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The Evolution of Xylem Elements

Tracheids are typical for most pteridophytes and gymnosperms, wood vessels for angiosperms and well-developed gymnosperms, like the Gnetales. Wood vessels of another origin can be found in Pteridium, a fern, Equisetum, horsetail, and the roots of some Marsilea-species. Wood vessels are missing in the primary xylem of some primitive angiosperms (species of the families Winteraceae, Monimiaceae, Chloranthaceae or Tetracentraceae [I. W. BAILEY, 1944]). It has already been mentioned that good proof exists for the origin of wood vessels from tracheids. Xylem fibres, too, can be regarded as modified tracheids.

The evolution of multicellular terrestrial plants demanded the development of supporting and vascular tissues. Both functions are best met by elongated cells, but hardly by just one cell type: the supporting function needs the formation of thick and stable cell walls, while the conducting function is best exerted by cells with permeable walls. The first solution to meet this paradox was the juxtaposition of both thickened walls and pits in one and the same cell type (tracheids). This proved to be insufficient, when huge terrestrial plants, like trees, with a high rate of transpiration began to develop. A new concept evolved. Supporting and conducting functions were separated. Xylem fibres can be regarded as derivatives of tracheids. Their walls are more stable than those of the tracheids, while their conductive function was largely lost; simultaneously the supporting function of the tracheids diminished, while their specialization into conductive elements was perfected.

Still, the question of the evolutionary relation between tracheids and wood vessels remained to be answered. An alternative to wood vessels stemming from tracheids could be that their origin is unlinked. At the beginning of the 1930th enough proof had accumulated to favour one of the two hypotheses. The Americans F. H. FROST (1930/31), V. I. CHEADLE and I. W. BAILEY (1940s/1950s) were able to verify the accuracy of the first assumption. The line of reasoning is based upon logically comprehensible preconditions that have to be met to guarantee a direct genetic link between both structures. At least two of the preconditions can be regarded as fulfilled in the comparative analysis of tracheids and wood vessels:

1. On the assumption that one structure is more primitive than the other and that both are determined by the same genetic program, the derived, more developed structure would have to have properties that are typical of the more primitive structure. If this was not the case, the assumption that both are genetically linked would have to be discarded unless both structures had diverged that much since their separation that all common features were lost. How did the findings and data of tracheids and wood vessels relate to these thoughts?

2. The walls of primitive tracheids like pteridophytes are usually opened by round or elongated pits with either weak or no borders at all. They occur in special areas at the ends of the cells. Similar patterns can be found in many gymnosperms. Stacked, grove-shaped pits are common. This arrangement is in many ways similar to the scalariform perforation of the vessels. The main difference is the existence of a middle lamina in tracheids that is missing in vessels.

3. Perforation plates are typical for primitive angiosperms, while a complete reduction of the end walls is a feature of further developed taxa. A juxtaposition of both forms is normal.

4. Tracheids reveal no clear end walls. The end wall is also missing in primitive vessels. It becomes ever more distinct during perfection (in ontogenesis !).

5. If a certain feature can be regarded as primitive, then it is very likely that another, closely associated one, is equally primitive.
   • Tracheids are thin, elongated cells, while the cells of wood vessels are short and have a wide lumen. Numerous transitions exist. The longer the wood vessels, the more primitive they are. Their walls have nearly always pits. Pit areas with round pits are looked upon as primitive, scalariform perforations as more advanced and a complete opening as the momentary final stage in the development from tracheids to wood vessels. The widening of the cell lumen is coupled to a strengthening of the secondary cell walls.

   • Bordered pits developed independently from the pits just mentioned. They are constructed rather simple in the tracheids of ferns and angiosperms and are highly developed in contemporary gymnosperms.