LICHENSLICHENS 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 look-alikes 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.

BRYOPHYTABRYOPHYTA ----- 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 male plant produces clusters of antheridia which contain thousands of ciliate sperm.

The female produces archegonia, each containing a 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 mossesMosses 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 Wisconsin.

Polytrichum commune one of the larger 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.

PTERIDOPHYTA

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 ‘SEED-LEAVES’.

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GYMNOSPERMS

SeedsFor 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.

CycadophytaCycads 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

GinkgophytaThis 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.

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.

Monocotyledon, 單子葉植物 Back to the Diversity of plants

　

The typical monocotyledon shows parallel venation (平行葉脈), 

i.e. the leaf veins run in parallel.

These large translucent (半透明)  leaves are lovely. They are very common in  Hong Kong. [Advanced learner : The scientific name is Alocasia odora.]

Another example of parallel venation.

Grasses are everywhere, they are some of the most successful species on earth.

Most Monocotyledons are herbs.

The " flowers " of grasses are small and they protrude into air.

Given poor resources, grasses still grow. 逆境求存

Many monocotyledons are important economic crops,   e.g. rice, wheat, maize (corn).

The photograph shows a cereal.

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.

Japanese short grain rice

S3 students need not learn these biological terms yet.

Bamboo is a monocotyledon.The photograph on the left shows the apexof a fast growing bamboo shoot.It is protected by overlapping leaf sheaths.

See other monocotyledons : Dried wheat

　

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