Affinities of Psilotales (With Diagram) | Pteridophyta | Botany

The Order Psilotales is characterized by a rootless, dichotomously branched sporophyte, which is partly underground and partly subaerial. The latter part may be leafless or leafy. The sporangia are in diads (e.g., Tmesipteris) or triads (e.g. Psilotum). The gametophytes are entirely subterranean.

The Order includes two genera: Psilotum and Tmesipteris.

Before the recognition of the group Psilophytineae, the Psilo­tales was included under Lycopodineae by Campbell, and under Equisetineae by Bower and as an independent series within the pteridophytes by Seward.

Affinity with Lycopodineae:

Campbell and his sup­porters mainly stressed on the resemblances between Psilotales and Lycopods in their gametophytic struc­ture, i.e., subterranean mycorhizal and radial form. The embedded na­ture of the antheridium resembles Lycopodium but unlike lycopods, there is a segregation of fertile and sterile regions. The sex organs and sperms differ in many important ways.

Affinity with Equisetineae:

Bower and his supporters claimed that Psilotales re­sembles Equisetineae in many important ways:

(i) Morphological and anatomical nature of the young shoot;

(ii) Ribbed stem;

(iii) Nature of the fructification which has been thought to be sporangiophoric in nature;

(iv) Similarity in function of sporogenous cells during the development of the sporangium, all the cells do not develop into spores, some supplying nutrition to the developing spores.

But, in many other characters it differs from Equisetales.

Affinity with Ophioglossales:

The group resembles Ophioglossales in its gametophytic structures which are similar to that of the sporophyte. In both the groups, the gametophytes are like the rhizomes in general appear­ance, in cylindrical form and dichotomous branching, in definite apical growth, in radial structure of rhizoids and in the presence of endophytic fungus.

The multiflagellate characters of sperms and absence of tape-tum along with other characters were considered by Seward to make it an independent series.

Affinity with Psilophytales:

Psilotales and Psilophytales are alike in many important cha­racters:

(i) The plant body is an axis without roots,

(ii) With emer­gences rather than leaves,

(iii) The sporangium on axis-tips,

(iv) The sporangium with massive and undifferentiated wall, and with central sterile columella.

Two orders resemble in many characters but Psilotales differ considerably from Psilophytales in the position of the chambered sporangium and in elaborated anatomical structures.

The members of this Order are known only in the living condi­tion. Psilotum is found in the tropical and subtropical regions, but Tmesipteris is somewhat restricted to Australia, New Zealand, Indonesia and some other islands in the Pacific ocean.