STRUCTURE OF SEED, TYPES & VIABILITY

STD-12      UNIT-6     CHA-2

REPRODUCTION IN FLOWERING PLANT

STRUCTURE OF SEED,  TYPES & VIABILITY

         

• Seeds are stimulated by the act of fertilization . 
• In angiosperms double fertilization produces two structures , a diploid zygote or oospore and a triploid primary endosperm cell . 
• The latter gives rise to a nutritive tissue called endosperm . 
• Zygote forms the embryo . 
• Endosperm provides nourishment to the growing embryo . 
• With the growth of embryo the central part of the endosperm is eaten up . Endosperm in turn corrodes over the nucellus . 
• In some seeds , the endosperm persists in the seed as food storage tissue . Such seeds are called endospermic or albuminous , e.g. , Castor , Maize , Wheat , Barley , rubber , coconut . 
• In this the endosperm is completely eaten up by growing embryo . 
• The food for later development of embryo is then stored in cotyledons which become massive . Such seeds are nonendospermic or exalbuminous , e.g .. Pea , Gram , Bean , Groundnut . 
• In some seeds remains of nucellus persist . The residual nucellus which persists in the seed is called perisperm , e.g. , Black pepper , Coffee , Castor , Cardamum , Nymphaea . 
• As the embryo reaches maturity its further growth is suspended due to development of growth inhibitors . abscission of funiculus or changes in integuments . 
• The cells of the integuments lose their protoplasm , develop thick and impermeable walls . 
• The integuments thus get transformed into seed coats , outer testa and inner tegmen . 
• The moisture content of seed decreases and reaches 10-15 % . 
• In this dry seed , the embryo occurs in state of inactivity called dormancy. 
• The micropyle of the ovule is changed in micropyle of seed . Through this pore , oxygen and water enter the seed at the time of germination .

Bean Seed 

• It is kidney - shaped brownish non endospermic dicotyledonous seed . 
• The surface is smooth . Concave surface is darker . 
• It has a whitish scar or hilum , a small pore or micropyle and a faint ridge or raphe . 

• A bulge of underlying radicle is observed on the opposite side of raphe . The seed is covered by a thick , tough , brownish seed coat or testa . 
• A thin papery transparent legmen lies below the testa . Seed coats enclose the embryo . There is no other structure . 
• Embryo axis or tigellum is curved . 
• It is covered by two massive cotyledons borne over it in the region called cotyledonary node . 
• One end of embryo axis called plumule lies embedded in between the two cotyledons . 
• It bears two small folded leaves . 
• The other end of embryo axis is radicle . It protrudes out of the cotyledons. 
• Part of the embryo axis lying between radicle and cotyledonary node is called hypocotyl while the part between the cotyledonary node and plumule is known as epicotyl . 
• Food is stored in the cotyledons .

Castor Seed 

• It is oblong mottled brown endospermic and dicotyledonous seed . 
• The narrow end bears a bilobed white spongy caruncle . 
• Both hilum and micropyle occur in this area . 
• Raphe develops from this part and proceeds towards the broad end where it bifurcates .

• A thick hard but brittle testa covers the seed . 
• A thin perisperm lies below it and around the kernel . 
• A white oily endosperm lies below the perisperm . It stores food reserve as oil drops and proteins . 
• Endosperm is source of castor oil . 
• Embryo lies in the centre of seed . It consists of a short embryo axis bearing two thin papery semitransparent oval cotyledons , a small indistinct plumule and a knob - shaped radicle . 
• Palmate venation occurs over the cotyledons . 

Maize Seed 

• It is a monocotyledonous , endospermic single seeded dry fruit called caryopsis . 
• The grain is conical and Mattened . Shallow husk occurs over the pointed end . 
• On one side the broader end bears a papilla representing remains of the style . 
• The same side has a depression in which a ridge indicates the position of underlying embryo . 

• Hilum and micropyle are absent since grain is a fruit and the seed is internal . 
• Color is variable . Surface is nearly smooth . 
• The covering of the grain is made of fused pericarp and testa . 
• 2/3 of the grain interior has food storage tissue of endosperm . 
• It is rich in starch . 
• A protein rich aleurone layer lies on the outside of endosperm . 
• Embryo lies on one side towards the upper pointed part . 
• A single large cotyledon lies lateral and parallel to the embryo axis . 
• It is called scutellum . Scutellum is attached to the middle part of embryo axis . Its outer layer in contact with endosperm is called epithelial layer . The layer secretes GA for formation of amylase from aleurone proteins during germination . 
• Embryo axis ends in plumule towards broader side and radicle towards pointed side . 
• Radicle has a root cap . 
• Plumule bears a few small leaves Sheaths derived from scutellum cover the two ends of embryo axis , undifferentiated coleorhiza over the radicle root cap region and hollow folial coleoptile over the plumule . 
• Area of embryo axis is between plumule and cotyledonary node is epicotyl while the area between cotyledonary node and radicle is called hypocotyl .

Viability of Seeds 

• The ability of seeds to retain the power of germination over a period of time is called viability of seeds . 
• A viable seed is , therefore , that seed which is capable of germination under suitable environmental conditions ( after the completion of dormancy , if it is present ) . 
• Viability may range from a few weaks to several years . 
• It is also influenced by conditions during storage and nongermination . 
• Excessive dry or damp weather and high temperature are known to reduce viability of all seeds . 

• Loss of viability is generally due to : 
• Exhaustion of food around the embryo . 
• Damage to embryo .
• Denaturation of enzymes . 
• Premature exhaustion of RNAs .

• Viability of several hundred years has been recently found out . 
• Some 2000 years old viable seeds of Phoenix dactylifera have been discovered during archaeological excavation of King Herod's palace near Dead sea . 
• About 10000 years old seeds of Lupinus arcticus taken out from arctic tundra have germinated and produced plants that flowered and bore fruits.

•  Viability of seeds can be known by two methods : 
• Ability to germinate .
• Testing their ability to respire . 
• All viable seeds respire . 
• This can be tested by immersing a section of seed containing the embryo in 0.1 % solution of triphenyl tetrazolium chloride . 
• The viable embryo will turn pink due to conversion of colourless triphenyl tetrazolium chloride into insoluble coloured dye called triphenyl formazan due to reduction .

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