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Differenitate polyembryony and parthenocarpyPolyembryony:In several genera of angiosperms more than one viable embryo in the seed has been observed. It is more frequent in Gymnosperms than in angiosperms. This condition arises in various ways:

(i) The oospore produces more than one embryo during development as a result of splitting. This is known as cleavage polyembryony and is very common in gymnosperms (e.g. Pinus) and angiosperms e.g. Allium, citrus and some orchids.

(ii) An ovule may contain more than one functional megaspore producing as many embryo sacs and oospheres which on fertilization give rise to more than one embryo in the seed.

(iii) In addition to the normal embryo produced from a fertilized oosphere, embryo may also be formed from the synergids (e.g. Dandelion) or rarely the antipodals with or without fertilization.

(iv) Additional embryos may be produced as a result of sporophytic budding by the division of the cells of the nucellus (e.g. lemon) or integument (e.g. onion).

Parthenocarpy:As a rule the development of the ovule into seeds and the enlargement of the ovary into fruit follow only after pollination and fertilization has occurred. In most species of flowering plants, the failure of pollination results in the failure of seed and fruit formation. This usually happens in mature of the pollinating insects are not active or destruction of pollen occurs. In many kinds of fruit trees, if heavy rains fall during the time when pollen is maturing the pollen grains are washed out of the flowers, pollination does not occur and as a consequence few or no flowers set fruits. In some plants fruits are formed even in the abundance of pollination and fertilization. Such fruits are always seed less. These are succulent and more desirable than normal seed containing fruits. They are said to be partheocarpic while the phenomenon concerned in their formation is termed Parthenocarpy. Parthenocarpy is of two types:

(a) Induced parthenocarpy: When seed less fruits are produced by spraying the stigmas of flowers with ether or water extract of pollen grains or by injecting such growth promoting substances as in indole acetic acid and naphthalene acetic acid into ovaries of plants.

(b) Natural parthenocarpy: When seed less fruits are produced without any special treatment from the ovaries in the absence of pollination and fertilization. Familiar examples are the naval oranges, grapes, bananas and pine apples.Posted by:Princessat00:02No comments: Email ThisBlogThis!Share to TwitterShare to FacebookShare to PinterestTags:Botany Describe Endosperm formation and its functionDevelopment of endosperm begins before that of embryo. It arises from the primary endosperm nucleus (also known as triple fusion nucleus) formed as a result of triple fusion of three nuclei i.e. two polar nuclei and a male nucleus. Primary endosperm nucleus is triploid (3N). It undergoes a series of divisions resulting in the formation of many free nuclei lying within the embryo sac. A vacuole appears in the centre so that the nuclei are pushed to the periphery.

Later on unless the endosperm is absorbed by the embryo in free nuclear stage, wall formation takes place starting from the periphery towards the centre of embryo sac and by further centripetal growth, the embryo sac is filled with a compact cellular tissue around the embryo without any intercellular spaces. This tissue is endosperm occasionally as in coconut a vacuole persists in the centre of the embryo sac as it is not completely filled by the endospermic tissue. This type of endosperm development since it involves the process of free nuclear division is called the nuclear type e.g. Fritillaria biflora. This is the most common method of endosperm development. In some plants as in Silphium Lacimatum, each division of the primary endosperm nucleus is followed by wall formation, so that the endosperm is formed at once. This type of endosperm development is known as cellular type.

FIG 395 Page 515 Angiosperms by G. L. Chopra

In some groups endosperm may develop as a few free nuclei only so that it may be regarded as suppressed as in Helobiales and orchids. Several members of Sympetalae a few families of Archichlamydae and monocotyledons show prominent endospmeric loustoria which form very efficient absorptive system.

Function of endosperm:

Endosperm serves as storage tissue as its cells are laden up with food material absorbed through the placenta. The reserve food is stored in large quantities in its cells. This may be deposited as hemi cellulose on the cell walls, when often become very thick as in date and so called vegetable ivory. In most of monocots and some dicots (e.g. castor, butter cup, cotton and all grasses) endosperm persists as permanent tissue in the seed. Such seeds are called albuminous or endospermic.Posted by:Princessat00:25No comments: Email ThisBlogThis!Share to TwitterShare to FacebookShare to PinterestTags:BotanyShort Note - Structure of Ovule (Megasporangium)An ovule (megasporangium) may arise from the placenta at the base of ovary or on the inner surface of the ovary. It is borne on a distinct stalk, the funicle which is attached to the body of the ovule at a point called hilum. The central and somewhat conical part of the ovule is nucellus. It consists of a mass of parenchymatous cells. It represents the metasporangium proper and is invested by two layered integuments in monocotyledons and primitive dicotyledons (Archicliamydeae) but by single layered integument in the higher dicotyledonous families (metachlamydeae); very rarely unitegmic and bitegmic ovules occur in the same family.

In many cases single integument has resulted due to the fusion of two separate primordial or by an elimination of one of the two integuments. Third integument in the form or aril is found in plants like Asphodehus and Trianthema, while Ricinus and several other plants of the family, Euphorbiaceae, show an integumentary proliferation called the caruncle at the micropylar end.

Very peculiar condition is reported in opuntia where the funicle becomes very long and completely surrounds the ovule, thus looking like third integument. In parasites like Santalum (Sandal wood) and Loranthus, there is no integument. The integuments arise from chalaza which constitutes the basal part of the nucellus. They grow upward in close contact with the nucellus and enclose the latter completely, leaving only a small opening, the micropyle at the apex. An embryo sac is embedded in the nucellus towards the micropylar end.

The embryo sac as will be seen latter bears the embryo and is the most important part of the ovule. In a mature ovule the embryo sac bears towards the micropylar end three protoplasts constituting the egg apparatus of those one is the egg or oosphere while the other two are the synergids or help cells. At the chalazal end are three antipodal cells enclosed in cell wall. In the centre of the embryo sac lies the secondary nucleus.

The vascular supply of the ovule is supplied by a vascular strand which runs through the funcile and extends upwards, but rarely beyond chalaza.

In some cases as in Gmynosperms the vascular bundles enter the integuments or even the nucellus also. Sometimes the integuments are green due to the presence of chlorophyll e.g. Amaryllis and bear stomata e.g. Nerium and Gassypium.Posted by:Princessat00:23No comments: Email ThisBlogThis!Share to TwitterShare to FacebookShare to PinterestTags:Botany Describe structure and development of Anther and Microsporangia

A cross sectionof young anther shows that each anther lobe contains two pollen sacs ormicrosporangia but the number may vary among different angiosperms from one tomany. As a rule microsporangia extend as longitudinally running sacs throughthe entire length of the anther. Later on as the anther matures, theintervening tissue breaks down between each pair of pollen sacs and the twopollen sacs coalense into one, thus forming two large cavities instead of fourin each anther. In Malvaceae anthers are one called (monotherous) as there aretwo instead of four pollen sacs per anther and these also coalesce at maturityto form single loculus. Very rarely four celled sacs may remain separate in a matureanther.
Within eachpollen sac of an angiosperm as within a Microsporangium of Salaginella or Pinusare produced small numbers of micropyle mother cells surrounded by aconspicuous layer of nutritive cells, the tapetum outside the tapetum is thewall of the sporangium consisting of a number of layers of cells. Outermostwall layer just beneath the epidermis shows characteristic band-like fibrousthickenings. This layer is the endothecium which assists mechanically in the dehiscenceof the anther opposite the partition between each pair of sporangia (pollensacs) the cells of endothecium are thin walled and constitute the stomium whichmarks the line of dehiscence or opening of ripe anther. Each microspore mothercell after two successive divisions of its nucleus divides to produce fourpollen grains (microspores). As this occurs and as the pollen grains mature,the tapetum disappears, the products of its disintegration are absorbed andused as food by the developing pollen grains.
Pollen grainsare liberated as a result of dehiscence of ripe anther which may take place invarious ways. In tulips, lily and Datura the opening is longitudinal e.g. theslit appears above the line of fusion of two pollen sacs; in others asbarberry, it is effected by uplifting valves; in still others by means of poresat the top as in tomato, potato and certain other grasses by transverse slitsin Hibiscus esculentus.Posted by:Princessat00:28No comments: Email ThisBlogThis!Share to TwitterShare to FacebookShare to PinterestTags:BotanyDescribe structure of ovule (Megasporangium)

An ovule (megasporangium)may arise from the placenta at the base of ovary or on the inner surface of theovary. It is borne on a distinct stalk, the funicle which is attached to thebody of the ovule at a point called hilum. The central and somewhat conicalpart of the ovule is nucellus. It consists of a mass of parenchymatous cells.It represents the metasporangium proper and is invested by two layeredinteguments in monocotyledons and primitive dicotyledons (Archicliamydeae) butby single layered integument in the higher dicotyledonous families(metachlamydeae); very rarely unitegmic and bitegmic ovules occur in the samefamily. In many cases single integument has resulted due to the fusion of twoseparate primordial or by an elimination of one of the two integuments. Thirdintegument in the form or aril is found in plants like Asphodehus andTrianthema, while Ricinus and several other plants of the family,Euphorbiaceae, show an integumentary proliferation called the caruncle at themicropylar end.
Very peculiarcondition is reported in opuntia where the funicle becomes very long andcompletely surrounds the ovule, thus looking like third integument. Inparasites like Santalum (Sandal wood) and Loranthus, there is no integument.The integuments arise from chalaza which constitutes the basal part of the nucellus.They grow upward in close contact with the nucellus and enclose the lattercompletely, leaving only a small opening, the micropyle at the apex. An embryosac is embedded in the nucellus towards the micropylar end.
The embryo sacas will be seen latter bears the embryo and is the most important part of theovule. In a mature ovule the embryo sac bears towards the micropylar end threeprotoplasts constituting the egg apparatus of those one is the egg or oospherewhile the other two are the synergids or help cells. At the chalazal end arethree antipodal cells enclosed in cell wall. In the centre of the embryo saclies the secondary nucleus.
The vascular supplyof the ovule is supplied by a vascular strand which runs through the funcileand extends upwards, but rarely beyond chalaza.

In some cases asin Gmynosperms the vascular bundles enter the integuments or even the nucellusalso. Sometimes the integuments are green due to the presence of chlorophylle.g. Amaryllis and bear stomata e.g. Nerium and Gassypium.Posted by:Princessat00:271 comment: Email ThisBlogThis!Share to TwitterShare to FacebookShare to PinterestTags:Botany Describe Endosperm formation and its function

Development ofendosperm begins before that of embryo. It arises from the primary endospermnucleus (also known as triple fusion nucleus) formed as a result of triple fusionof three nuclei i.e. two polar nuclei and a male nucleus. Primary endospermnucleus is triploid (3N). It undergoes a series of divisions resulting in theformation of many free nuclei lying within the embryo sac. A vacuole appears inthe centre so that the nuclei are pushed to the periphery. Later on unless theendosperm is absorbed by the embryo in free nuclear stage, wall formation takesplace starting from the periphery towards the centre of embryo sac and byfurther centripetal growth, the embryo sac is filled with a compact cellulartissue around the embryo without any intercellular spaces. This tissue isendosperm occasionally as in coconut a vacuole persists in the centre of theembryo sac as it is not completely filled by the endospermic tissue. This typeof endosperm development since it involves the process of free nuclear divisionis called the nuclear type e.g. Fritillaria biflora. This is the most commonmethod of endosperm development. In some plants as in Silphium Lacimatum, eachdivision of the primary endosperm nucleus is followed by wall formation, sothat the endosperm is formed at once. This type of endosperm development isknown as cellular type.
In some groupsendosperm may develop as a few free nuclei only so that it may be regarded assuppressed as in Helobiales and orchids. Several members of Sympetalae a fewfamilies of Archichlamydae and monocotyledons show prominent endospmericloustoria which form very efficient absorptive system.
Function ofendosperm:
Endosperm servesas storage tissue as its cells are laden up with food material absorbed throughthe placenta. The reserve food is stored in large quantities in its cells. Thismay be deposited as hemi cellulose on the cell walls, when often become verythick as in date and so called vegetable ivory. In most of monocots and somedicots (e.g. castor, butter cup, cotton and all grasses) endosperm persists aspermanent tissue in the seed. Such seeds are called albuminous or endospermic.Posted by:Princessat00:32No comments: Email ThisBlogThis!Share to TwitterShare to FacebookShare to PinterestTags:BotanyDescribe polyembryony and parthenocarpy

In severalgenera of angiosperms more than one viable embryo in the seed has beenobserved. It is more frequent in Gymnosperms than in angiosperms. Thiscondition arises in various ways:
(i) The oosporeproduces more than one embryo during development as a result of splitting. Thisis known as cleavage polyembryony and is very common in gymnosperms (e.g.Pinus) and angiosperms e.g. Allium, citrus and some orchids.
(ii) An ovulemay contain more than one functional megaspore producing as many embryo sacsand oospheres which on fertilization give rise to more than one embryo in theseed.
(ii) In additionto the normal embryo produced from a fertilized oosphere, embryo may also beformed from the synergids (e.g. Dandelion) or rarely the antipodals with orwithout fertilization.
(iv) Additionalembryos may be produced as a result of sporophytic budding by the division of thecells of the nucellus (e.g. lemon) or integument (e.g. onion).
Parthenocarpy:
As a rule thedevelopment of the ovule into seeds and the enlargement of the ovary into fruitfollow only after pollination and fertilization has occurred. In most speciesof flowering plants, the failure of pollination results in the failure of seedand fruit formation. This usually happens in mature of the pollinating insectsare not active or destruction of pollen occurs. In many kinds of fruit trees,if heavy rains fall during the time when pollen is maturing the pollen grainsare washed out of the flowers, pollination does not occur and as a consequence fewor no flowers set fruits. In some plants fruits are formed even in the abundanceof pollination and fertilization. Such fruits are always seed less. These aresucculent and more desirable than normal seed containing fruits. They are saidto be partheocarpic while the phenomenon concerned in their formation is termedParthenocarpy. Parthenocarpy is of two types:
(a) Inducedparthenocarpy: When seed less fruits are produced by spraying the stigmas offlowers with ether or water extract of pollen grains or by injecting suchgrowth promoting substances as in indole acetic acid and naphthalene aceticacid into ovaries of plants.
(b) Naturalparthenocarpy: When seed less fruits are produced without any special treatmentfrom the ovaries in the absence of pollination and fertilization. Familiarexamples are the naval oranges, grapes, bananas and pine apples.
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