unique features of the psilotales / PSILOTUM
Why is this small order, the Psilotales, so unique among vascular
plants.
(a) The Psilotales are an order showing essentially primitive
features. The plants are rootless; the development from the zygote shows that
this condition is genuinely primitive and not a result of reduction due to
specialization of habit.
The plant body is relatively unspecialized anatomically.
The sporophyte is in many respects like a primitive simple thallus, the main
continuous axis weakly differentiated into underground rhizome and leafy aerial
shoot.
The gametophyte is likewise a simple branching undifferentiated thallus.
(b) The resemblance of the two generations of the life cycle is
more striking in this order than in any other group of Pteridophytes.
The
vascular gametophyte is unique in the Plant Kingdom.
(c) The Psilotales have no known fossil history in the geological
record.
Their nearest relatives are a very primitive early Devonian order of
fossils, the Psilophytales.
It is indeed a puzzle as to why the undoubtedly
primitive and ancient Psilotales should have survived without leaving us any
faint clue in the fossil record as to ‘what they are or whence they came’.
(d) The genera of this order exemplify discontinuous geographical
distribution.
This occurrence pattern at the ends of the earth probably
indicates that the Psilotales are merely relics of a once more continuous and widespread
group.
(e) In Pteridophytes the gametophyte generation has the haploid or
n. number of chromosomes; the sporophyte has the diploid or 2n. number which is
halved by a reduction division or meiosis at spore formation.
The cytology of
this order is extremely interesting.
The chromosome count for the sporophytic
cells of the Psilotales is exceedingly high.
For Tmesipteristhe
diploid chromosome number is about 200 (Yeates, 1925), although Manton, 1950,
reports a sporophytic number of 400.
It seems likely therefore that polyploidy,
i.e., multiplication of the diploid number, occurs in this genus.
The fact that
there is more than one nucleolus in the nuclei of some specimens, established
by Yeates, also supports this.
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