Evolutionary Trends in Extant Plants

Evolutionary life-form trends among extant plants can be inferred sometimes based on the phylogenetic distribution of specific anatomical features that correlate with habitat preference. Leaf form, shape, and internal anatomy have been used to classify fossil as well as modern-day floras according to climatic conditions. Likewise, the anatomical details ofthe xylem in stems, roots, and leaves in conjunction with phylogenetic hypotheses have been used to adduce evolutionary trends.

For example, broad surveys of xylem anatomy indicate that species with shorter, narrower vessel members possessing simple perforation plates tend to grow in drier habitats than related species with longer, wider-vessel members. A higher proportion of species with scalariform perforation plates are generally found in mesic habitats, and species growing in xeric habitats tend to have vessel members with simple perforation plants. In the case of species with wide geographic distributions and vessel members possessing scalariform perforation plates, fewer bars per plate are typically observed for individuals growing in drier habitats. These and other trends in xylem appear to be adaptive in terms of the hydraulic and mechanical demands placed on roots, stems, and leaves. For some phylogenetic groups, like the monocotyledons, it is possible to infer evolutionary trends in habitat preference by examining the distribution of the different vessel perforation plates (and their occurrence in the roots, stems, and leaves) among related plant families (Figure 4).

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