Download Plant Biology: introduction to the module

Introduction to plants
All life on earth depends on plants. Without plants
ecosystems would soon grind to a halt.
Animals (and fungi) are parasitic on plants.
Taxonomy - definitions
Always start by setting out what you’re talking about!
Plants are widely taken to be the green things that make flowers
Kingdom Plantae – algae, mosses, ferns, conifers, flowering plants
In fact the boundaries are rather unclear, especially at the single-celled
level. Modern taxonomies do no try to shoehorn unicellular eukaryotes
into kingdoms alongside multicellular forms but prefer to handle them as
a distinct group (with 27 phyla at the last count). Within plants, the term
‘alga’ is not a monophyletic group, with red alga very different to brown
and green classes.
Basics
Plants (like animals and fungi) are eukaryotes. Plants are primitively
photosynthetic, relying on organelles called chloroplasts to capture
light energy. (A few plants have lost this ability and are parasitic on
other plants). Most have highly structured bodies, with green
material growing upwards and roots growing down. Their cell walls
are reinforced with tough polymers, notably cellulose and lignin.
All are capable of sexual reproduction, and their classification is
heavily based on studies of their reproductive organs. All exhibit a
phenomenon called alternation of generations, which you may well
be unaware of and which we will look at closely later in the lecture.
In this course
I will focus mainly on the familiar ‘higher’ forms, flowering plants Algae
(and fungi) I will callously exclude, on grounds of time and factual
overload!
We will need a taxonomic structure to hang names on, so I propose to
start by introducing the major taxa you need to be familiar with. I am
sure that these divisions will remain valid – I am not sure that the exact
taxonomic levels will remain unchanged in the next 20 years. Geological
dates are also prone to being extended backwards as new fossils are
found.
“Each new edition of this book requires revision of most of the
phylogenetic trees to reflect the ongoing revolution in
systematics.”! Campbell Reece & Mitchell (1999) Biology (5th Edition)
Addison Wesley
(On p. 584 they show a family tree of photosynthetic eukaryotes which allows
for 4 different definitions of ‘plant’!)
Classification of the plants we will cover in this course.
Bryophytes
(mosses,
liverworts)
Pteridophytes
Ferns and
allies
Gymnosperms:
Conifers,
cycads etc
Angiosperms:
Flowering
plants
Mesozoic
120MYBP
Carboniferous
350MYBP
Devonian
400MYBP
Seeds
Vascular tissues (tracheids or derivatives)
Bryophytes
These are the mosses and liverworts, both relatively common wellknown groups of non-flowering plants typical of permanently damp
areas. (Actually a few specialise in dry open sites – fire sites, bare
concrete etc). The dominant phase is a leafy form (the gametophyte),
which is genetically different to the stalked pods that produce its spores.
A typical moss,
showing the spore
capsule, which is a
genetically different
plant to the green
fronds from which it
grows.
(More later..)
The hepatica: Liverworts. These also make genetically
distinct spore-dispersing individuals, but here the spores are
dispersed from an umbrella-like structure, while the main plant (the
gametophyte) is generally flattened, plate-like.
sporophytes
c. 2 cm
The common liverwort
Marchantia
Pteridophytes: ferns, horsetails,
club mosses and allies.
Pteridophytes are the group of plants which first (as far as we can tell…)
developed the tracheid cells which permit stems to rise high above any
water supply, and as such were the first colonists of dry land, at least 400
MYBP. We have a good fossil record of them (in fact our industry has
depended on burning this fossil record since the inception of the
industrial revolution!). The facets which fossilise show that apart from
the extinction of the giant forms, this group has changed little since the
Devonian.
Like mosses these plants have two genetically distinct phases in their life
cycle, but here the dominant phase is the sporophyte, the familiar fern
leaves etc.
Ferns
These are ancient but still successful forms, in which the spore-bearing
stage is very familiar. Bracken Pteridium aquilinum is one of the most
widespread and pernicious weeds on the planet! We still have tree ferns,
native to Gondwanaland (Australasia, South America, Africa) but now
widely planted in tropical, subtropical and frost-free temperate areas. In
all cases spores are shed from the underside of the leaves (fronds).
Bracken Pteridium aquilinum
A tree fern Dicksonia antarctica
Horsetails: Equisetacea
These plants are every-day miracles. There
are only about 15 species in the world, all
in the genus Equisetum. It has changed
hardly at all since the carboniferous period.
I know of a Carboniferous site in Yorkshire
where one can find 2m high horsetails still
standing, fossilised in a cliff, looking
exactly like living forms (only rather
bigger, though giant horsetail E. telmateia
can grow nearly this tall). Also known as
Lego plants, because the stems comes apart
at the nodes.
Horsetails, contd.
The needle-like leaves are
reinforced with silica, and have
been used as pan scrubs. Few
animals find them palatable.
For all their ancientness and oddity
they are a serious weed, with
immensely deep root systems and
an ability to shrug off herbicides.
Gardeners’ Question Time (BBC)
advice on how to respond to
horsetails in your garden
Sell your house, in winter when
the stems aren’t visible.
Club mosses:
Lycopodiacea
These rather nondescript crawling
plants are nowadays confined to a
minor role in northern forests on
acid soils. Present in the UK but
easily overlooked. The sole
survivors of a large group including
vast forest-forming trees in the
carboniferous, the first terrestrial
forests. The have a vascular
system, and always one vein
running along the leaf axis.
Gymnosperms
This group contains many well-known plants, including all coniferous
trees (pines, larch, spruce etc), yews and allies, along with other ‘living
fossils’ the cycads, plus a few simple plain oddities thrown in to keep
botanists happy.
Gymnosperm means ‘naked seed’, and indeed in this group the
fertilised seed protrudes from the cone/aril. They have apparently lost
the sporophyte generation (but see later), and are now trees which shed
viable seed that germinates to make a new tree – the pattern of seed
germination which we are familiar with. They have tracheids allowing
water to be sucked to great heights: the this group contains probably
the largest (Sequoia) and oldest (Bristlecone pine, Pinus aristata)
organisms in the world.
UK Gymnosperms
We have 3 spp native to the UK: Scots pine Pinus sylvestris, Yew
Taxus baccata and Juniper Juniperus communis.
Do you know where they grow?
Scots pine – native to Scotland but widely naturalised on sandy acid
soils
Yew – dense ancient forests on southern chalk
Juniper – an oddly disjunct distribution, present but under Yew on
chalk and in the acid uplands.
Most people know one group of gymnosperms; the
conifers. Literally the cone bearers – these are
pines, spruces, larches, firs etc.
Cones – correctly stobili (1
strobilus) are sexual organs
either shedding pollen (male
cones) or bearing ovaries,
awaiting fertilisation by windblown pollen (female cones).
In fact all gymnosperms, plus
male pine cone
lycoods, have similar
Female pine cone
structures. You will meet the
terms microsporangia (pollenproducing organs) and
megasporangia (egg producing
organs).
Conifers
Cycads
These plants look rather like stunted palms, or
possibly rather tough tree ferns, but are neither. They
are gymnosperms that have changed little since the
Jurassic period, when they were dominant land cover
and presumably staple food for herbivorous
dinosaurs.
Now they are thinly scattered in tropical areas, some
highly endangered.
Males and females plants are separate, using a winddispersed pollen to fertilise their cones. The male
gamete is notable for using cilia to swim towards the
egg (the ‘highest’ occurrence of cilia in the plant
kingdom).
Some cycads fix atmospheric nitrogen using a
symbiosis with blue-green algae living in their stems
and roots.
Ginkgo biloba – the wonderful discovery
People had since the early days of fossil hunting
been recovering well-preserved fossil leaves from
Ancient (Jurassic and earlier) which looked like an
unrolled pine needle. No living plant matched this
pattern.
Then in 1691 the German Engelbert Kaempfer
discovered strange trees with exactly this unfamiliar
leaf form in Japan, cultivated in temple gardens.
They proved to be living specimens of Ginkgo, one
male and one female. Thankfully their seed was
fertile, and has now been widely propagated. The
oldest in the UK is in Oxford botanic gardens
(pruned and now rather small for its age). Generally
males are planted as the female flower is rather
sticky and smelly. (Sex is coded by an X-Y
chromosome system, as in mammals).
Welwitschia mirabilis
This is certainly one of the strangest plants in the world, whose
classification inside the gymnosperms has long been assumed but is
confirmed by DNA analyses.
It lives only in the Namib desert, South Africa, in a region where rain
never falls. Instead it relies on the mist that condenses in coastal regions
where cold currents from the southern oceans well up against the desert.
Welwitschia has only 2 leaves, long strap-like ones that grow perpetually
from their base while the ends become frayed and tatty. It is dioecious.
Angiosperms: Monocotyledons
and Dicotyledons
Flowering plants (phylum Anthophyta) come in two fundamentally
different ‘designs’ or classes, known as the Monocotyledons and
Dicotyledons. Or Monocots and Dicots in botanical jargon.
Formally these are defined by the number of seed leaves, or
cotyledons, that emerge when the seed 1st germinates. In
Monocotyledons it is 1, in Dicotyledons it is 2. Coinciding with
this are a series of other characteristics which are so consistent that
everyone seemed happy that these are monophyletic groups,
splitting from the gymnosperms about 130 MYBP (early
Cretaceous).
Unifying features of
monocots and dicots
Class
Monocotyledon
Number of cotyledons1
Leaf veination
parallel
Flower parts
in 3s
Roots
a fibrous bundle
Stem
herbaceous
Arrangement of
vascular bundles
scattered
Pollen pores
1
Dicotyledon
2
branching
in 4s/5s
A tap root with laterals
woody
in a ring
3
Monocotyledons
This truly monophyletic group contains all
grasses, sedges, rushes, bamboo etc. Orchids.
Pineapples and allies (the bromeliads). Lilies,
and their succulent relatives Aloes. Few trees
but including bananas and palms.
Dicotyledons.
Actually the
eudicotyledons plus a few others…
It is here that I have to confess to a certain oversimplification. Neat
though the division was, recent (late 1990s) DNA work has shown that
the group known as ‘Dicots’ consists of 4 groups, all as unrelated to each
other as they are to the monocots. Fortunately, virtually all the ones you
are likely to meet are in a good monophyletic group, now called the
Eudicotyledons.
(Sometimes DNA research makes a good simple system needlessly
complicated..)
Eudicotyledons
Here we have most gardens flowers, all
herbs, cacti, climbers, and most trees.
The awkward other 3 groups
For sake of completeness I need to tell you of
the other 3 ex-dicots.
One group will be familiar and has long been
thought of as primitive: the water lilies
Nymphales. All are aquatic with floating leaves:
Nymphaea
Drimys winteri
A second early offshoot is the winterales,
containing Winters bark Drimys and Star Anise
Illicium.
Amborella trichopoda
Until the Deep Green project looked at its DNA, Amborella was an
incredibly obscure and unimportant member of the laurel family only
found in the wild forests of New Caledonia.
It turned out to be closest to the root of all flowering plants, pre-dating
the split between monocots and the others. It has its own class now!
Current angiosperm taxonomy.
(Until they sequence something else really odd…)
Eudicots
Monocots
Winterales
Anthophyta:
Water lilies
Amborella
Alternating generations:
sporophytes and gametophytes
One of the more unusual insights into plant
biology comes from the study of their life
cycle, specifically in a pattern of alternation
that is easily seen among the ‘lower’ forms
but becomes progressively less obvious as
one moves from ferns to cycads to conifers
and flowering plants.
This pattern consists of having two
genetically distinct generations, each giving
rise to the other, and is called the alternation
of generations.
Gametophyte
Sperm or eggs
0 0
Sporophyte
(sheds spores)
Haploid N/diploid 2N?
There is one more feature to include here to understand a generalised
account of the alternation of plant generations.
This concerns the chromosomal composition: does the cell have 1 set of
chromosomes (haploid) or two (diploid)? Where in the cycle does the
crucial genetic shuffling act called meiosis occur?
Gametophyte
Gametophyte
Haploid N + N
Haploid
0 0
N
Meiosis to make haploid spores
Sporophyte 2N
(Diploid)
This alternation is clearly seen in the non-vascular plants (the
non-tracheophyta), mosses and liverworts.
Here we see the dominant (at least the biggest, most visually
conspicuous) part of the life cycle is the sporophyte, which
shed spores from various shaped capsules. The taxonomy of
these plants relies largely on this phase.
These spores germinate to produce a flat green individual, looking
very much like a smooth lichen. This is the gametophyte, and all
external gametophytes tend to look pretty much the same. There are
no field guides to gametophytes, as a fern is hard to tell from a
liverwort or a moss – until sporophytes appear.
This pattern can be seen right up through the plant kingdom. Various
modifications occur in the algae, such as the condition where the
sporophyte and gametophyte look identical (until you examine their
reproductive cells or count their chromosomes). This condition is
called isomorphic alternation of generations.
In mosses and liverworts the gametophyte thallus is the largest and
most metabolically significant phase. These are most easily seen in
liverworts, where the umbrella-like sporophytes sprout from an
expanse of flat green thallus. In these plants the spore is the main
agent of dispersal in time and space.
In these plants the male gametes are motile, swimming with cilia, and
can plausibly be called sperm.
In Pteridophytes the gametophyte generation (haploid) is still a distinct
individual (looking and behaving remarkably like a liverwort
gametophyte), but this is overshadowed, both literally and
metaphorically, by the much larger (diploid) sporophyte generation.
Fern collectors in the UK have collected-out some rare ferns, but their
gametophyte stage hung on overlooked and allowed the population to reemerge.
This pattern appears to halt with the arrival of seed plants –
gymnosperms and angiosperms.In fact we can see these as the extreme
development of the dominance of the 2N sporophyte generation. The
trick is to see the sporophyte as making a 1N gametophyte generation by
meiosis which is retained as an internal individual, an egg or a pollen
grain. The pollen is then dispersed, and on germination it releases a male
gamete which fertilises the egg. And the sperm? In cycads the gamete is
a ciliated ball which swims majestically towards the egg – elsewhere it is
replaced by a simple pollen tube along which the nucleus is propelled.
The alternation of generations –a summary
Taxon
Sporophyte 2N
gametophyte N
Bryophyta
small capsules
dominant leafy phase
Pteridophyta Large leafy stage
small, liverwort-like
Seed plants
tissue within reproductive
organs
Only visible phase