Affinities of Pteridophytes
Affinities of Pteridophytes
The members of pteridophytes are said to have an intermediate position between bryophytes and higher vascular plants (gymnosperms and angiosperms). The simplest living pteridophytes (e.g. Psilotales, Lycopodiales) resemble with foliose bryophytes in many of their basic characters and may have evolved from the same ancestral stock but they are nevertheless very diverse and show a great deal of specialization. The pteridophytes resemble with the spermatophytes (gymnosperms and angiosperms) in having complex internal organization of the sporophyte, but differ from them in the lack of seed habit. Such specialized features justify independent identity of pteridophytes.
Similarities between Pteridophytes and Bryophtes :
- Both pteridophytes and bryophytes posses heteromorphic alternation of generation.
- The sexual reproduction is oogamous. The male and female sex organs are known as antheridia and archegonia respectively.
- The sex organs have a protective layer of sterile cells (jacket). The male gamete (antherozoid) is flagellated and motile and the female gamete (egg) is non-motile.
- Water is essential for opening of the mature sex organs and fertilization.
- Many pteridophytes are homosporous i.e. they produce only one type of spores (e.g., Lycopodium,, Equisetum Psilotum Polypodium, etc.) like bryophytes.
- Certain pteridophytes like Rhynia and Psilotum possess rhizoids as they are present in bryophytes.
Differences between Bryophytes and Pteridophytes :
- The most striking feature the differentiates pteridophytes from bryophytes in that the plant body in pteridophytes is sporophytic (2N) and in the bryophytes, it is gametophytic (N).
- The plant body of pteridophytes consist of root, stem and leaves, whereas in bryophytes it is thalloid or foliose.
- The vascular system of pteridophytes is well developed and composed of xylem and phloem whereas the vascular tissue in bryophytes are absent.
- Vegetative reproduction in quite common in bryophytes than in pteridophytes.
- Bryophytes are always, homosporous, whereas many pteridophytes (e.g. Selaginella) show heterospory.
- in pteridophytes the sporophyte is large, long lived and independent of the gametophytes but in bryophytes, it is completely or partly dependent on the gametophyte.
- The sporophyte of some bryophytes has elaters along with spores but elaters are altogether absent in pteridophytes.
Similarities of Pteridophytes with Gymnosperms:
- The plant body in body the groups is sporophytic, differentiated into root, stem and leaves.
- The young leaves show circinate vernation in both.
- The vascular system is well developed and made up of xylem and phloem in both the groups. The xylem is without vessels (except Gnetales in gymnosperms) and the phloem without companion cells.
- Like gymnosperms, many pteridophytes show homosporous condition.
- The antherozoides of some gymnosperms (e.g. Cycas, Ginkgo) are flagellated like those of pteridophytes.
- In some pteridophytes (e.g. Selaginella), the megaspore is retained within the megasporangium (see habit tendency), a permanent feature of gymnosperms.
- There is regular alternation of sporophytic and gametophytic generations in both the groups.
Differences between Pteridophytes and Gymnosperms:
- Pteridophytes usually flourish in most and shaded places, whereas gymnosperms enjoy xerophytic habitats.
- Gymnosperms have tap roots but in pteridophytes the tap root is soon replaced by adventitious roots.
- Pteridophytes lack vascular cambium (hence do not show secondary growth), while it is present in gymnosperms (hence, they show secondary growth).
- The archegonium in pteridophytes has neck canal cells and venter canal cell but they are absent in gymnosperms.
- In homosporous, pteridophytes, the development of gametophyte starts after the dispersal of spores from the sporangium. But in gymnosperms the female gametophyte develops in situ, i.e. inside the megasporangium (ovule). The ovule is permanently attached to the sporophyte and the microspores are liberated at various stages of development of the male gametophyte.
- The pollen grain (microspore) of gymnosperm develops pollen tube on germination, but no such structure is formed in pteridophytes.
- The gametophyte of pteridophytes is independent of the sporophyte, whereas it is dependent on the sporophyte in gymnosperms.
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