Expert: Dana Krempels, Ph.D. - 2/24/2008

Question
Hi Dana,

I'm wondering, in which group of plants (Bryophytes, Seedless vascular plants, gymnosperms, or angiosperms) did leaves first evolve. I know the lycophytes (seedless vascular) have microphylls, so is that when leaves first evolved?

If so, how did bryophytes and earlier plants photosynthesize?

thanks

Answer
Dear Lisa,

Plant organs (root, stem and leaf) are defined by the presence of true vascular tissue.  Because bryophytes lack vascular tissue, their structures that superficially resemble leaves are not trechnically classifed as true leaves, though they serve similar function of gas exchange and photosynthesis.

Liverworts and mosses (and hornworts) have a body called a "thallus" because it's not really differentiated into true leaves, roots, and stems (though some mosses have the precursors of vascular tissue in their stalks and leaf-like structures).  But these structures could certainly be considered reminiscent of the ancestral structures that eventually led to true leaves.  They're loaded with chloroplasts, and they have stomates.  Transmission of photosynthates occurs from cell to cell in these simple plants, since there is no true phloem.

The most primitive vascular plants were the first ones to, technically at least, evolve true leaves containing xylem and phloem.  So the ancestors of modern club "mosses" (seedless vascular plants) would be good candidates for the first true leaf-bearers.  But ancestral bryophytes had the beginnings from which these structures did evolve.

Hope that helps!

Dana