<P> Vessels first evolved during the dry, low CO periods of the Late Permian, in the horsetails, ferns and Selaginellales independently, and later appeared in the mid Cretaceous in angiosperms and gnetophytes . Vessels allow the same cross-sectional area of wood to transport around a hundred times more water than tracheids! This allowed plants to fill more of their stems with structural fibres, and also opened a new niche to vines, which could transport water without being as thick as the tree they grew on . Despite these advantages, tracheid - based wood is a lot lighter, thus cheaper to make, as vessels need to be much more reinforced to avoid cavitation . </P> <P> Leaves are the primary photosynthetic organs of a modern plant . The origin of leaves was almost certainly triggered by falling concentrations of atmospheric CO during the Devonian period, increasing the efficiency with which carbon dioxide could be captured for photosynthesis . </P> <P> Leaves certainly evolved more than once . Based on their structure, they are classified into two types: microphylls, which lack complex venation and may have originated as spiny outgrowths known as enations, and megaphylls, which are large and have complex venation that may have arisen from the modification of groups of branches . It has been proposed that these structures arose independently . Megaphylls, according to Walter Zimmerman's telome theory, have evolved from plants that showed a three - dimensional branching architecture, through three transformations--overtopping, which led to the lateral position typical of leaves, planation, which involved formation of a planar architecture, webbing or fusion, which united the planar branches, thus leading to the formation of a proper leaf lamina . All three steps happened multiple times in the evolution of today's leaves . </P> <P> It is widely believed that the telome theory is well supported by fossil evidence . However, Wolfgang Hagemann questioned it for morphological and ecological reasons and proposed an alternative theory . Whereas according to the telome theory the most primitive land plants have a three - dimensional branching system of radially symmetrical axes (telomes), according to Hagemann's alternative the opposite is proposed: the most primitive land plants that gave rise to vascular plants were flat, thalloid, leaf - like, without axes, somewhat like a liverwort or fern prothallus . Axes such as stems and roots evolved later as new organs . Rolf Sattler proposed an overarching process - oriented view that leaves some limited room for both the telome theory and Hagemann's alternative and in addition takes into consideration the whole continuum between dorsiventral (flat) and radial (cylindrical) structures that can be found in fossil and living land plants . This view is supported by research in molecular genetics . Thus, James (2009) concluded that "it is now widely accepted that...radiality (characteristic of axes such as stems) and dorsiventrality (characteristic of leaves) are but extremes of a continuous spectrum . In fact, it is simply the timing of the KNOX gene expression!" </P>

When did land plants first appear on earth