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Biology of Dryland Plants

Mauseth, James D. [1].

Anatomy of stem-succulent dryland plants.

Evolution of succulence has numerous developmental consequences. As stem volume increases, absolute water-storage capacity increases and S/V decreases if shape does not change, reducing the tendency to lose water. But succulent shoots shrink during droughts and swell after rains, putting their epidermis at risk of being torn; a pleated surface (having ribs or tubercles) allows volume to change while surface area remains constant. During drought, water is lost preferentially from water-storage cells and transferred to photosynthetic cells: storage cells have thin undulate or plicate walls but chlorenchyma cells have thicker walls. Evolutionary broadening of cortex moves epidermis away from stem xylem, putting epidermis at risk of desiccation as diffusion becomes too slow to keep it hydrated. Cacti have cortical bundles, so their cortex is free to evolve to be extremely broad. Epidermis is persistent, not ephemeral. Bark formation is delayed and superficial and does not shed cortex tissues. Succulent stems weigh more per meter, so succulent shoots are less branched: whereas an ordinary tree is produced by thousands of apical meristems, a succulent stem is produced by few. Having fewer apical meristems, each meristem cell must divide more times, increasing replication errors within meristem cells themselves. As succulent plants lost their leaves, they also lost their leaf venation, which unloads water from xylem: stem-succulent plants unload water from secondary xylem, a tissue that typically transports water but does not unload it. Water should be stored in both cortex (readily available to chlorenchyma) and wood (where it reduces cavitation risk or is used to refill cavitated vessels). Most succulents have increased wood parenchyma, but cacti also have wide-band tracheids that shrink as they donate water, thus reducing their volume to match the volume of water they contain, eliminating the possibility of cavitating.


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Related Links:
Research on cacti in Mauseth lab


1 - The University of Texas, Integrative Biology, 1 University Station A6700, Austin, Texas, 78712, USA

Keywords:
cactus
xeric
anatomy
meristems
vessel refilling.

Presentation Type: Symposium or Colloquium Presentation
Session: 41-4
Location: Salon K - Austin Grand Ballroom/Hilton
Date: Wednesday, August 17th, 2005
Time: 9:00 AM
Abstract ID:44


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