Telome Theory

The "Telome theory" of Walter Zimmermann (1930, 1952) is the most accepted theory that is based on fossil record and synthesizes the major steps in the evolution of vascular plants.
It describes how the primitive type of vascular plants developed from Rhynia like plants.

1. TELOME THEORY Submitted By: Deepak Das Roll No M.Sc. Life Sciences (Specialization in Plant Sciences)

2. Contents Introduction Meaning of Telome Theory Elementary Processes Concept of Telome Theory in Evolutionary processes Merits Demerits

3. Introduction Aquatic plants → Land plants Early vascular land plants Evolved from Rhynia Rhynia

4. Telome theory was first proposed by – Walt Zimmermann in 1930 Based upon : Fossil record Evolutionary modification : True Leaf True Stem True Root Complex Vascular System Protected Sporangia

5. Meaning of Telome Theory Telome - “Single nerved ultimate terminal portion of a dichotomising axis ” 2 types of Telome : Fertile Telome – Terminated by Sporangium Sterile Telome – Without Sporangium The connecting axex between two telomes - Mesomes Several telomes grouped together to form Telome truss/ Syntelome Syntelome : (i) Phylloid Truss – Only Sterile telomes (ii) Fertile Truss – Only fertile telomes

6. A figure of Rhyniaceae showing different telomes Kenrick , P. (2002). The telome theory.  Developmental genetics and plant evolution ,

7. Process of Telome Theory Rhynia group of plants underwent certain evolutionary processes Overtopping Planation Syngenesis Reduction Curvation

8. Overtopping One of the two dichotomising branches outgrows the other and become larger and mechanically stronger Larger branch forms – Axis Overtopped branch forms – lateral branches Overtopping process

9. Planation Planation Process Rearrangement of telomes and mesomes from a three dimensional pattern to a single plane

10. Syngenesis Syngenesis Process Telomes and mesomes fuse tangentially Parenchymatous tissue is developed in between Known as webbing process In stem of Selaginella polystelic condition due to - simple parenchymous webbing

11. Reduction Reduction Process Activity of terminal meristem of each telome supressed resulting into much shorter branches Responsible for – Microphyllous leaves of lycopsida and Sphenopsida Needle like leaves of conifers

12. Curvation Recurvation process The fertile Telomes become curved or bend downwards Two sub processes Incurvation - downward shifting of the sporangia from terminal or ventral surface of the leaf in Pteropsida Recurvation - downward bending of the sporangia occurs along with the sporangial stalks in Sphenopsida

13. Concept of Telome Theory Origin of Sporophylls in Lycopsida Planation of the fertile and sterile Telomes Reduction of the mesomes and in number of spornagia Protolepidodendron and Psilotum – like axillary sporangia and single veined leaves are evolved

14. Origin of Sporophylls in Sphenopsida Recurvation and Syngenesis processes are involved The intermediate developmental stages are seen in some fossil genera like Calamophyton , Hyenia , Protocalamostachys and Asterocalamites

15. Origin of Sporophylls in Pteropsida Overtopping , reduction, syngenesis and incurvation are the processes involved Pinnately -veined sporophylls with marginal sori developed by the lateral fusion of mesomes In many ferns, the sori are shifted downwards due to the incurvation process

16. Merits of Telome Theory Describes the origin and evolution of sporophytes of land plants Structure of the sporophytes of the most primitive known plants is defined The exact relationship between the root, stem and leaves Connects the living and fossil plants.

17. Demerits of Telome Theory Does not explain how a telome -like characteristic body has been developed [ Bower (1935 )] Telome theory does not explain the whorled or spiral arrangement of sporangia [ Thomas (1950 )] For origin of Lycopsida , it is somewhat hypothetical [ Andrews (1960 )] Does not provide an acceptable origin of all leafy structures It does not explain the derivation of the dictyostelic condition [ Stewart (1964 )]

18. Reference Beerling , D. J., & Fleming, A. J. (2007). Zimmermann's telome theory of megaphyll leaf evolution: a molecular and cellular critique.  Current opinion in plant biology ,  10 (1), 4-12. Kenrick , P. (2002). The telome theory.  Developmental genetics and plant evolution , 365-387. O.P.Sharma (2016), Series on “Diversity of Microbes and Cryptogams – Pteridophyta ” ( pp 28.1-28.5) New Delhi, India, McGraw Hill Education Pvt. Ltd. Vashishta , Sinha , Kumar (2006), Pteridophyta ( pp 32-36) New Delhi, India, S. Chand & Co Pvt. Ltd.

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