Download 1 Goals – Experience plant diversity, learn about important

IB 201 LABORATORY EXERCISE 10
PLANT EVOLUTION AND DIVERSITY
Goals – Experience plant diversity, learn about important evolutionary innovations in
plant history, understand how the modular body form of most plants has led to a different
evolutionary pattern than is seen in animals (lack of body plans). Be able to identify
examples of adaptive radiation and convergence using a phylogenetic framework..
This lab will introduce you to major groups of plants other than algae. You will use the
information on plant taxa and phylogeny to trace the evolution of plant structures. The
lab will take place at the Plant Biology Greenhouse. Directions to the Greenhouse are
located on the previous page. The public parts of the conservatory consist of the
conservatory (a large, landscaped area with a circular path) and a series of rooms off of
the hallway running out of the conservatory. Each room contains plants with a particular
theme (desert plants, aquatic plants, etc.).
PART I: PLANT EVOLUTIONARY HISTORY AND THE MAJOR PLANT TAXA.
Origin of Plants – The plants are among the most diverse and certainly the most
conspicuous of terrestrial organisms. What we think of as plants are more technically
known as embryophytes. The embryophytes are a monophyletic group that fall within
the green algae. Algae is a general name for organisms capable of photosynthesis (other
than the plants). Most algae are simple organisms composed of a single cell or a simple
colony of cells. The green algae are a monophyletic group (if the embryophytes are
included), united by a number of cellular and biochemical characteristics, most notably
the presence of the photosynthetic pigment, chlorophyll b. There are several other groups
of algae with different photosynthetic pigments. Some of the these are only distantly
related to the green plants (green algae and embryotphytes).
Green algae green algae embryophytes green algae
green algae
The phylogeny above shows the relationship to the green algae in a schematic fashion.
The important point is that the green algae and the embryophytes form a monophyletic
group but that the green algae are paraphyletic without the embryophytes.
Embryophytes are distinguished by the presence of embryos, offspring in early stages of
development that grow within one parent. This was a crucial adaptation that allowed
plants to live in terrestrial environments.
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Important Characteristics of Embryophytes
Before continuing with the lab you should briefly review the terms that follow. Material
explaining these terms in more detail will be available, along with illustrations, on the
course web site shortly before spring break. Make sure you are familiar with alternation
of generations and what is meant by a sporophyte and a gametophyte. You should also
familiarize yourself with these parts of a plant: antheridia, archegonia, sporangia, strobili,
microphyll, macrophyll, petal, sepal, anther, stigma, style, and ovary.
Reproduction. All embryophytes have two different multicellular stages in their life
cycle. The haploid stage is known as the gametophyte while the diploid stage is known
the sporophyte. A plant life cycle is shown schematically in the following diagram
Spore
meiosis
mitosis
Sporangia
Sporophyte
Gametophyte
Archegonia
Antheridia
Egg
Sperm
mitosis
Zygote
The stages in bold are diploid, the stage that are underlined are haploid. A considerable
amount of detail has been removed. The archegonia and antheridia are gamete producing
structures. They are roughly equivalent to the ovaries and testes of animals, except,
being already haploid, they produce gametes through mitosis rather than meiosis. The
sporangia are structures that produce spores. In some plants all spores produce identical
gametophytes that contain both antheridia and archegonia. In other plants there are two
kinds of sporangia that produce two kinds of spores. Microspores produce
microgametophytes that have only antheridia (i.e. they are male). Megaspores produce
megagametophytes that only have archegonia (i.e. they are female). All seed plants have
both micro and megagametophytes.
The Plant Body. A plant is composed of two major sections: the root and the shoot. The
shoot can be further divided into stems and leaves. Movement of water and nutrients
between different parts of the plant is accomplished through the vascular system, which is
composed of xylem and phloem cells.
Plants can take on a variety of growth forms. Herbaceous plants have no wood
(secondary growth) and are generally fairly small. Vines have elongated stems that use
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other plants for support. Shrubs have multiple woody stems and grow to a moderate size.
Trees have a single woody stem and grow to a large size. Succulents have leaves, stems,
and/or roots that have been modified to store water and nutrients.
In the following sections we will discuss the major groups of embryophytes. Sections
that are preceded by ** are not monophyletic groups but are used to present general
characteristics. Monophyletic taxa are indicated by an alphanumeric code (e.g. A1, B3)
**NONVASCULAR PLANTS
Three different taxa of plants lack true vascular tissue (xylem and phloem) that transports
food and nutrients within plant bodies. They lack roots and do not absorb nutrients or
water from the soil. These taxa do not form a monophyletic group. The most familiar
group of non-vascular plants is the mosses. Mosses have leaf like structures and cells
that are capable of limited transportation of water and nutrients (these cells are not the
same as the phloem and xylem of vascular plants). The other major group of nonvascular plants is the liverworts. Some liverworts resemble mosses but the most
commonly observed species have a flattened green body called a thallus. All nonvascular
plants are small, low-growing organisms. All non-vascular plants have a dominant
gametophyte. In other words the plant is haploid and the sporophyte is a small structure
attached to the gametophyte. In mosses the sporophyte is a stalk that projects upwards
from the top of a ‘stem’.
A1 - Bryophyta – Mosses
B1 – Hepatophyta – Liverworts (Marchantia is one example in the greenhouse)
C1 – Anthocerophyta – Hornworts (no examples in greenhouse)
**TRACHEOPHYTA OTHER THAN ANGIOSPERMS
All of the rest of the plants have vascular systems and are known collectively as the
Tracheophyta. The great majority of the Tracheophyta are in a single group, the
angiosperms or flowering plants. In this section we will consider all the other groups.
D1 - Lycopodiophytes – These plants mostly resemble large mosses (common names
include club moss and spike moss). The Lycopodiophytes have a true vascular system
and their reproductive system is similar to the ferns (see next section). The dominant
form of the plant is the sporophyte. They are widespread and often common plants
(particularly in the moist tropics) but fairly inconspicuous. The most obvious difference
between the Lycopodiophytes and most ferns are that the club and spike mosses have
microphyllous leaves that only have a single vein. Representatives of both Lycopodium
and Selaginella are in the greenhouse.
E1 - Pteridiophytes and other seedless tracheophyta – The Pteridiophytes are the ferns.
They are vascular plants that reproduce without seeds. Closely related to the ferns are the
horsetails (Spenophyta, genus Equisetum) and Psilotum, which is probably an unusual
fern but has historically been classified in its own group. All the seedless vascular plants
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have similar reproductive systems. The sporophyte is dominant (ferns are diploid unlike
mosses). Spores are released into the environment and develop into small inconspicuous
gametophytes that produce eggs and/or sperm. Sperm has to swim into archegonia to
fertilize eggs making the plants dependent on water for sexual reproduction (non-vascular
plants also need water for sperm to travel from antheridia to archegonia. There are many
ferns in the greenhouse – most are in the fern room.
The remaining groups are all seed plants. Unlike the plants described above, the
gametophytes are not free living organisms. The megagametophytes are encased in
ovules (part of the maternal plant). The microgametophytes are tiny structures consisting
of only 2-3 cells known as pollen. The embryonic sporophyte develops inside the ovule
which becomes a seed after fertilization.
F1 - Cycads – These plants resemble palm trees but reproduce using cones (different
cones produce pollen and ovules). They are very slow growing plants that are restricted
to the tropics and are generally not very common or widespread. Cycads were the
dominant group of plants during the Mesozoic (the age of dinosaurs). They differ from
all other seed plants except Gingko in having sperm that can swim. Cycads also differ
from all other woody plants in having a single unbranched stem. They have extremely
slow rates of growth and very soft wood. Cycas, Ceratiozamia, Zamia, and Dioon are
examples in the greenhouse.
G1 - Conifers and H1 Gingko. These two groups of plants are closely related although
in many ways they seem quite different. Unlike cycads their growth form is very similar
to that of trees in the Angiosperm group. Gingko was a common plant in the Mesozoic
but only a single species exists today and it is extinct in nature (it is a common urban tree
in cool climates). There are no specimens of Gingko in the greenhouse but it can be seen
on campus – look for it later in the semester when it has leaves. Each leaf is fan shaped
with un-branching veins that radiate out from the base. Like cycads Gingko has separate
male and female trees. Male trees are much more commonly grown than female trees
because the fleshy covering of the ovule has an unpleasant smell.
The conifers are the conspicuous non-flowering plants. They include many common
garden trees and shrubs as well as abundant trees of northern forests. Any number of
species can be seen on or near campus (Spruce, Fir, Pine, Juniper, Yew, etc.). Most
conifers have leaves that are modified into needles or flattened scale-like structures.
Conifers do not have mobile sperm and reproduce using sperm nuclei like flowering
plants. Both pollen and ovules are produced in cones. Araucaria (Norfok Island Pine)
and Podocarpus (a broad-leaved conifer) are both in the greenhouse.
J1 - Gnetophytes. This is a small but bizarre group containing three very different kinds
of plants: Welwitschia, Ephedra, and Gnetum. All three of these can be seen in the
greenhouse (in the desert room, the economic plant room, and the tropical shrub room
respectively). The Gnetophyta are very closely related to the angiosperms. They share
some similarities in the structure of their vascular tissue as well as details of fertilization
with the Angiosperms.
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K1- ANGIOSPERMS
The angiosperms are by far the dominant group of plants on earth with over 90% of the
species (most of the rest of the species are either ferns or mosses). They include all the
plants not mentioned above. The defining characteristic of the flowering plants is that the
ovules are encased in an ovary (part of the flower). This ovary becomes the fruit when
the ovule becomes the seed.
Major Groups of Angiosperms
The flowering plants were traditionally divided into 2 major groups: the dicots and the
monocots. The characteristic that distinguished the two groups is the number of
cotyledons (the embryonic leaves that are seen in seedling plants): monocots having one
cotyledon and dicots having two. A number of other characters (floral structures, leaf
venation, distribution of vascular bundles in stems and roots) also tended to separate the
angiosperms into these two groups. However more detailed phylogenetic analyses have
consistently found that the dicots are not a monophyletic group and that the dicot
condition evolved prior to the origin of the angiosperms (all of the other seed plant
groups with the exception of conifers are composed of plants with two cotyledons).
These analyses have consistently supported two major monophyletic groups of
angiosperms: the monocots and the eudicots (tricolpates). There are a number of other
smaller groups of flowering plants whose relationship to each other and the monocots and
eudicots is uncertain. These plants are typically divided into two informal groups: the
‘paleoherbs’ and the ‘magnoliids’.
K2 - Monocots
The monocots are the most well defined group of angiosperms. In addition to the single
cotyledon.the monocots typically have parallel veined leaves (secondarily lost in many
species with large leaves), and have vascular bundles that are scattered throughout the
stem. The root system is adventitious (not developing from embryonic root tissue but
growing from the stem). Monocots lack the ability to form true wood (secondary growth)
and are typically herbaceous with floral parts in sets of three. Grasses, palms, orchids,
lilies, and the aroids (many common house plants including Philodendron) are all
examples of monocots. The monocots contain close to 100,000 species.
K3 - Eudicots (Tricolpates)
The eudicots are the largest single group of plants in the world. Evidence of the
monophyly of this group largely comes from molecular data. The only morphological
character uniting the group is the structure of the pollen grains. The eudicots have many
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typical dicot characters (e.g. two cotyledons, stem vascular bundles in a ring, branching
veins in leaves) but these are presumed to be basal angiosperm characters.
There are well over 100,000 species of eudicots. They range from tiny herbs to large
trees and virtually every other type of growth form. Any angiosperm with a woody stem
or highly branched leaves is likely to be a eudicot.
**‘Paleoherbs’
The paleoherbs are a group of dicots that share some monocot characteristics.
Relationships among the four main groups are uncertain and it is not known if they form
a monophyletic group or if they are several groups closely related to the monocots. The
paleoherbs are similar to monocots in being mostly herbaceous. They are also primarily
found in aquatic and moist forest habitats, similar to plants in the earliest branches of the
monocot phylogeny. Waterlilies are the most familiar example of a ‘paleoherb’
**‘Magnoliids’
The magnoliids are several groups of trees and shrubs that have what appear to be
ancestral characteristics for the angiosperms. Most notable are the large number of floral
parts with little fusion of those parts. It is uncertain whether they are a monophyletic
sister taxon to the eudicots, a paraphyletic group that is basal to the eudicots or a
paraphyletic group that is basal to the rest of the angiosperms.
Exercise 1: Scavenger Hunt to Trace Plant Evolution. Using the material above as well
as the phylogeny and list of taxa in the appendix do the following for the following list of
plant taxa.
Bryophyta (mosses) – find 1 specimen
Hepatophyta (liverworts) – find 1 specimen
Anthocerophyta (hornworts) – none in greenhouse
Lycopodiophytes – find 1 specimen
Pteridiophytes – find 2 specimens from different families
Psilotum – find one specimen
Sphenophyta – find 1 specimen
Cycadophyta– find 1 specimen
Gingko – none in greenhouse
Coniferophyta - – find 1 specimen (in greenhouse or outside)
Gnetophyta – find all three taxa
Monocots – find three specimens from different families
Eudicots – find three specimens from different families
Find an example of either a paleoherb or a magnoliid.
A. Find the specimens as listed above and create a table listing them with a brief
description of each in your lab notebook. Include all the taxonomic information on the
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label plus whatever else you need to oomplete the rest of the exercise. Have your species
checked by your TA.
B. Go to the Tree of Life web page for embryophytes
http://tolweb.org/tree?group=Embryophytes&contgroup=Green_plants
And to the Tree of Life Web page for seed plants
http://tolweb.org/tree?group=Spermatopsida&contgroup=Embryophytes
Use the phylogenies on these pages to produce a phylogeny for your non-angiosperm
plants from exercise A. Based on your observations mark down the points of any major
evolutionary transitions (vascular tissue, wood, seeds, etc.).
C. Based on your observations and out of class reading write a one paragraph description
for each of two plant families (one non-angiosperm, one angiosperm). Include
information on reproduction, growth form, and basic ecology.
PART II: CONVERGENCE AND ADAPTIVE RADIATION.
In comparison with animals plants are both more and less variable in structure. Plants of
the same species show much more variation in form than animals. The size and number
of stems, branches, leaves, etc. are highly variable in plants. The number of limbs and
other organs is generally much less variable in animals. In contrast the diversity animal
form is high across different taxa. An arthropod is clearly distinguishable from a
vertebrate and an amphibian is clearly distinguishable from a mammal based on overall
body plan as well as numerous more subtle details. In contrast plants that appear similar
may be completely unrelated. The evolution of similar form in evolutionary distant
organisms is known as convergence.
Convergence is known in animals, for example in the evolution of similar body shape and
digging limbs in three different groups of mammals: moles (family Talpidae, order
Insectivora), golden mole (family Chrysochloridae, order Insectivora), and marsupial
moles (family Notoryctidae, order Marsupialia). Convergence is very common in plants.
Growth forms such as vines, succulents, and epiphytes have evolved repeatedly.
The inverse of convergence is adaptive radiation. The Galapagos finches used in the
computer simulation in lab 8 are an example of adaptive radiation. So are the many
different forms of Drosophila that have evolved on the Hawaiian islands and elsewhere.
Many different forms evolve from a single common ancestor. This has occurred
commonly in plants.
Exercise 2. While touring the greenhouse identify two examples of convergence
(members of different families with the same body) form and one example of adaptive
(one family with at least three members showing different growth forms). Show these
examples to your TA before leaving. Do some research and write a paragraph on each of
your three examples describing the pattern of evolution in each case.
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