Origin of Pteridophytes (Origin of Vascular Cryptogams)

Origin of Pteridophytes

Origin of Pteridophytes

To explain the origin of the independent sporophyta and are based on two assumptions:

  1. The vascular cryptogams originated from the bryophytes. Bower (1935), Lignier (1903), and Zimmermann (1930, 1938) regard Bryophytes and Pteridopytes as two divergent lines originating from a hypothetical group of archegoniate land plants. Campbell (1895) and Smith (1958) believed them to have originated from an anthocerotean type of a bryophyte.
  2. The adherents of the second view believe algae to be the ancestors of the vascular cryptogams. The proponents of this idea trace the origin of the pteridophytes from various algal groups. Scott (1900), Bohlin (1901), Lotsy (1909), Chruch (1919), Fritsch (1916, 1945), Eames (1936), Arnold (1947), Gregus (1955). Andrews (1956), Axelrod (1959) and Mehra (1968) are the chief proponents of the idea of algal origin of the pteridophytes.

Anthocerotean Theory

It was put forth by Campbell (1859) and later supported by Smith (1938). Presence of intercalary meristem and photosynthetic tissue in the sporophyte of Anthoceros lead Campbell (1895, 1899) to put forth the view that such sporophytes are very near to the independent sporophytes of the vascular cryptogams.

He conjectured, “Were the foot of the sporophytes in Anthoceros prolonged into a root penetrating the earth, it would become quite independent of the gametophyte, and were a special assimilative organ of leaf developed, a condition directly comparable to the sporophyte of the lower Pteridophytes or ferns would result”. The discovery of the rootless, leafless and dichotomously branched psilophytales led Smith (1938) advance his views favouring Campbell’s views. Smith followed the origin of vascular cryptogams from Anthoceros-like ancestors along the following assumptions:

  1. The intercalary meristem of Anthoceros-like sporophytes shifted to the apex and initiated dichotomous branching.
  2. Dichotomous branching necessitated the formation or terminal sporangia like those of the Psilophytales (Rhynia and Psilophyton).
  3. Metamorphosis of the anthocerotean columella into a conductive strand.
  4. Occurrence of conducting tissue in the lower portions of the sporophyte of Anthoceros fusiformis (Campbell, 1924) and restriction of spore formation to apical regions alone.

Campbell (1939) compared the simple gametophytes of Anthoceros with the green algae. Smith also pointed towards some superficial resemblances between the embryos of Psilotum and Tmesipteris with those of Anthoceros.

This theory has not received wide acclaim and has almost been discarded by the modern botanists.

The Strobilus Theory

It was put forth by Bower in 1894 and supported the origin of sporophytes of vascular cryptogams from the liverworts. According to this theory the simple sporophyte of the liverworts evolved into an elaborate and a complex structure along the following lines:

  1. There was progresive sterilisation of the sporogenous tissue in the liverwort sporophyte. It led to the formation of alternating sterile and fertile regions.
  2. The fertile regions or sporangia became superficial in position.
  3. The sterile parts gave rise to lateral projections called the enations. The enations were either below the sporangia or besides them. At this stage the entire sporophyte looked like a strobilus. The sporangia lie in the axils or at the bases of the enations. Such a condition was visualised in Phylloglossum, Isoetes and those species of Lycopodium where all the leaves are sporophylls.
  4. Ultimately the sporangia shifted on to the enations. The enations could them be called the sporangiophores or the sporophylls.
  5. Further sterilisation led to the disappearance of sporangia on the sporophylls. This step led to the formation of sterile lateral appendages called the leaves.
  6. The central columella-like part increased considerably in size and became an axis or the stem bearing leaves and sporophylls.
  7. The leaves and sporophylls later became large and elaborate.
  8. The roots developed from the base of the stem and stamped the sporophyte as an independent plant.

This theory has also been discarded and has only historical importance. Discovery of a number of extinct plants belonging to the psilophytales does not support the existence of such a simple sporophyte as pictured by Bower. All these primitive land plants possess branched stems that were not small. The enation theory is even now believed because it explains the origin of micophyllous leaves, but Bower has postulated that the microphylls which originated as enations later developed into large megaphyllus in his strobilus theory. This latter part has not appealed the morphologists. Later, Bower, however, realised this and regarded his enation theory to explain only the origin of microphyllous leaves. He proposed another sequence leading to the origin of megaphyllous leaves, which later became the basis of Zimmermann’s telome concept.

Protocorm Theory

This theory was put forth by Treub in 1884. He regarded the protocorm of the Lycopodium embryos as primitive organ and considered it to be an ancestral feature that has been retained in some contemporary species (Phylloglossum and Lycopodium). He postulated that it might have been of common occurrence in the ancestors of the vascular cryptogams. This idea did not receive the general approval of the morphologists and is now of historical importance only. Bower (1907, 1935) regarded the protocorm as “opportunist growth” and not a primitive feature.

Phyton Theory

This theory was put forward by Celakovsky in 1901. He denied the existence of the stem as an independent member. His theory was closely related to the strobilus theory of Bower because he postulated that the early land plant, when in a strobilus stage was nothing but a cluster of leaves or sporophylls. Axis developed later from the sporophyll bases. Schoute (1931) in his paper entitled “On Phytonism” severely criticised Celakovsky’s memoir and stated that his theory has only constructed a scheme of thought without giving any decisive proof in support of his ideas, which can be explained otherwise also.

Wolff (1759) and Gaudichaud (1841) were the earlier authors who gave the idea of phytonism’ to the plant morphologists. Wolff made a very frank statement that he saw nothing in the plant except the leaves and stem. He considered the roots to be portions of the stem. The stem was regarded to be a mere prolongation of the leaf stalks. His views were later elaborated by Gaudichaud (1841) and Celakovsky (1901).

Algal Origin of the Vascular Cryptogams (Pteridophytes)

  1. Greguss’s Hypothesis:

    Greguss put forth his views in 1955. He traced the origin of vascular plants from the three major divisions of algae, i.e., Chlorophyta, Phaeophyta and Rhodophyta. He did not take into consideration the pigment constitution, the products of photosynthesis, and the presence or absence of cilia in the reproductive unties. He laid stress on the branching of the organisms i.e., whether it is dichotomous, monopodial or verticillate.

He derived mosses from Chlorophyceae and liverworts from the Phaeophyceae. In the same way he traced the origin of the Rhynia and Horneophyton from the Chlorophyceae and Psilotum and Tmesipteris from the Phaeophyceae. Lam (1957) criticised Greguss’s hypothesis and regarded it as based on unsound footigns. He almost disregarded the phylogenetic relationship of the plant groups while tracing their origin e.g., Psilophytales and Psilotales that are quite closely related to each other have been regarded as descendants of two different algal groups.

  1. Andrews’ Hypothesis:

    Andrews (1956, 1959) also believed in the polyphyletic origin of vascular plants. His conclusions are based on the discovery of some peculiar fossils that are not vascular plants but can be regarded as descendants of marine algae that made several attempts to conquer land. These are nematothallus, Crocalophyton and Protosalvinia. The former was discovered by Lang (1937) from the upper Silurian of. England and possessed firm cuticle covering its body. Their spores were also cutinized and had a resistant wall. Crocalophyton possessed tracheid- like structures in its body. Proto-salvinia was discovered from Upper Devonian of Ohio. Its body was made up of distinct short stalk bearing a spherical and bilobed head. It was covered with cuticle and possessed distinct conceptacles bearing spore tetrads. The spore walls were thick and resistant.

Occurrence of such plants led Andrews to believe that a number of algal groups attempted to conquer land. All these algal groups gave rise to the different types of vascular plants. His belief was strengthened due to diversity in the morphology and structure of the various groups of land plants such as Lycophyta, Psilophyta, Articulatae (Sphenophyta) and Pterophyta. He considered that morphological and structural variations presented by these main division of vascular cryptogams are due to their origin along various independent liens of evolution from different algae rather than from 3 or 4 groups. He gave a vivid description of the important fossils from the early Palaeozoic in 1959 and then commented on Zimmermann’s telome concept. He supported his concept regarding the evolution of the pteropsids and sphenopsids, but disregarded his concept for the evolution of the lycopsids and dismissed it as a purely hypothetic approach.

  1. Leclereq’s Hypothesis:

    Leclereq (1954, 1956) also believed in the polyphyletic origin of the vascular-cryptogams. She postulated that various divisions (Psilophyta, Lycophyta, Sphenophyta, Pterophyta) originated along independent lines running down separately into an ancestral algal complex. As a result of her palaeopalynological studies of the early land plants that inhabited the earth from the upper Devonian down to the Cambrian, she concluded that the land plants originated in the pre-Cambrian period. Her studies on the form and structure of the spores in the upper and middle Devonian revealed a striking correlation between these spores and the megaflora. She also studied the spores from lower horizons like Ordovician and cambrian period and concluded that land plants exists in those ages. They must, therefore, have originated in the Pre-Cambrian periods. She also reviewed the extinct land plants belonging to the middle Devonian, upper Silurian and Cambrian periods. She found that the Rhyne fossils (Rhynia, Horneophyton) that were discovered by Kidston and Lang were of quite simple organisation as compared to those which occurred in earlier periods like the upper Silurian. The fossils belonging to this period were quite complex and elaborate and, therefore, led her to conclude that simple psilophytes like Rhynia and Horneophyton are not primitive and ancestral, but represent the last remnants of a dying race that might have flourished in the earlier ages (Pre-Ordovician). She concluded that the psilophytes like Rhynia and Horneophyton are the last and recent in the series that had an independent origin from the algal stock.

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