According to the telome theory, leaves (megaphylls) originated from isotomous, cruciate branches of dichotomous shoot systems (e.g., Rhynia). Reduced growth in alternating branches of successive dichotomies, overtopping, formed lateral branch sets. The cruciate branches were reoriented to one plane (planation) followed by lateral fusion (syngenesis or webbing) to form flat, isotomously veined laminae. Revision of the telome concept here is based in part on development of primary leaves in some ferns (e.g.,Woodwardia virginica) that have shown minimal change since their origin. In a Rhynia type shoot system, reduced growth in alternating branches of successive dichotomies (anisotomy) accomplished overtopping and the establishment of lateral indeterminate sets of isotomous, cruciate branches. In each set, progressive growth reduction basipetally established a determinate branch system, the most primitive expression of a new structure--the leaf. With a change from three to two division planes in the apical cell of each initial lateral truss, the branches became flat instead of cylindrical (fasciation). The elements of each lateral truss thus appeared collectively as a deeply divided, simple leaf with dichotomous cruciate lobes. A gradual or sudden increase in the rate of basipetal growth reduction produced a shallow-lobed lamina, not by lateral fusion of adjacent free elements (webbing), but by progressive equalization of lobe and interlobe growth. The intact blade in early ontogeny was curved because of the cruciate placement of the second dichotomy. Flattening occurred at the third set of dichotomies where basipetal reduction of growth and lamina development forced planation on the cruciate system. Divided and compound leaves evolved later with a resurgence of anisotomous growth within simple laminae.

Key words: Leaf origin, Megaphyll, Telome Theory