Telome Theory in Pteridophytes: Merits & Demerits

Telome theory by Zimmermann is an important part of the research on the evolution of sporophytes in pteridophytes. It was believed that the primitive sporophytic plants had an axial nature and the leaves and roots evolved only later. These assumptions were stronger with the discovery of Rhynia which prompted many botanists to explain the process of further evolution. Among the various available theories, Zimmermann’s Telome Theory is the most important.

According to this Telome Theory, all vascular plants evolved from plants such as Rhynia which had no root or leaves but had only an axial body with sterile and fertile branches.

Telome Theory in Pteridophytes

The dichotomously branched axial plant body was divided into the terminals called telomes and the stem portion was called mesomes. Since the branches were either sterile or fertile, they were termed accordingly.

The sterile branches were phylloids and the fertile ones are called fertile telomes. The fertile telomes end in terminal sporangia. The sporangium had a stalk and was traversed by a sole vascular strand till the base.

As the evolution progressed, the dichotomous branches became sympodial by fusing two or more telomes. Such fused telomes became syntelomes or telome trusses.

The fused fertile telomes resulted in sporangial trusses while the sterile telomes formed the phylloid trusses. When different types of telomes fuse, they form mixed syntelomes or mixed telome trusses.

Zimmermann’s Postulates

The primitive vascular cryptogams originated from green algae.
The unicellular green algae formed a parenchymatous thallus through various divisions in all planes.
Later, an erect branched body was formed with the development of meristematic tissues that branched radially.
The appearance of alternative generations came next followed by dichotomously branched sporophytes.
According to Zimmermann, the algal ancestors were the ones who led to the evolution of vascular cryptogams in the Devonian and Silurian periods.
The evolved plants had no roots or leaves but had terminal sporangia with a vascular system of simple protostele.
The aerial and simpler stems had stomata while the roots had rhizoids or hairs.
Many of these sporophytes also had a subterranean plant portion called rhizomes.
The evolution of such primitive vascular plants into higher vascular cryptogams happened through three different lines called Lycopsid, Sphenopsid, and Pteridopsid.
This evolution happened through different progressive steps of organogenesis that occurred at various degrees in different groups of these cryptogams.

Steps of Telome Theory

According to Zimmermann, there were five elementary processes involved in the evolution of the cryptogams. They are,

Overtopping
Plantation
Syngenesis ir Webbing
Reduction
Recurving

Overtopping

The dichotomous branches started developing into unequal dichotomies with long and short branches. These short branches evolved into lateral shoots. These lateral shoots help in the development of a sympodial axis and later into a monopodial axis with lateral branches. It was these later branches that developed into leaves.

Plantation

Plantation is the process where the dichotomous branches that were present in different planes progressed into the same plane. It happened gradually at right angles and formed an important step in the evolution of leaves.

Syngenesis or Webbing

Syngenesis is the process where the adjacent telomes and mesomes connect with each other and fuse to form a webbing. The telomes and mesomes that came together developed parenchymatous tissues that helped join them. Here it was not just the tissues that were fused but also the steles as well. Syngenesis is of two types.

Foliar syngenesis

It is the fusion of the apical meristems of the telomes. This fusion led to the formation of marginal meristems which gave rise to lamina with veins. It was these meristems that enabled the fusion of the telomes and web formation and the latent foliar appendages.

When the foliar syngenesis is followed by overtopping, the resultant leaves are of pinnate venation. If the foliar syngenesis is accompanied by the webbing of vascular strands, it results in net veined leaves or reticulate venation. Overtopping, plantation, and foliar syngenesis, together helped in the evolution of megaphyllous leaves.

Axial syngenesis

The fusion of the branches refers to axial syngenesis. When they fuse, the protosteles in them fuse to form the more complex stelar organization. If the fusion is the result of parenchymatous webs, the vascular type thus produced is polystelic. The fusion of the branches happening in different methods produces different types of steles such as eustele, siphonostele, solenostele, etc.

Reduction

Reduction is the process that helped in the evolution of unbranched or simple microphyllous leaves as seen in Lycopodium, Isoetes, Selaginella, etc. The process involves the reduction of the syntelome into a simpler needle-like appendage on the lateral side.

Recurving

In recurving, the fertile telomes with erect sporangia curved into an inverted position. While Zimmermann called it recurvation, Wilson found out that there are two processes involved here.
Recurvation when the sporangia bends downwards as seen in Sphenophyta.
Incurvation is where the bent sporangia shift its position further towards the ventral surfaces of the foliar appendages. This led to the condition as seen in common ferns.
Later, Wardlaw brought all these processes under one term- recurvation. Zimmermann also found that the reduction process helped in the development of microphyllous leaves. The evolution of megaphyllous leaves was a result of the combined efforts of overtopping, plantation, and foliar syngenesis.

Origin of Sporophylls

According to Zimmermann, there are three divisions of the evolution of sporophylls in pteridophytes.

Sphenopsida or Arthrophyta

Zimmermann visualized that recurvation and syngenesis resulted in the evolution of sporangiophores in this division. The recurvation of the sporophytes was followed by the fusion and flattening of telomes and mesomes to produce a peltate disc. The intermediate processes formed the conditions as seen in fossils such as Hyneia, Calamophyton, Protocalamostachys, Eviostachya, etc.

Plantation followed by reduction resulted in sterile leaves in this division. The Calamophyton and Asterocalamites showed intermediate stages.

Lycophyta

In Lycophyta, sporangia are seen in the axils of microphylls and Zimmermann explains the steps that resulted in this condition.

The fertile and sterile telomes are aggregated.
The number of mesomes and the sporangia were reduced leading to the development of a single leaf with a single sporangium in its axil.
The intermediate type was seen with bifid tips of sporophylls and leaves in the extinct type of Protolepidodendron.

Filicophyta or Pteridophyta

In this division, the development of megasporophylls was a result of overtopping, reduction, and foliar syngenesis which led to the formation of pinnate sporophyll. The sporangia was positioned marginally and recurvation shifted it from the margins to the ventral side.

The intermediate stages were seen in extinct genera such as Botryopteris, Pseudosporochnus, and Stauropteris. Zimmermann added that the roots were developed from the primitive rhizomes before the evolution of leaves.

Merits of Telome Theory

Zimmermann’s theory was based on the comparison of living genera and fossil plants.
His assumptions were based on phyletic relationships between various living and extinct plants and were correct too.
The five elementary processes of evolution of sporophytes in pteridophytes might have been the same for the primitive land plants as well.
These steps simplify the theory of evolution of simple as well as complex plant parts.
Telome theory explains that there is an axis on which the sporophyte sits. The axis has an underground potion called roots and an aerial part called shoot. The appendages of the shoot are the sporophylls, sporangia, and sterile leaves.

Demerits of Telome Theory

The lack of reference to any extinct species to confirm the origin of microphyllous leaves of Lycophyta.
Telome theory applies only to a few pteropsids but that explains only the origin of this group and not of Lycopsids.

Enation Theory of Bower

Bower proposed the Enation Theory in 1935 about the origin of the microphyllous leaves. According to Enation theory, the microphyllous leaves originated as superficial outgrowths from the stem. These outgrowths are called enation and they don’t have any vascular strands. The enations continued to increase in its size and the vascular strand reached just its base.

Later the leaf traces entered the enations and traversed its length. This vascular strand was unbranched and enervating the characteristic of microphyllous leaves.

As per the Enation Theory, the microphylls originated by progressive elaboration process and not by reduction, as proposed by Zimmermann. Psilophyton, an extinct cryptogam of order Psilophytales, represents the first stage in this process. Asteroxylon forms the next stage with leaf traces reaching the base of the lateral appendage. In the species Drepanophycus, the leaf trace enters the lateral appendage.