‘Telome Theory’ of Sporophyte Evolution in Pteridophytes
Evolution of Sporophyte in Pteridophytes
According to a general belief the earliest and the primitive sporophyte of the land plants was axial in nature. The leaves and branches were evolved later by a modification of the central stem. These primitive land plants were supposed to have rootless and dichotomously branched green stem. The vascular system was a simple protostele. Some of the ultimate branches terminated in sporangia. This assumption is supported by the discovery of some simple plants in the upper Silurian and Devonian periods of earth’s history. (Rhynia, Horneophyton, Psilophyton, Asteroxylon and others) Discovery of Such plants led may botanists to propose theories explaining the appearance of leaves, branches, root and sporangia on the sporophyte. These theories tend to explain the process of elaboration of the primitive sporophyte into the modern sporophyte possessing roots, leaves and variously disposed sporangia. Of all these theories Telome theory of Zimmermann (1930) has attracted great attention and has a large number of supporters.
This theory was first proposed by Zimmermann in 1930 and later elaborated by him in 1938 (Manual of Pteridology, 558-618). Zimmermann postulated that all the vascular plants evolved from a very simple, leafless and a dichotomously branched ancestral type like Rhynia. Such a plant is made up of sterile and fertile branches. The fertile branches terminated in sporangia. The simple and terminal parts of a dichotomously branched stem were designated as telomes. The telomes meet each other at the point of forking. Portions of the stem below the forking were called mesomes. The telomes were classified as sterile or vegetative telomes also called phylloids and fertile telomes. The fertile telome consists of a terminal and a stalked sporangium usually traversed by a single vascular strand up to its base. As evolution proceeded there was a change from dichotomous to sympodial branching as a result of the fusion of two or more telomes called telomes trusses or syntelomes. The fertile telome trusses were called the sporangia trusses and sterile telome trusses were called the phylloid trusses. Both the types of syntelomes also occurred together to form mixed telome trusses or mixed syntelomes.
Zimmermann postulated that the primitive vascular cryptogams originated from the green algae. The unicellular green algae divided in all planes to from a parenchymatous thallus. Later meristematic tissues developed and erect radially constructed branches came into existence. It was followed by the appearance of distinct alternation generations. The sporophyte branched dichotomously and possessed a central conducting strand. Such algal ancestors, according to Zimmermann led to the evolution of early vascular plants of the upper Silurian and Devonian periods. They possessed heteromorphic alternation of generations and their sporophyte were simple and dichotomously branched. Some of the branches terminated in sporangia. They possessed no leaves and had no roots. The vascular system was a simple protostele. The aerial and simple stem possessed stomata. The function of roots was performed by hair or rhizoids. These sporophytes had subterranean portions called the rhizomes.
These early land plants evolved into the higher vascular cryptogams along three independent lines of evolution. These were the Lycopsid, Sphenopsid and Pteropsid trends of evolution. During the course of this evolution certain elementary processes of progressive differentiation took place in the sporophytes. These can be listed as:
(i) Overtopping, (ii) Planation, (iii) Webbing or syngenesis, (iv) reduction and (v) recurvation. All these elementary processes of organogenesis were believed to have occurred in varying degrees in the different taxonomic groups. These will be discussed separately.
The equally dichotomizing axes developed unequal dichotomy.This resulted in the formation of short and long branches. The short branches appeared as lateral shoots. This led to the development of a sympodial axis which ultimately changed to a monopodial axis with lateral branches. These lateral branches metamorphosed into leaves.
The equal dichotomies were originally in more than one plane. They were arranged in plan successively at right angles. During the process of planation, the dichotomies became arranged in a single plane. It is an important process that led to the evolution of the leaf.
- Syngenesis or Webbing- As the name indicates the adjacent telomes and mesomes were connected with each other by the development of a parenchmatous tissue between them. This is also called parenchmatous webbing. During this process the steles of the concerned telome also fused. Syngenesis was considered to be of two types by Zimmermann.
- Foliar Syngenesis- During this process, there is coalescence or fusion of apical meristems of the telomes. When these meristems fuse to form a marginal meristem, a lamina with veins develops. The activity of these meristems leads to the formation of parenchymatous webs between the telomes. Such developments result in the formation of lateral foliar appendages. In case of foliar syngenesis is accompanied by overtopping the leaves with pinnate venation are formed. Sometimes foliar syngenesis is accompanied by fusion of the vascular strands. These results in development of net-veined leaves or leaves with reticulate venation. Overtopping, planation and foliar webbing brought about the evolution of a megaphyllous leaf.
Zimmermann visualised that during this process there was absorption of a number of branches into a single stout axis. The fusing branches had protosteles and their fusion led to the appearance of a complex stelar organization. In case, the shoots united by the formation of parenchymatous webs the resulting stelar organisation was a polystelic condition. In case during axial syngenesis, the steles anastomosed in different manners resultant stellar organization was of diverse types, e.g. siphonostele, eustele, solenostele.
It is supposed to have brought about the evolution of simple and unbranched microphyllous leaves of the lycopods (Lycopodium, Selaginella, Isoetes). It was brought about by the reduction of the syntelome to a single needle-like lateral appendage.
During this process, the fertile telomes were supposed to become reflexed. As a result, the sporangium assumes an inverted position. Zimmermann called this process as incurvation. Wilson (1953) recognized two processes.
- Recurvation- During this process, the sporangia bent downwards as in sphenophyta.
- Incurvation- This led to the shifting of the sporangia to the ventral surfaces of the foliar appendages thus bringing about a condition found in the ferns.
Wardlaw (1952), however, pointed out that all these changes can be embraced within one term, which he calls as recurvation.
While discussing the above listed five elementary processes of organogenesis as postulated by Zimmermann, it becomes evident that microphyllous leaves originated as a result of the process of reduction. The megaphyllous leaves of ferns (Filicophyta) originated as a result of combined process of overtopping, planation and foliar syngenesis. We shall now discuss the origin of the fertile leaves or the sporophyll as visualized by Zimmermann.
Origin of Sporophylls
The origin of sporophylls in the three main divisions of the vascular cryptogams has been visualized differently by Zimmermann.
Spheniophyta or Arthrophyta-
Recurvation and syngenesis were supposed to be the elementary processes that led to the origin of the sporangiophores the members of this division. Recurvation led to the downward bending of the sporangia. It was followed by fusion and flattening of the telomes and mesomes to form a peltate disc. Intermediate stages leading to this condition existed in the fossil members of this division. e.g. Calamophyton, Hyenia, Eviostachya, and Protocalamostachys, Planation followed by reduction, however, led to the development of the sierle leaves of this division. The intermediate stages are met with in the fossile genera Calamophyton and Asterocalamites. The Telome theory gives a satisfactory explanation of the evolution of sporangiophores and sterile leaves in the Sporophyte.
The sporangia are usually borne in the axils of microphylls in the lycophyta. Such a position was derived by Zimmermann along following steps :
- There was aggregation of fertile and sterile telomes.
- There was reduction in the number of mesomes and the sporangia. This led to the development of a single leaf with a single sporangium in its axil.
The bifid tips of the sporophylls and leaves of the extinct genus Protolepidodendron may be cited as an example of an intermediate type.
Filicophyta or Pterophyta-
The elementary processes of overtoppiag, reduction and foliar syngenesis lead to the development of the megaphyllous sporophylls of the ferns. As a result of overtopping planation and syngenesis, a pinnate sporophylls was evolved. The sporangia were marginal in position. Later recurvation led to the shifting of the sporangia from the margins to the ventral side, intermediate type were provided by the extinct genera Pseudoporochnus, Stauroptris and Botoryopteris.
Zimmermann postulated that the roots developed from the creeping rhizomes of the primitive or ancestral types before the evolution of the leaves.
Merits of the Telome Theory
It is an outcome of a master mind that has portrayed, in a skilful manner the origin and evolution of the sporophytes of land plants. Zimmermann has based his theory mostly on account of comparative study of the fossil as well as living genera of the vascular plants. Many of his assumption are correct and based on exact phyletic relationships between the various groups of plants both live and extinct. His five elementary processes of planation, overtopping, syngenesis, reduction recurvation give us a unified concept of the manner in which evolution might have proceeded in the land plants. These processes explain in a simple and lucidway as to how the primitiveland plants led to the evolution of both the simple and complex land plants of today. It explains in a nice manner that the entire sporophyte is an axis that has an underground portion called the root and an arial part called the shoot. The appendages of shoot that is the sporophylls, sporangia and sterile leaves are nothing but modified parts of the stem. While building up his theory Zimmermann has taken into consideration the morphological aspects of the various groups of extinct and livng groups of plants.
Demerits of the Telome Theory
This theory is open to criticism in its application to the origin of Lycophyta (Lycopsida). The origin of microphyllous leaves of the Lycophyta by reduction of telome trusses is not exemplified by any living or extinct vascular plant. Andrews (1960) has expressed his views on the theory in the following words. “Zimmermann’s scheme for the pteropsids or at least some pteropsids, has much supporting evidence; his concept for the articulates may be valid, but we are only on the verge of understanding the origin of this group; his content for the lycopside is, so far as I am aware, purely hypothetical.
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