Describe the merits and demerits of telome theory.

Some of the merits and demerits of Telome theory are as follows:

The interpretation of the plant body as a branched axis with its divisions specialized for various functions, but all like in fundamental structure, is called the telome theory. The telome theory is due to the work of Zimmermann.

He goes back to the earliest and simplest type of vascular plant known, that in the Psilophytales in which the body consists of branching leafless axes, which are all substantially alike. In some cases these may be arranged dichotomously in others monopodially, and the difference may have arisen by ‘overtopping’, but in either case these naked axes as the primitive units of construction of the cormophytes, which is thus regarded as primarily a system of axes.

Each of the terminal branches in such a system is called a telome. Each telome is an ultimate branch on an older axis or mesome. Some telomes are sterile; others bear terminal sporangia and are therefore fertile. Telomes also tend to unite into groups, called, syntelomes, which may be all sterile, all fertile or mixed.

According to telome theory the earliest levees are flattened telomes or phylloids, and the sporangium in all cases, together with its stalk or sporangiophore, is a fertile telome. Compound sporangiophores, like those of Equisetum, are syntelomes. The phylloids are the prototypes of the microphyllous foliage leaves.

In the most primitive conditions the sterile and fertile telomes were apparently quite independent of each other. To axillary position which the sporangial telomes occupy in Lycopsida has been arrived at by secondary shifting of the telomes and shows three ways in which the change may have taken place.

The plant body is thus a branching axis, its specialized underground segments are roots its aerial tips, if fertile, sporangia, if sterile, phylloids. “Leaves” consist under this interpretation, of one or more sterile telomes associated in many cases with one or more fertile telomes.

Leaf morphology thus supports the division of vascular plants into four major groups the Psilopsida are leafless or show leaves only partly differentiated; the leaf of the Lycopsida is an enation enlarged and elaborated; that of the Sphenopsida is a minor branch system, that of Pteropsida major one. The leaf is thus of different nature in vaseular plants; the evolutionary development of a photosynthetic organ has come about in more than one way.

Merits:-

This is a simple concept and explains most of the morphological problems about different organs of a plant. According to Bierhorst (1971) this theory is too simple and too easily applicable but unfortunately its excessive use has greatly diminished its value.

Demerits:-

1. Telome has been considered as a readymade unit. This difficulty was realized by Zimmermann (1949, 52) and subsequently he recognized several other elementary processes as 

     (a) interconnection of cells; 

     (b) rotation of cell axis; 

     (c) differentiation of apical cell, etc., which have led to the formation of an apical cell with      three cutting faces. However, these elementary processes do not satisfy the plant        morphologists (Puri, 1956).

2. Many other plants of much greater complexity, than Rhynia fossils have been discovered in beds of the same age or even earlier, e.g., Zosterophyllum, Baragnnathia (Leclercq, 1954) Lyon Hueber (1964), Hueber and Banks (1967) and Lyon (1964) observed lateral sporangia on short vascularized stalks in Psilophyton and Asteroxylon respectively instead of usual terminal sporangia.

3. According to the supporters of the telome theory all the leaves in plants are telomic in nature. Enation theory on the contrary considers microphyllous leaves as only outgrowths of the stem (Bower, 1935).

4. The polystelic condition in the axis of plants is supposed to have developed due to parenchymatic syngenesis of several monostelic axes. The actinostelic condition is supposed to be the product of radial fusion of steles in polystelic axis. Such an explanation is diametrically opposed to the widely accepted concept of stelar theory (Stewart, 1964).

5. Andrews (1963) has given a series of diagrams of Palaeozoic seeds to explain the origin of cupule. Pettitt (1970) found them to be more or less of the same age.

6. The theory has received little attention by angiosperm centred morphologists. Its application to stamens (Puri, 1947, 1951, 1955), venation pattern of leaves (Foster, 1950), morphological nature of angiosperm leaves and sporophylls, and carpels (Eames 1961) have been criticized from time to time.

                To sum up it would be worth to quote the reaction of Andrews and Eames. Andrews (1961) says, “Zimmermann’s scheme for the pteropsids, or atleast some pteropsids, has much supporting evidence, his concept for the articulates may be valid, but we are only in the verge of understanding the origin of this group, his concept for the lycopsids is, so far as I am aware purely hypothetical.”

Eames (1961) says, “consideration of the primitive plant body as made up of basic units, telomes, is doubtless of value for the understanding of more primitive taxa, but its value in the interpretation of the higher taxa, where axis and appendages have become established as morphological units, is doubtful.”