An earliest record of vascular plants is from Devonian of Canada (Gaspe Peninsula, Quebec). Although the compressions were fragmentary, a reconstruction revealed slender axes arising from a subterranean rhizome system (Fig. 7.3A).
The rhizome lacked roots but had rhizoids. The aerial axes were dichotomously branched with scale-like appendages and terminated into sporangia. Because of its similarity with Psilotum. the plant was named Psilophyton princeps.
In a redescription of fossil remains, the sporangia were shown in pairs at the apices of slender branches. This indicated that fossil specimen comprised more than one taxon. In a re-examination of original specimen Psilophyton princeps is described to have pseudomonopodial branched axis that was either naked or spinous with a central vascular strand, and sporangia borne terminally on lateral branches.
True to many discoveries this significant finding of first vascular plant was without an impact, because botanists at that time were not prepared to accept such an unusual kind of plant described by a geologist. However, after a gap of over half a century the significance of Psilophyton became apparent when similar and well-preserved vascular plants were redescribed from lower devonian red sandstone cherts of village Rhynie.
It was realized that plants of such a simple organization did exist and the group Psilophytales was instituted to include them. The chief components of Rhynie flora are Rhynia (Fig 7.3B) and Horneophyton (Fig. 7.3C, D). These fossils are described from petrifactions; the sediment that formed chert (Quartz-like rock). The preservation was so perfect that not only cellular details but subcellular details could be seen.
Originally described as Rhynia gwynne-vaughanii (Fig. 7.3B), it was a small plant of up to 17 cm in height. The aerial dichotomous axes arose from a subterranean horizontal rhizome. The rhizome system lacked roots, instead had rhizoids. The aerial axes terminated into fusiform sporangia and lacked leaves.
It, however, had hemispherical projections which are considered to be adventitious branches. Another species described from Rhynie chert was R. major. It was a robust species attaining the height of up to three times that of R. gwynne- vaughanii. Otherwise these species were similar to each other.
Anatomically, on the outside of Rhynia axes is a well-defined epidermis (Fig. 7.3E) with sparsely distributed stomata, which lack in rhizome. Interestingly, the cortex is divisible into outer and inner regions. The outer cortex is narrow band of elongated mesophyll-like cells.
The inner cortex is quite extensive, consisting of circular cells with inter-cellular spaces connected to the exterior through stomata; it is considered to be the assimilatory zone. Surrounding the central cylinder of tracheids is a zone of thin-walled cells interpreted as ‘phloem’.
Endodermis and pericycle are not recognizable. Longitudinal section of axis passing through phloem-like zone reveals the occurrence of numerous pores. This is considered to be an evidence for phloem nature of this zone, which has a conducting function. In the central cylinder are seen annular or helically thickened tracheids.
The sporangia were massive with many thick-walled-cells and without a dehiscence mechanism.
Horneophyton lignerii (Fig. 7.3C) was very small and differed from Rhynia in as much as in its sporangia it had a central sterile region, the columella (Fig. 7.3D).
Rhynia — The Oldest Vascular Plant:
Devonian Rhynie flora in general and Rhynia in particular are not considered to be a representative of earliest land plants. This is because a more complex form (Asteroxylon) coexisted with Rhynia in Devonian. More complex forms preceded them in fossil record (see next section, Silurian Vascular Plant). Therefore, the primitive status of Rhynia is questionable.
It is very likely that Rhynia attained its present structure and without evolving further had existed in this form for millions of years. In that case, it is truly primitive. This explanation is within the realm of possibility, as many plants are known which have existed for millions of years without any significant change. Alternately, if Rhynia is a reduced form from a more complex land plant, then it is not a primitive plant.
More significant about Rhynia is its simple nature and it is unquestionable.
The significant features of Rhynia are:
(a) Lack of differentiation into stem and root;
(b) Uniform dichotomous branching;
(c) Appendages that cannot be called leaves;
(d) Simple stele, a slender strand of tracheids;
(e) Sporangia with modified branch tips.
Therefore, Rhynia is a type which is as simple as one could conceive a vascular plant to be. Irrespective of whether Rhynia is a reduced form from complex ancestors or is truly a primitive form, in its simple organization it represents a ‘stage’ in the evolution of vascular land plants.
Silurian Vascular Plant:
A plant traced from Silurian of Australia stands as a landmark in palaeobotany.
Cooksonia is the smallest and simplest of vascular plants and at present it is accepted as the oldest known vascular plant. It is reported from Wales, USA, Canada, Scotland and Czechoslovakia. Specimens from Wales are from mid-Silurian; others are from lower Devonian.
Cooksonia plants were slender naked dichotomously branched axes (Fig. 7.3F) about 5-7 cm in length and 1-2 mm in diameter, with a thin vascular strand. Some specimens were with a vascular strand of annular tracheids. The axes terminated into sporangia nearly spherical in C. hemispherica and wider than long in C. pertonii.