The epidermis is also interrupted by the presence of stomata. In homophyllous isophyllous species the stomata are present on outer as well as inner epidermis amphistomatic but in heterophyllous anisophyllous species the stomata are mostly restricted on the lower epidermis hypostomatic. It occupies a wide zone between the epidermis and the vascular bundle. It is usually made up of thin walled chlorenchymatous cells which may be with or without intercellular spaces. In the centre of the leaf is situated only a single concentric vascular bundle made up of only xylem and phloem.
The vascular bundle is surrounded on all sides by a sclerenchymatous sheath. In a few species like L. The morphological nature of gemmae is still not fully known. The gemmae when fall on ground, develop root primodia and soon form the root. Species with creeping stem reproduces vegetatively by the death and decay of older parts of the stem up to the point of branching. This separates the branches which later on grow independently. This tip portion of the rhizome acts as resting bud which in the coming spring resumes growth and develops into a new plant.
In several epiphytic species fragments of the plant body are capable of giving rise to new plants. Lycopodium is a sporophytic plant and reproduces sexually. The plants are homosporous i.
These spores are produced in sporangia which, in turn, are produced on fertile leaves known as sporophylls. Usually the sporophylls are grouped together to form a compact structure known as strobili Sing. In the primitive species of the sub-genus Urostachya every leaf on the plant is a sporophyll or at least potentially so and the fertile and sterile zones alternate. The sporophylls are loosely arranged. In species of Rhopalostachya and in some species of Urostachya the leaves of the apical portion of the branches only bear sporangia and are called sporophylls.
The rest behave as vegetative leaves. The sporophylls may be of the same size or of different size from the foliage leaves in different species. The arrangement of sporophylls is same on the central axis as that of the vegetative leaves on the stem i.
The position of the sporangium is also different in different species. The sporangia may be axillary and protected with the help of sporophylls e.
Longitudinal section L. On both sides of the strobilus axis are present sporophylls Fig. Each sporophyll bears only one sporangium Fig. All the sporangia are similar in structure and are arranged acropetally in a strobilus i. Sporangia are sac-like structures but usually kidney shaped in appearance Fig. Sometimes they are sub-spherical in appearance. Their colour varies from orange to yellow. Each sporangium consists of a basal short massive stalk i.
The body of the sporangium consists of 3 or more layers of wall surrounding a cavity. The inner most layer of the wall of sporangium is called as tapetum Fig. In the young sporangium inside the wall is present a mass of sporogenous cells which in due course of development form spore mother cells which by meiotic divisions, produce haploid tetrad of spores.
The spores at maturity separate from each other. The wall of the sporangium is provided with a transverse strip of cells known as stomium from where the sporangium at maturity splits into 2 valves and the spores are dispersed away in the air. The spores produced by a sporangium are all alike homosporous. They are small, rounded or even spherical structures. The surface of the spores is usually rough due to the presence of reticulate ridges or knob like protrusions.
Each spore is provided with a triradiate ridge Fig. A small amount of chlorophyll may or may not be present in spores. Reserve food is in the form of oil in the spores. Development of sporangium and formation of spores. Bower had studied the development of sporangium in Lycopodium. The sporangium develops from a small group of superficial cells arranged in a transverse row on the adaxial side of the sporophyll near the base.
Its development is of eusporangiate type. These superficial cells are called sporangial initials Fig. These cells divide by periclinal divisions forming an outer and inner layer of cells.
The outer cells divide periclinally and anticlinally forming three celled thick wall of the sporangium Fig. The inner layer or archesporial cells divide in all directions forming a group of cells known as sporogenous tissue which finally give rise to spore mother cells. During these developments the inner-most layer of wall is differentiated as a nutritive layer and is known as tapetum.
It is a persistent structure and rich in reserve food material. Each spore mother cell undergoes a process of meiosis thus producing a tetrad of spores haploid with tetrahedral arrangement. These spores later on separate from the tetrad, as a result of which, a large number of spores are produced inside each mature sporangium. Dehiscence of sporangium and liberation of spores. As the sporangium approaches towards maturity, a transverse row of cells is differentiated near the apical portion from the wall of a sporangium known as stomium.
The walls of the cell of stomium thicken and differ from the walls of other cells of the sporangium. As the sporangium loses water, it creates a pressure on the wall which leads to the appearance of slit in the stomium as a result of which the wall splits opens into two halves and the spores are disseminated by air current. The development of the gametophyte prothallus takes place from the haploid spores which are the unit of gametophytic generation.
Each spore is unicellular, uninucleate haploid structure, 0. Chlorophyll may or may not be present in different species. In few species spores may germinate within a few days after liberation but in some species the spores germinate when they are years old and the development of gametophyte upto formation of mature sex organs may take a time of 8 months to 6 or even 15 years. The rate of the formation of photosynthetic tissue is usually proportional to the rate of growth of gametophyte.
Both the male and female reproductive organs are produced on the same gametophyte. The male sex organs are produced earlier than female sex organs. Usually at the time of germination of spore, it swells up to absorb water. First the spore divides into two unequal cells by a lenticular division, forming a very small lens shaped cell known as rhizoidal cell and a bigger cell Fig. This rhizoidal cell takes no part in further development of gametophyte and is a colourless structure.
At this two celled stage the spore will rupture at the triradiate ridge. Second division divides the bigger cell into two equal halves, the cell near the rhizoidal cell is known as basal cell and the other one is known as upper cell Fig. The upper cell further divides by two successive divisions in such a way as to form an apical cell with two cutting faces Fig.
At this stage the gametophyte is 5 celled structures and the symbiotic phycomycetous fungus mycorrhizal fungus attacks it. If this fungus fails to attack at this stage, further development of gametophyte stops. This infection takes place through the basal cell. During further course of development of gametophyte the apical cell further divides to form six or morecells which later on develop into meristematic cells.
These cells, by further divisions form a multicellular structure, the gametophyte prothallus Fig 10 E-H. Gametophyte is partially aerial and partly in soil.
The prothalli are usually 2 to 3 millimetre in height and millimetre in diameter. The gametophytes prothalli grow at the surface of the ground and consist of a colourless basal portion buried in soil and a conspicuous upright, fleshy, green aerial portion having lobes Fig.
The sex organs develop between the green expanding lobes. The prothallus itself is a nourishing body. The underground part contains endophytic fungus e.
The gametophyte is wholly subterranean and totally saprophytic i. It is tuberous and without lobes. It may be one to two centimentre long or wide and is top shaped, conical or discoid in shape Fig. The endophytic fungus is present. Sex organs are formed on the upper surface e. The gametophyte is subterranean, saprophytic and colourless. As is the case with many of the ferns the common names for clubmosses have been much more stable than the scientific names, several of which have been changed in the last thirty years.
Several species are frequently encountered in the common understory forests of the Adirondacks of New York and in forests in New England. A ll members of the group that exist today are small plants, typically less than 10 cm in height. But in the past members of the group were much larger and formed forests. The group originated over million years ago in the Paleozoic and the phylum is the oldest group of vascular plants that still has members today. Tree forms up to 35 m in height were common at the end of the Paleozoic, roughly million years ago, and were important in forming deposits that are sources of coal and oil.
All the tree forms disappeared at the end of the Paleozoic. The clubmosses form a distinct group that is generally recognized at the phylum level Lycopodiophyta.
The other groups are the ferns , horsetails and wisk ferns some people lump these three groups together into one phylum. The Lycopodiophyta includes three groups, clubmosses, spikemosses and quillworts.
On some clubmoss and spikemoss species the leaves are overlapping and resemble those of cedar, which gives some species a common name of ground cedar.
Each sporophyll bears a solitary sporangium on the upper side at the basal portion. Each sporangium is yellow or orange coloured and provided with sterile jacket layer of layers of cells thick. Within the jacket layer is the fertile sporogenous tissue provided with nutritive tissue known as tapetum. The sporogenous tissue later differentiates into spore mother cells , each of which by meiotic division produces spore-tetrad. Lycopodium is homosporous, i. As soon as the spores are developed, haploid n gametophytic generation begins.
Spore is the first cell of the gametophyte. Spores are very small, tetrahedral and provided with two thin walls — outer exine and an inner intine. Each spore contains a single nucleus and fats and oils as reserve food materials. Lycopodium is homosporous, hence the germination of spore produces homothallic gametophytic plant body or prothalli singular : Prothallus. Depending upon their nature, the prothalli of Lycopodium is of 3-types —.
First Type — In this type, the prothallus is very small mm long , cylindrical or ovoid in shape, short lived, green in colour and develops on the surface of the ground.
Such type of prothallus is found in tropical species. Second Type — In this type, the prothallus is much larger cm long , more or less tuberous or carrot shaped, long lived, yellowish in colour or almost colourless and sub-terranean. Such type of prothallus is found in creeping species. Third Type — This type of prothallus is intermediate between first and second types. This type of prothallus have irregularly shaped tuberous body about 2mm in diameter , colourless and sporophytic in nature.
Such type of prothallus is commonly found in epiphytic species. Since the prothallus is homothallic, it bears both male and female sex organs, i. Antheridia:- Antheridia arise in several numbers in a gametophytic plant body.
They remain either wholly embedded in gametophytic tissue or projected slightly. They are generally oval in shape. Each antheridium is surrounded by a jacket layer of one-celled in thickness.
Inside the jacket layer lies numerous sperm mother cells , which directly metamorphosed into small, cubical, biflagellate sperm. Archegonia:- Archegonia also arise in numbers in the gametophytic plant body. They also remain sunken with only their neck projecting outwardly. A mature archegonium consists of a neck , composed of neck canal cells , and a narrow venter , composed of one ventral canal cell and an egg cell. Fertilization:- At maturity, the neck cells of the archegonium separate and the neck canal cells and ventral canal cell disorganize, leaving a passage for the entry of sperm.
The sperm after liberation from the antheridium makes its way through the neck and finally reaches the egg. On reaching t he egg, one sperm fuses with the egg to complete the fertilization. As a result of fertilization, a diploid zygote 2n is formed. With the formation of zygote, diploid sporophytic generation begins. Unknown November 1, at AM. Unknown July 19, at PM. The genus is mainly confined to temperate regions, and is found to grow in moist, cool and shady places. About 11 species of Marchantia have been reported from India mainly growing in Himalayas and very few species occur in plains and hills.
Of these the common species are — M.
0コメント