Soru

In late-nineteenth-century botanical physiology, explaining the ascent of sap to the crowns of tall trees posed a severe mechanical paradox. Atmospheric pressure can sustain a water column of barely ten meters under a vacuum, yet sequoias and eucalyptus trees elevate water past one hundred meters. Early investigators postulated "vitalist" mechanisms, asserting that living cells flanking the xylem vessels acted as rhythmic metabolic pumps to propel fluid upward. However, Eduard Strasburger demonstrated that tall tree stems treated with lethal picric acid continued transpirational ascent for weeks, decisively refuting the requirement of living metabolic tissue. In response, Henry Dixon and John Joly formulated the cohesion-tension theory, which posited that water columns exist in a metastable liquid state under immense negative pressure (tension). According to this model, evaporation from sub-stomatal leaf cavities generates a pulling force transmitted continuously down the xylem conduit, reliant entirely upon the strong intermolecular hydrogen bonding (cohesion) between water molecules and adhesion to xylem walls. While critics objected that cavitation—the spontaneous nucleative formation of air bubbles (embolisms) under tension—would inevitably rupture the column and permanently arrest transport, proponents demonstrated that bordered pits in xylem tracheids operate as capillary safety valves, isolating embolized vessels and diverting flow through adjacent saturated channels.

Based on the passage, which of the following statements regarding the mechanisms of fluid transport in plants are most strongly supported? Select all that apply.