LICHENS
LICHENS ARE ORGANISMS FORMED BY SYMBIOTIC ASSOCIATION BETWEEN FUNGI AND ALGAE. BOTH ALGAE AND FUNGI
BENEFIT FROM THE ASSOCIATION. THE FUNGUS ALWAYS ENVELOPES THE ALGAL PARTNER. THE FUNGUS IS DEPENDENT TO
THE ALGA FOR FOOD,WHEREAS ALGA RECEIVES WATER,MINERALS AND PROTECTION (AGAINST DESICCATION) FROM THE
FUNGUS.
LICHENS ARE WIDELY DISTRIBUTED AND GROW IN VARIED HABITATS,I.E. ON LEAVES,THE BARK OF TREES UNDER SHADE, ON
DECAYING WOOD, ON ROCKS, AND ON THE SOIL. THEY SURVIVE IN EXTREME ENVIRONMENTALCONDITIONS ,SUCH AS HOT
DESERTS, ON DRY EXPOSED ROCKS AND ALSO ON SNOW COVERED MOUNTAIN TOPS. IN INDIA ,LICHENS ARE VERY COMMON
IN THE EASTERN HIMALAYAS.
MOST LICHENS PRODUCE ‘LICHEN ACIDS’ GIVING THE THALLUS A DISTINCTIVE COLORATION, GREY –GREEN OR BLUE.
LICHENS EXIBIT BOTH ASEXUal AND SEXUAL REPRODUCTION.
LICHENS ACT AS INDICATOR OF AIR POLLUTION AS THEY ARE SENSITIVE TO INDUSTRIAL POLLUTION.
PARMELIA,USNEA AND CLADONIA ARE GOOD EXAMPLES OF LICHENS.
The Fungus Kingdom: Lichens
(Last modified: 15 July 1997)
If the other fungi are nature's recyclers, lichens are nature's pioneers. Lichens find their homes in some
of the most barren and inhospitable parts of the world. From there they slowly begin the process of
creating a foundation for habitation by others.
Lichens are among the most fascinating organisms on this planet. Their very structure is unique: a
symbioses of two organisms -- a fungus and algae -- so complete that they behave and look like an
entirely new being. A lichen can literally eat stones, survive severe cold, and remain dormant for long
periods without harm.
Lichens rank among the least well known forms of life. Common names, when available, typically apply
to the entire genus rather than to indvidual species. Classification of lichens is undergoing change as
well. In fact, Mycologists now suggest eliminating the Lichens as a Phylum and, instead, reclassifying
each invidual lichen according to its fungal component -- mostly Sac Fungi (Ascomycota). Never-the-less,
lichens look so different from other fungi that they deserve separate treatment here.
Lichens can be divided into three basic forms: crustose, or crust-like; foliose or leaf-like; andfruticoseor
stalked.
All lichens are believed to be edible (or at least not poisonous) except for Wolf Moss, shown below.
Rock-tripe Lichens (not shown) are even considered a delicacy by the Japanese.
Crustose Lichens
Crustose lichens are flaky or crust-like. They can be found covering rocks, soil,
bark, etc. -- often forming brilliantly colored streaks.
The yellow ones pictured here on a granite stone in the Rocky mountains, are
probably Common Yolk Lichens (Acarospora spp.); the red,Caloplaca spp.; and
green, Lecanora spp.. The little buttons to the left are a magnification of the red streaks above.
Foliose Lichens
Foliose (leaf-like)
lichens can be papery
thin or, in more
advanced forms, netted
branch-like. Branched
foliose lichens have a
distinct top and bottom
surface, thus
differentiating them from
most fruticose lichens.
This can be seen clearly
in the Pseudocyphellaria anthraspisphoto, above left;
the Hypogymnia imshaugii on the right has a puffed body with a
black undersurface.
The dainty buttons on the Pseudocyphellaria Lichen are quite showy. If you look carefully of
the Hypogymnia you will see that it too is fruiting with less conspicuous green funnel-shaped
mushrooms.
Fruticose Lichens
Fruticose lichens are the most highly developed lichens. Their
branches are much closer in form to "true" branches although,
unlike most plants, the lichen branch has no specialized
vascular system for transporting fluids.
The British Soldier Lichen (Cladonia macilenta), left, is one
of the showiest fruiting lichens: even though the mushrooms
are quite tiny, their bright color and distinctive form makes
them stand out
in their forest
habitat.
Old Man's Beard (Usnea spp., right and below) is a common green-grey lichen seen hanging
from trees. A beginner may call all such lichens "Old Man's Beard" but, in fact, several lookalikes do exists (see below).
The fruiting Old Man's Beard (Usnea arizonica) below, right is another typical Usnea with a
hairy appearance. If you look carefully at the lower right portion of the photo you will notice the
wiry white inner pith of a broken branch. This is an identifying characteristic of all Usneaspecies:
the inner cord becomes exposed when you grasp the ends of a branch and pull apart until it
breaks.
Two similar-looking lichens are the yellow-orange Wolf Moss lichen (Letharia vulpina, below,
left) -- named, oddly, because it was used to poison wolves -- and its bright green cousin Letharia
columbiana (below, right) which graces the trees and floor of Montane conifer forests here in
Northern
California.
Lace Lichens (Ramalina menziesii, not pictured), can be seen hanging in thick curtains covering
entire Oak trees with their flat lace-like growth.
BRYOPHYTA
BRYOPHYTA ----- AMPHIBIANS OF THE PLANT KINGDOM.---------------BODY IS
DIFFERENTIATED TO FORM STEM AND LEAF-LIKE STRUCTURES.
MAINLY THREE TYPES OF BRYOPHYTA----------(i)LIVERWORTS------RICCIA, MARCHANTIA,
PELLIA (ii)HORNWORTS--ANTHOCEROS (iii) MOSSES----SPHAGNUM (PEAT MOSS) ,
FUNARIA(TREE MOSS)
OTHER COMMON BRYOPHYTA -- RICCIA, MARCHANTIA, FUNARIA ETC.
BRYOPHYTA
The Bryophyta or mosses, unlike the liverworts, are present in most terrestrial habitats (even
deserts) and may sometimes be the dominant plant life.
As with the liverworts the plant that we commonly see is the gametophyte. It shows the
beginnings of differentiation of stem and leaves - but no roots. Mosses may have rhizoids and
these may be multicellular but they do little more than hold the plant down.
The stem shows some internal differentiation into hydroids and leptoids which are like xylem
and phloem of higher plants but very simply organized with no connection to leaves or
branching stems.
The leaves are mostly one cell thick; sometimes the midrib is several cells thick but this does
not contain conducting tissue so it is not equivalent to the vein of a leaf.
Male and female gametophytes look identical except when they produce reproductive
structures.
The female produces
The male plant produces clusters of antheridia which contain
archegonia, each containing a
thousands of ciliate sperm.
single egg.
Fertilization is dependent on water - sperm are splashed or swim to the archegonia. The
zygote grows into the diploid sporophyte which remains attached to the female gametophyte
It is a leafless stem with a seta or foot at one end, drawing nutrients from the gametophyte. At
the other end is a capsule in which meiosis occurs to form spores.
The archegonium grows around the developing sporophyte for a while but becomes separated
from the gametophyte and is carried up to form a cap or calyptra over the sporangium.
Curiously, the sporangia of some mosses have stomata much like those on the leaves of
vascular plants.
Immature moss capsules with calyptra
The calyptra is lost when the sporangium is
mature as is the operculum or lid on the end of
the capsule.
Underneath the operculum there are often peristome teeth which open under dry conditions and
control spore release A spore germinates to produce a filamentous protonema which sooner or
later produces buds that grow into new gametophytes.
Ecology of mosses
Mosses require abundant water for growth and reproduction. They can tolerate dry spells by
drying out or,in the case of mosses like Sphagnum, by holding huge amounts of water in dead
cells in the leaves.
They look pretty lowly and insignificant, but
have become dominant in particular habitats
andSphagnum itself is said to occupy 1% of the
earth's surface (half the area of the USA).
Because of its ability to soak up blood and its
relative freedom from bacterial
contamination Sphagnumwas used in dressings.
The moss itself is used in some horticultural
media and it is an important source of peat.
This is a sign that I saw at a rest stop on I-90 in Polytrichum commune one of the larger
Wisconsin.
mosses with mature sporophytes
If you have tried to grow a lawn in a shady location you have probably been troubled by
mosses as weeds. Like many lower organisms they are very sensitive to copper salts and can be
controlled in this way. On the other hand mosses are green and better adapted to shade than
most grasses, so maybe we should accept them in this situation.
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PTERIDOPHYTA
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PTERIDOPHYTA INTERMEDIATE BETWEEN BRYOPHYTA AND HIGHER SEED – BEARING PLANTS.---------BODY IS
DIFFERENTIATED IN TO ROOTS , STEM ,AND LEAVES.
EXAMPLE------MARSILEA,FERNS,HORSE –TAILS,CLUBMOSSES ETC.
FIRST PLANT HAVING VASCULAR TISSUES.
PTERIDOPHYTAS ARE SEEDLESS PLANT.
MAJOR GROUP OF PERIDOPHYTA ARE -------(i)CLUBMOSSES------e.g. LYCOPODIUM, SELAGINELLA (ii)HORSETAILS---e.Gequisetum (III)FERNS----e.g.SALVINIA,AZOLLA,DRYOPTERIES,PTERIS
CRYPTOGAME (CRYPTO---------HIDDEN ,GAMOUS------MARRIAGE)
: THE REPRODUCTIVE ORGANS IN THALLOPHYTA, ,THE
BRYOPHYTA, AND THE PTERIDOPHYTA ARE HIDDEN AND ARE VERY CONSPICUOUS.
EXTERNAL FLOWERS AND SEEDS ARE ABSENT AND THEY HAVE NAKED EMBRYOS CALLED SPORES.
PHANEROGAME(PHANEROS---------VISIBLE)
KINGDOM- PHANEROGAME ( PLANT WITH SEED AND FLOWER)
DIVISION------GYMNOSPERMS (SEEDS NAKED) AND
ANGIOSPERMS (SEEDS INSIDE FRUITS)
CLASS OF ANGIOSPERMS-----------DI- COTYLEDONS (SEEDS WITH 2 COTYLEDONS) AND MONO –COTYLEDONS (SEEDS
WITH SINGLE COTYLEDONS)
COTYLEDONS-----------PLANT EMBRYOS IN SEEDS HAVE STRUCTURES CALLED COTYLEDONS. COTYLEDONS ARE CALLED ‘SEEDLEAVES’.
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GYMNOSPERMS
Seeds
For the lower vascular plants the important evolutionary development was in the water and
food conducting tissues of the sporophyte. As we move on through the plant kingdom the
next important development was the seed. The free living gametophyte is a vulnerable phase
of the life cycle. Reproduction by seeds is a less chancy procedure and has other advantages
for plant survival and dispersal. Seeds can be remarkably tolerant of environmental extremes
heat, cold and drought. Unlike free-living gametophytes seeds can postpone their
development until conditions are right. And, of course, we find them very convenient for
plant propagation.
Already in the coal-measure forests there were plants that reproduced by seeds. Some were
the so-called "seed ferns". none of which survive. Others were the ancestors of the plants we
now know collectively as "gymnosperms". In these plants the seeds are not enclosed in an
ovary, as in the flowering plants; they grow on the surface of a modified leaf in a strobilus or
cone. "Gymnosperm" means naked seed.
Alternation of generations is still involved in the reproduction of these plants. They are all
heterosporous: the microspores are shed as pollen, whereas the megaspore germinates in the
strobilus to produce the female gametophyte. The archegonia in this gametophyte get
fertilized by sperm from the male gametophyte and the zygote grows to produce an embryo
which is enclosed in a seed coat of tissue from the parent plant.
Gymnosperms were the dominant land plants in the age of dinosaurs, the Cretaceous
and Jurassicperiods. The surviving gymnosperms in the Coniferophyta, Cycadophyta and
Ginkgophyta are similar in their woody habit and pattern of seed development but are not
closely related.
Coniferophyta
Conifer leaves are needle or scale-like.
They result from the downsizing of true
megaphylls and unlike the microphylls of
lower plants they are connected to the
vascular system of the stem. Conifers are
often large and can dominate the plant life
in some ecosystems because their stems
continue to expand in width as well as
length throughout the life of the plant. The
older parts of the stem become woody,
which provides a further distinction from
the seedless vascular plants of which there
are no surviving woody representatives.
The wood of conifers is more primitive than that in angiosperm trees. It contains tracheids
but no vessel elements, and there is generally less ray parenchyma in conifer wood than in
dicot wood.
Conifer life cycle
Microspores and megaspores are formed on sporophylls in male and female cones
respectively. Each scale in the male cone has two sporangia in which meiosis occurs to
produce tetrads of spores, just as in a fern sporangium. Male gametophyte development
starts in the microspore (or pollen grain) before it is shed. Mitotic divisions result in two
prothallial cells, a tube cell and a generative cell. The sporangium breaks open to shed the
immature gametophytes which are carried on the wind and may chance to arrive at a
sporophyll on a female cone.
Old male cones of Pinus nigra and (right)
L.S. of male cone
In the female cone each scale
bears two megasporangia ovules in which a single mother
cell undergoes meiosis to
produce four megaspores.
One megaspore develops into
the female gametophyte which
contains thousands of cells and
is considerably larger than the
male gametophyte.
Male gametophyte development
has to wait up to a year for the
female gametophyte to mature
and produce two or three
archegonia with egg cells.
The pollen tube grows slowly through a pore in the integument of the megasporangium and
eventually the generative cell divides to produce two sperm cells. One of these fertilizes an
egg cell to produce a zygote. Usually only one archegonium will produce a zygote in each
megagametophyte so that there is only one embryo per seed.
The mature seed consists of three generations of tissues: maternal sporophyte tissue (seed coat
and nucellus), gametophyte and daughter sporophyte (embryo) - After about two years the
mature seeds are shed. Conifer seedlings have several needle-like cotyledons in a whorl and
the seedlings produce scattered leaves until adult foliage develops.
Because seed development takes such a long time it is often possible to find three years' cones
on pine trees
Unlike the seedless vascular plants, conifers are more prevalent in cooler regions and in xeric
habitats. Because of their leaf and stem anatomy they are better adapted to drought than most
broadleaved trees. In addition many conifers have evolved cold hardiness so that above ground
structures can persist even in harsh environments. The bristlecone pines that grow on
mountain ridges in California are an extreme example.
Of course conifers are also important economically as a tree crop for pulp and timber. Their
ability to grow in areas that are unsuitable for other crop production is an asset for this
purpose. Similarly, since most (though not all) conifers are evergreen they are valued as
landscape plants, particularly in areas like Ohio where few broadleaved evergreens can
withstand the winter. The evergreen habit does have its disadvantages since premature leaf
death caused by pollution, disease or insect attack can be more damaging than in plants which
produce a complete new flush of leaves each spring.
Cycadophyta
Cycads or similar plants were the food of herbivorous dinosaurs and the fate of both of these
groups of organisms was probably closely linked. They survive as a few species of tropical
palm-like trees, including one which is native to the USA, Zamia pumila the cardboard palm.
This is found on sandy soils in Florida and is sometimes grown as a foliage
plant. Cycas species are larger and are often used as ornamentals in tropical areas. The cycads
can be viewed as beneficial as they form symbiotic associations with nitrogen fixing bacteria,
but they have also been the subject of extermination programs since they are highly toxic to
livestock. Their life cycle is rather similar to the conifers' but they have free-swimming sperm
(a primitive feature) and sometimes they are pollinated by insects (an advanced feature)
Cycas revoluta with male cones
Zamia pumila with female cone
Ginkgophyta
This is a monotypic division, a single species of a single genus, Ginkgo biloba the maidenhair
tree. Several relatives are known as fossils dating back to Pennsylvanian times. Ginkgo
biloba was preserved in the gardens of Buddhist monasteries in China and Japan where it was
encountered by Westerners in the eighteenth century. It has turned out to be a valuable street
tree because of its unusual foliage and tolerance of pollution.
Ginkgo biloba with mature female strobili
GYMNOSPERMS (GYMNOS=NAKED,SPERMA= SEED)
EXAMPLE-----------------CYCAS(LEAVES MAY BE LARGE AND PINNATE),PINUS(LEAVES MAY BE NEEDLE LIKE)
GYMNOSPERM DO NOT HAVE EITHER FLOWERS OR FRUITS.
THE POLLEN GRAINS ARE DISPERSED BY WIND.
WATER IS NOT NEEDED FOR SEXUAL REPRODUCTION.
POLYEMBRYONY ( DEVELOPMENT OF MORE THAN ONE EMBRYO)IS VERY COMMON.
LIVING GYMNOSPERMS ARE DIVIDED INTO FOUR CLASSES,BUT CYCADES AND CONIFERS ARE MORE COMMON.
Monocotyledons
Monocotyledons make a seed with a seed coat.
Inside the seed there is plant embryo with a primitive
root and a supply of food for the new plant. The food
supply is called the endosperm, and it is all in one
piece. When the little plant comes out, the root goes
down and a single spire goes up. The young plant
uses the food resources of the endosperm to provide
energy for growth.
The endosperm part (the inside of the corn kernel in
our example) is the single seed leaf: the
monocotyledon The parent plant has stored food in this kernel so
that the young plant will have energy resources that it can use to
start building itself up as the seed germinates.
Monocotyledons have other distinctive characteristics.
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The veins in their leaves are parallel to each other. Look at the picture of the trillium. The big veins in the leaves are all
going in the same direction.
o Look at the purple crocuses at the top of the page. They also show parallel veins in their petals and long, thin
leaves.
o Crocuses are also monocotyledons.
The parts of the flowers of monocotyledons are arranged in threes or multiples of threes. Note that the trillium has three
petals, and the crocus flowers have six petals. The stamens also follow this pattern.
Examples of monocotyledons are the grasses, such as rice, corn, wheat, and sugar cane. The lily family is also a family
of monocotyledons. Lilies have six petals on their flowers.
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Monocotyledon, 單子葉植物
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Back to the Diversity of plants
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parallel venation (平行葉脈),
i.e. the leaf veins run in parallel.
The typical monocotyledon shows
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These large translucent (半透明) leaves are lovely. They are very common in Hong Kong.
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[Advanced learner : The scientific name is Alocasia odora.]
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Another example of parallel venation.
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Grasses are everywhere, they are some of the most successful species on earth.
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Most Monocotyledons are herbs.
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The " flowers " of grasses are small and they protrude into air.
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Given poor resources, grasses still grow. 逆境求存
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Many monocotyledons are important
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economic crops,
e.g. rice, wheat, maize (corn).
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The photograph shows a cereal.
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Maize (corn)
 is one of the most common monocotyledonous crops.
Each grain on the cob (a few are shown above) is a single fruit with a single
seed. Each seed has only one cotyledon. As each seed comes from an
independent event of fertilisation between the male and female part,
therefore the genetic characteristics among the seeds are different.
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Japanese short grain rice
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S3 students need not learn these biological terms yet.
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Bamboo is a monocotyledon.
The photograph on the left shows the apex
of a fast growing bamboo shoot.
It is protected by overlapping leaf sheaths.
See other monocotyledons : Dried wheat
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Classification of plants / diversity of plants menu