Figure 2.21. Representative water-vascular cells in the vegetative body of Bromeliaceae. (A) One end of the type of tracheid widely distributed through the family. (B) Vessel element with multiperforate end plate that also occurs in most Bromeliaceae. (C) Vessel element in root of Pitcairnia sp. illustrating the most advanced structure for tracheary function present in the family. Redrawn from Cheadle (1955).
alone of the surveyed taxa exhibited vessel elements with transverse, simple perforate plates (Fig. 2.21). Members of this genus feature some of the most mesomorphic foliage described so far for Bromeliaceae, organs that oblige substantial streams of moisture to support relatively vigorous transpiration and photosynthesis (Chapter 4). Narrower cells lacking end walls (tracheids) or vessel elements with multiperforate end plates characterized the rest of the sample (Fig. 2.21).
Xylem strands made up exclusively of tracheids, or vessel elements with multiperforate rather than uniperforate end plates, seem best suited for most Bromeliaceae because so many species use water sparingly and succulence is so common (Table 4.1). So what can tracheary anatomy tell us about the evolutionary status of the family and the relationships among its species? Cells that just happen to t Cheadle s de nition as primitive probably resist cavitation by reason of the same supposedly archaic structure. On the other hand, xylem tensions recorded for Bromeliaceae have always been low, although most records come from succulent types that by virtue of this circumstance rarely, if ever, experience sufficient dehydration to rupture water columns in xylem capillaries (Chapter 4; e.g., Fig. 4.18). Clearly, Cheadle s story remains incomplete.
A biophysical critique mindful of the relationship between xylem anatomy and hydrodynamics is needed to explain why tracheary cell structure differs in the root vs. the stem and leaf of the bromeliads and Liliopsida in general. Is Cheadle s (1953) hypothesis supported, or do the environments of these organs and the demands placed upon them by the rest of the plant differ enough to explain his ndings without recourse to evolutionary history? How does tracheary cell morphology among the monocots, and the bromeliads in particular, relate to hydraulic demand and the maintenance of capillary integrity (safety) in different organs of the same plant and among species adapted to different growing conditions?
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