Gymnosperms : General Characteristics of Gymnosperms | Botany Notes

Gymnosperms : General Characteristics of Gymnosperms | Botany Notes

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• Mostly living gymnosperms are evergreen trees or shrubs with xerophytic adaptations.

• Plant body is sporophytic and differentiated into root, stem and leaves

• Plant possesses well developed tap root system. Some roots have symbiotic relationship with algae (Cycas) & fungi (Pinus)

• Stem in Gymnosperms :

Erect, Profusely branched (except Cycas) & woody.

• Presence of leaf scars on the stem is a characteristic feature of gymnosperms

classification of sporne

Leaves

Leaves are dimorphic.

Foliage leaves: green, simple, needle shaped or pinnately compound

Scaly leaves: minute & deciduous. Ephedra possesses only scaly leaves. Roots are di to polyarch.

• In the stem tanniniferous cells are frequently present

• Young stem has a ring of collateral and open vascular bundles.

Xylem consists of trachieds & xylem parenchyma. Vessels are absent except in the members of Gnetales

Phloem consists of sieve tubes & phloem parenchyma. Companion cells are absent

Wood: Manoxylic in Cycas & Pycnoxylic in Pinus  

Leaves with thick cuticle & sunken stomata. Mesophyll may be undifferentiated(Pinus) or differentiated into palisade & spongy parenchyma( Cycas)

• Most gymnospermic leaves don’t have lateral veins & lateral translocation takes place with the help of transfusion tissue.

• They are heterosporous – Megaspores & Microspores aggregate to form cones

Cones : are usually monosporangiate except in Ephedra intermedia

• Male cones :-

short lived & microsporangia develop on the abaxial side  of the microsporophyll

                          – No. varies from 2 (Pinus)to many(Cycas)

                           – development is eusporangiate

• Female cones:-

formed by the aggregation of megasporophyll

                           – remain persistent on the plant for several year

• The megasporangium (ovule) is a naked structure on the megasporophyll

• Ovules are orthotropous & unitegmic but bitegmic in Gnetales

• Ovular integument is three layered, outer and inner layers are fleshy & middle is stony

Microspore

Microsporeare liberated in different developmental stages as: 3 celled in Cycas4 celled in Pinus 5 celled in Ephedra.

• Male gametes are non motile except in Cycas & Ginkgo.

Archegonia

• The no. of archegonia in a female gametophyte is variable as they are several in Cycas while only 2 in Pinus

• Archegonia has a single egg and venter canal cell; neck canal cells however absent

Pollination

• Pollination is direct & all gymnosperms are wind pollinated •Fertilization is siphonogamic

Zygote

• Zygote formed is the mother cell of next sporophytic generation & development of embryo is meroblastic.

• Development of endosperm is before fertilization and hence endosperm is haploid 

Polyembryony

• There is marked tendency of polyembryony, which may arise by fertilization of more than 1 eggs or by the division of the zygote but only 1 one embryo attains maturity.

• The naked ovule develops into seed & the ovular integument form seed coat

Classification by sporne(1965)

Gymnosperms are divided into three classes and nine orders:

Clases

Cycadopsida

  • Manoxylic wood.
  • Unbranched or poorly branched trunks.
  • Leaves large and pinnate.

Coniferopsida

  • Pycnoxylic wood.
  • Large sized trees.
  • Profusely branched stems.
  • Simple leaves.

Gnetopsida

  • Link between gymnosperms and angiosperms.
  • vessels are present.
  • companion cells are present.

    

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