The telome theory was formulated by Walter Zimmermann in the 1930’s and developed over the next 30 years (Zimmermann 1930, 1931, 1938, 1952, 1959, 1965) with a good general explanation for English-speaking audiences by Wilson (1953). Fundamentally, the theory attempts to explain the origins of all major plant organs as derivations of the simple structures of the earliest land plants of the upper Silurian and lower Devonian. These plants had already switched from the bryophyte photosynthetic gametophyte to a photosynthetic and persistent sporophyte. These small upright plants were nothing more than dichomotously-branching axes (stems and roots), some of which terminated in sporangia. Zimmermann referred to the intermediate vegetative axes as mesomes and the terminal axes he called telomes. See Figures 1-2.

Zimmermann explained the formation of megaphylls, the complex leaves of the types that are seen in ferns and seed plants, as being reduced branch systems in which overtopping (unequal branching) and planation (the organization of the side branches into a planar arrangement) had occurred. Following that, flat webs of tissue developed between the planar branches. Figure 3 shows approximate times and examples of fossils that illustrate those stages. Evidence for this explanation can be seen in ferns today in which sporangia are only found on veins of leaves (Figure 4). The interpretation is that veins are highly reduced axes with fertile telomes (the small branch bearing the sporangium) branching off of the mesome (the vein of the leaf). Further evidence can be seen in the vascular stele. When vascular tissue branches from the stele, it leaves a gap (called a leaf gap) above it. The presence of a leaf gap defines a megaphyll no matter how small it might be (Figure 5).

Zimmermann also proposed that in many plants selective pressure to prevent herbivory and/or increase photosynthetic surface area led to the development of enations and spines. These secondarily were innervated by vascular tissue which was not derived from a branch system. Many of the early plants showed spines and other types of structures that clearly were not leaves. Look at the illustration of Sawdonia below (Figure 6). Its axes had spines and lateral sporangia, a consequence of overtopping the fertile telomes by the vegetative or sterile telomes. Figure 7 illustrates a scenario by which

There is no general agreement on the power of the Telome Theory to explain the origins of leaves. Some (e.g. Kenrick and Crane, 1997; Galtier, 2010) affirm the independent evolutionary origins of microphylls and megaphylls but believe that Zimmermann pushed his theory too far and should not be used to explain the origins of stele types and seeds. Vasco et al. (2016) provide evidence that both types of leaves have a deep molecular relationship but the sporangial developmental program led to apparant differences. Using the same developmental regulatory gene families (e.g. KANADI and Class III HD-Zips), Zumajo-Cardona et al. (2019) observe that the regulation of leaf development is fundamentally different in microphyllous plants relative to other other vascular plants.