Parallelism and Reversibility in Xylem Evolution a Review

The irreversibility of the major trends of xylem evolution, such as the origin of vessels in primitive angiosperms with long fusiform initials, and the shifts from scalariform to simple perforations and from tracheids to libriform fibres, has long been accepted by wood anatomists. Parallel development of these and other xylem features is generally accepted, and is suggested by the distribution patterns of the fibre and perforation plate type. Some recent phylogenetic analyses of seed plants suggest that there also have been some reversals in these general trends. The likelihood and extent of parallel origins and reversions of the major trends in xylem specialization are explored here by analysing a number of published hypotheses on the phylogenetic relationships within wood anatomically diverse major clades of angiosperms, and within some individual families. On the basis of these analyses, it appears that for these major Baileyan transformation series, parallelisms were more than twice as common as reversals. Functional adaptations to increased efficiency and safety of hydraulic architecture can largely explain the high incidence of parallelisms in xylem evolution.

Morphology of the outer integument in three primitive angiosperm families

Scanning electron microscope examinations were performed to characterize the developmental morphology of the outer integument in several species of the three families Annonaceae (Cananga odorata var. fruticosa, Goniothalamus macrophyllus, and Stelechocarpus burahol), Eupomatiaceae (Eupomatia laurina), and Winteraceae (four Tasmannia species). In all species examined, the inner integument arises as an annular ring, but the outer integument initiates as a semiannular outgrowth interrupted on the concave side of the funiculus; the outer integument then becomes hood-shaped. The inner integument overtops the outer one at maturity, and the micropyle consists only of an endostome. Data from the present and previous studies support the hypothesis that the outer integument is hood-shaped in magnolialean angiosperms and that the bilaterally symmetrical, bladelike outer integument is homologous with the ovuliferous sporophyll of the glossopterids. The micropyle consisting of an endostome might be in a primitive state, compared with a bistomic one. Key words: developmental morphology, integument, micropyle, phylogenesis, primitive angiosperms, scanning electron microscopy.

Vessel Occlusions in Plants: Morphological, Functional and Evolutionary Aspects

The minimum pit aperture diameter of a vessel-parenchyma pit pair was found as the decisive wood anatomical feature for vessel occlusion by either tyloses or gums. Based on this observation, as well as on considerations and established knowledge about the functional significance of vessel occlusions, an evolutionary hypothesis is presented. In order to withstand microorganisms and embolisms, plants are able to occlude their vessels with tyloses or gums. The most primitive Angiosperms show tylosis formation. With the decreasing pit sizes in the vessel wall during plant evolution, gum formation was developed, whereas the increasing vessel sizes led in some plants to renewed tylosis formation.