EMBRYO - STRUCTURE, TYPES & DEVELOPMENT

STD-12      UNIT-6     CHA-2

REPRODUCTION IN FLOWERING PLANT

EMBRYO - STRUCTURE, TYPES & DEVELOPMENT 

• Embryogeny is the sum total of changes that occur during the development of a mature embryo from a zygote or cospore . 

Embryogeny in Dicots . 

• In a typical dicot  the zygote elongates and then divides by a transverse wall into two unequal cells ( Schulz and Jensen , 1969 ) . 
• The larger basal cell is called suspensor cell . 
• The other towards the antipodal end is termed as terminal cell or embryo cell . 
• The suspensor cell divides transversely a few times to produce a filamentous suspensor of 6-10 cells . 
• The suspensor helps in pushing the embryo in the endosperm . 
• The first cell of the suspensor towards the micropylar end becomes swollen and functions as a haustorium . 
• The haustorium has wall ingrowths similar to transfer cells ( Schulz and Jensen , 1969 ) . 
• The last cell of the suspensor at the end adjacent to the embryo is known as hypophysis . 
• Hypophysis later gives rise to the radicle and root cap . 
• The embryo cell undergoes two vertical divisions ( quadrant stage ) and one transverse division to form eight cells arranged in two tiers ( octant stage ) - epibasal ( terminal ) and hypobasal ( near the suspensor ) . 
• The epibasal cells eventually form the two cotyledons and the plumule . 
• The hypobasal cells produce the hypocotyl except its tip . 
• The eight embryonic cells or octants divide periclinally to produce an outer layer of protoderm or dermatogen . 
• The inner cells differentiate further into procambium ( = plerome ) and ground meristem ( = periblem ) . 
• Protoderm forms epidermis , procambium gives rise to stele or vascular strand and ground meristem produces cortex and pith . 
• Initialiy the embryo is globular and undifferentiated . 
• Early embryo with radial symmetry is called proembryo . 
• It is transformed into embryo with the development of radicle , plumule and cotyledons . 
• Two cotyledons differentiate from the sides with a faint plumule in the centre . 
• At this time the embryo becomes heart - shaped . The rate of growth of the cotyledons is very high so that they elongate tremendously while the plumule remains as a small mound of undifferentiated tissue . 

• Structure of Dicot Embryo : 
• A typical dicotyledonous embryo  consists of an embryonal axis and two cotyledons . 
• The part of embryonal axis above the level of cotyledons is called epicotyl 
• It terminates with the stem tip , called plumule ( future shoot ) . 
• The part below the level of cotyledons is called hypocotyl which terminates in the root tip called radicle ( future root ) . 
• The root tip is covered with a root cap ( calyptra ) .
• In Capsella bursa - pastoris , the elongating cotyledons curve due to the curving of the ovule itself . With the growth of embryo , the ovule enlarges.
• Its integuments ultimately become hard to form protective converings . Now the embryo undergoes rest and the ovule gets transformed into seed. 
• In some plants the embryo remains in the globular or spherical form even at the time of seed shedding without showing any distinction of plumule , radicle and cotyledons , e.g. , Orobanche , Orchids , Utricularia . 

Embryogeny in Monocots . 

• The zygote or cospore elongates and then divides trans versely to form basal and terminal cells . 
• The basal cell ( towards micropylar end ) produces a large swollen , vesicular suspensor cell . 
• It may function as haustorium . 
• The terminal cell divides by another transverse wall to form two cells . 
• The top cell after a series of divisions forms plumule and a single cotyledon . Cotyledon called scutellum , grows rapidly and pushes the terminal plumule to one side . 
• The plumule comes to lie in a depression . The middle cell , after many divisions forms hypocotyl and radicle . It also adds a few cells to the suspensor . 
• In some cereals both plumule and radicle get covered by sheaths developed from scutellum called coleoptile and coleorhiza respectively . 

• Structure of Monocot Embryo .
• The embryos of monocotyledons ( Fig ) have only one cotyledon . In grass family ( Gramineae ) , this cotyledon is called scutellum . 
• It is situated towards lateral side of embryonal axis . 
• This axis at its lower end has radicle and root cap enclosed in a sheath called coleorhiza . 
• The part of axis above the level of attachement of scutellum is called epicotyl . 
• It has as shoot apex and few leaf primordia enclosed in a hollow foliar structure called coleoptile.
• Epiblast represents rudiments of second cotyledon .

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