use these different views to reconstruct in your mind's eye a
three-dimensional image of the plant structure under study.
IV. Apical Meristems
One of the most important differences between the ways that plants and
animals grow is that the growth of the animal body is determinate - once it
reaches its mature size, it stops growing. Plant growth, by contrast, is
indeterminate: plants get larger and produce new organs (stems, leaves,
roots, and sporangia) almost indefinitely. How do they do it? At the end of
each branch and root is an active growth center called an apical meristem.
These meristems are regions of cells called initials, which have unlimited
potential to keep dividing. (Can you think of any analogs in the animal
body?) Here we take a close look at the apical meristems of two monocots:
the shoot apical meristem of Elodea (water-weed) and the root apical
meristem of Allium (onion).
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A. Shoot apical meristem of Elodea
1. Study a prepared slide of an Elodea stem tip. (Examine it under lower
magnification to get the "big picture", then switch to higher
magnification for the observation of fine details.) Most prominent will
be long, thin leaves surrounding the shoot apex. In the center of these
young leaves you will find the apical meristem flanked by leaf primordia.
You can pick out the meristem itself because the cells there have several
characteristic features:
a. they are quite small, and more or less cuboidal
b. the nucleus is proportionately larger than it is in cells outside the
meristem
c. the cytoplasm is quite dense, appearing granular and dark
In the axils of the leaves you will find the axillary buds. These are a
characteristic feature of angiosperms - we don't find them in the lower
vascular plants. Note that each axillary bud is like a miniature replica of
the shoot apical meristem, differing from it in position but not in
structure. Each axillary bud may eventually give rise to a new stem (a
branch), which will in turn have meristematic axillary buds in the axils of
its leaves. You see, plants are very much modular organisms; this is one
of the important ways that they differ from animals.
B. Root apical meristem of Zea mays
1. Now examine a prepared slide of the Zea mays root tip. You should
observe several discrete regions or zones on the slide. Under low power,
focus on the very tip of the root. Identify the root cap, a group of loosely
fitting cells covering the root apex. The cells of the root cap, though
living, are worn away as the root forces itself through the soil. Just
behind the root cap, and protected by it, are the cells of the root apical
meristem. Note that these cells, like those of the shoot apical meristem
you just examined, are small and have dense cytoplasm. Remember that
this is a region of active cell division: look for dividing (mitotic) cells
with dark-staining chromosomes in a star-like array.
A little further down, away from the region of cell division, is a region in
which the cells have stopped dividing. Some of them are noticeably
elongate. This is the region of cell elongation; cells in this vicinity have
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stopped dividing but they are still in the process of differentiating.
Perhaps you can pick out three discrete areas of differentiating cells;
these correspond to the three tissue systems discussed previously:
1 - the protoderm - the outermost layer of cells is the protoderm, those
cells that will differentiate into the epidermis; these show pronounced
elongation.
2 - the procambium - at the center of the section are the notably elongate
cells of the procambium, those cells that are differentiating into the
tissues of the vascular system (xylem and phloem). Those tracheary
elements that complete their development first (the protoxylem) start to
develop lignified secondary thickenings while they are still in the
region of cell elongation; hence they have helical or annular secondary
thickenings so they can continue to elongate. Because these slides
were not stained with safranin, the secondary thickenings are difficult
to see; look carefully. If you still haven't seen good helical secondary
thickenings, take a look at the Lycopodium (clubmoss) strobilus long
sections we've set out. Ask your T.A. for help if you can't find them.
3 - the ground meristem - between the protoderm and the procambium
are the cells of the ground meristem, those that are differentiating into
the ground tissues, cortex and pith, if present. (We seldom find pith in
roots, however there is some in the roots of onion and other monocots.)
Don't be misled by the term "ground meristem"; here in the region of
cell elongation, cells have stopped dividing and hence are not truly
meristematic. They are cells whose future course of development is at
least partly determined.
Shoot apices also have protoderm, procambium and ground meristem,
however tissues at the shoot apex don't differentiate in regular zones - as
they do near the root apex - and so are more difficult to see.
Finally, examine the cells and tissues in the vicinity of the root hairs.
This is the region of maturation. Cells in this region have acquired their
mature features, including the root hairs just mentioned (these are
features of young but fully differentiated cells of the epidermis). Cells
are no longer elongating in this region; hence tracheary elements that
complete their differentiation here (the metaxylem) have nearly
continuous secondary walls. Again, you probably won't find them in
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these root tip slides; if you haven't seen good tracheary elements with
nearly continuous secondary walls (they're called scalariform-pitted
tracheids), the Lycopodium strobilus slide is a good place to find them.
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