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Modeling fluid flow in Medullosa, an anatomically unusual Carboniferous seed plant
- Jonathan P. Wilson, Andrew H. Knoll, N. Michele Holbrook, Charles R. Marshall
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- Journal:
- Paleobiology / Volume 34 / Issue 4 / Fall 2008
- Published online by Cambridge University Press:
- 08 April 2016, pp. 472-493
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Medullosa stands apart from most Paleozoic seed plants in its combination of large leaf area, complex vascular structure, and extremely large water-conducting cells. To investigate the hydraulic consequences of these anatomical features and to compare them with other seed plants, we have adapted a model of water transport in xylem cells that accounts for resistance to flow from the lumen, pits, and pit membranes, and that can be used to compare extinct and extant plants in a quantitative way. Application of this model to Medullosa, the Paleozoic coniferophyte Cordaites, and the extant conifer Pinus shows that medullosan tracheids had the capacity to transport water at volume flow rates more comparable to those of angiosperm vessels than to those characteristic of ancient and modern coniferophyte tracheids. Tracheid structure in Medullosa, including the large pit membrane area per tracheid and the high ratio of tracheid diameter to wall thickness, suggests that its xylem cells operated at significant risk of embolism and implosion, making this plant unlikely to survive significant water stress These features further suggest that tracheids could not have furnished significant structural support, requiring either that other tissues supported these plants or that at least some medullosans were vines. In combination with high tracheid conductivity, distinctive anatomical characters of Medullosa such as the anomalous growth of vascular cambium and the large number of leaf traces that enter each petiole base suggest vascular adaptations to meet the evapotranspiration demands of its large leaves. The evolution of highly efficient conducting cells dictates a need to supply structural support via other tissues, both in tracheid-based stem seed plants and in vessel-bearing angiosperms.
10 - Drought responses of neotropical dry forest trees
- Edited by Stephen H. Bullock, Harold A. Mooney, Stanford University, California, Ernesto Medina
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- Book:
- Seasonally Dry Tropical Forests
- Published online:
- 07 September 2010
- Print publication:
- 24 November 1995, pp 243-276
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Summary
Introduction
Many neotropical dry forests are dominated by trees that shed their foliage and remain leafless for a substantial period each year. Because the majority of deciduous species drop their leaves during the dry season and renew their canopies with the onset of the rains, the question of how these trees cope with seasonal reductions in soil moisture and increases in evaporative demand is most simply answered by calling them ‘drought avoiders’ (sensu Levitt, 1972). This categorization, however, gives little insight into the conditions, constraints and consequences that accompany the deciduous habit in these forests. Furthermore, the coexistence of even a small number of evergreen species indicates that the deciduous habit is not unconditionally imposed by the environment and that patterns of leaf fall and renewal must be viewed as part of an integrated response to environmental conditions. Seasonality in water availability clearly plays a major role in structuring patterns of activity and growth in this biome, but there have been few studies of the dominant life form (and fewer of life-form diversity; see Medina, Chapter 9). In this chapter, we address characteristics of trees of tropical dry and deciduous forests that influence their patterns of water use.
Plants respond to changes in resource availability on several scales. Our review considers three such levels: ‘structure’ encompasses features that remain relatively constant throughout the life of a plant, such as rooting patterns or stem hydraulic properties; ‘physiology’ focuses on parameters that influence diurnal patterns of water use and gas exchange; while ‘phenology’ considers seasonal patterns of meristem activity.
3 - Biomechanical studies of vines
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- By Francis E. Putz, University of Florida, N. Michele Holbrook, Stanford University
- Edited by Francis E. Putz, University of Florida, Harold A. Mooney, Stanford University, California
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- Book:
- The Biology of Vines
- Published online:
- 05 November 2011
- Print publication:
- 31 January 1992, pp 73-98
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Summary
Although vines and self-supporting plants differ physiologically, anatomically, and phenologically, the primary differences are biomechanical. Lacking the capacity to hold themselves upright, climbing plants are constrained by their capacity to (i) encounter suitable structure(s) on which to climb; (ii) ascend efficiently; and, (iii) survive the inevitable mechanical demise of their supports. In this chapter, we address some of the biomechanical features of climbing plants within the context of their life history. We emphasize that vines have broken free from the constraints of having to be self-supporting, only to suffer new restrictions associated with mechanical dependence. Most of our comments pertain better to vines that climb by twining or with tendrils than to plants that climb with the aid of adventitious roots or other adhesive structures. Our bias towards woody vines (= lianas) in tropical forests (Figure 3.1) also will be apparent.
Locating suitable supports
Availability of suitable supports is a major constraint on the height growth of forest vines. The vast majority of vine ‘seedlings’ (and vines that have reached the top of their support) produce searcher shoots that grow up, bend over, and are successively replaced as each, in turn, fails to encounter a support (Palm, 1827; Darwin, 1867; Putz, 1984). In lowland forest in Panama, vines experimentally supplied with artificial trellises grew to be much taller than did control individuals (Putz, 1984). Vines can increase their likelihood of encountering a suitable support by: (i) actively searching; (ii) waiting (searching in time); and (iii) being able to ascend a wide range of support sizes.