Monday, January 28, 2019

Gymnosperm: General characteristics

   Gymnosperm 


 Characteristic feature
Distribution

throughout the temperate and tropical regions and even in Arctic zone
in India mostly found in hilly areas very few species grow in Plains like cycadales.
Living gymnosperm are perineal xerophytic, Woody evergreen plant none of them are annual.

External features

plant body is sporophytic and differentiated into root, leaf & stem
the gymnosperm sequoia world tallest tree ( the giant Redwood) 125 M height and 30 M with 4000 year old in Redwood Park( California)
smallest gymnosperm Zamia with underground tuberous stem .
Tap root system,  sometimes symbiotic relationship with algae and fungi found
in cycas root show symbiotic relationship with algae
in pinus root show symbiotic relationship with fungi.
stem usually erect profusely branched( unbranched in cycas) tuberous in zamia
Leave scars on stem  are characteristic feature of  gymnosperms.
the arrangement of leaves is spiral on stem or cyclic in some gymnosperm. The foliage of leaves of some coniferous are modified into long needles.
Leaves maybe monomorphic( that is one kind leaves) and dimorphic (that is of two kind of leaves)
in dimorphic, the foliage leaves are evergreen and protected by the cuticle whereas the scale leaves a minute and dioecious.

Internal feature 


the vascular cylinder of roots maybe diarch or polyarch
stem possess  Collateral endarch and open vascular bundle arranged in ring
secondary growth occurs. secondary growth of occur by the activity of cambium
xylem cosist of only tracheids and xylem parenchyma.vessels are absent (gnetals except).
Phloem have sieve tube &phloem parenchyma but companion cells are absent.
Stem show distinct secondary growth
Woods are manoxylic type in cycas
Pycnoxylic in pinus
Leaves -Mesophyll of leaf is undifferentiated or differentiated into palisade and spongy parenchyma.
Stomata are sunken.
Most of the gymnospermic leaves do not  have ventral veins and translocation of nutirents take place by transfusion tissue.

 Reproduction

Cones are usually monosporangiated but (ephedra species) are bisporangiated.
Male microsporangia develop on abaxial side of micro sporophyll.
Eusporangiate
Megasporophyll may be similar to foliage leaves or cauline(pinus).
Ovular integument have 3 parts-outer layer and inner layer is flashy but middle layer is stony.

heterosporous plant usually produce two different type of spores  microspheres that is male megaspore that is female spores are found in sporangium.
The microsporangium are born on lower surface of microsporophyll and megasporangium are born naked on megasporophyll
microsporangium maybe arrange indefinite group called sori their number may be reduced to 2 in pinus
microsporophyll are aggregated in a form of compact structure called male cone and megasporophyll usually aggregate in a form of female cone in some cases megasporophyll are falling in nature and not aggregated in a form of cones

the male cone is short life and female cone are long life .female remain in plant until maturity and reaping of seeds.

microsporophyll contain microsporangia. sporangium contain microspores  (distributed by wind always) microspores leavues microsporangia after 3 states normally but it diffeentr in different genes.

On germination

micropile + Pollen form tube cells. microspores form male gametophyte that is haploid (one or two gametes)
pollen grains get direct connected with a ovule  and deposit on Pollen chamber which after an unfavourable condition germinate here.
Each Pollen Grain from Pollen tube have two functions
1-haustorial  that used to suck food and 2- sperm career
That type of structure and functioning of pollen tube is termed as sphonogamic .
female cone - megasporophyll which contain megasporangia  (ovule)
ovules are orthotropous and   unintermic but bitegmic in gnetales
haploid megaspores developed into female gametophyte which remain surrounded by integumin  that is endosperm.
mature female gametophyte have( two or more or none) archegonia.this phenomena is called polyembryony .
more then two mature female gametophyte in archigonia called polyembryony.
each archegonia have single cell ,rentals Canal cell but neck Canal cells are absent .
male sperm carry to archigonia through Pollen tube( in cycas Pollen tube only used to suck food from endosperm).
female nuclei and male nuclei combine to form a zygote that is haploid zygote is first cell of sporophytic generation.

The young sporophyte


zygote developed into embryo
meroblastic development only Basil part develop to embryo middle and upper part developed into different things
embryo develop at terminal part of the pressure which pushes the developing embryo into endosperm
endosperm always haploid in nature because it develop before the fertilization MBA is
endoscopic that means shoot Apex is directed opposite to micropile.
the basic characteristic feature of gymnosperm is occurrence of polyembryony but only one embryo develops and mature
mature embryo differentiated into leaf root stem number of cotyledons  in gymnosperms are one two or more 
Cotlyodons are maybe in green in colour while still in close in seeds
no true fruit are found in gymnosperm.


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