Question

This passage is adapted from an essay on eighteenth-century natural philosophy and early pneumatic chemistry.

In the summer of 1779, Dutch-born physician Jan Ingenhousz rented a small villa near London to conduct a concentrated series of over five hundred experiments on plant physiology. Building upon Joseph Priestley’s 1771 observation that vegetation possessed the restorative capacity to purify air fouled by combustion or animal respiration, Ingenhousz sought to isolate the precise physical conditions under which this phenomenon occurred. Priestley had mistakenly concluded that the mere presence of living plant tissue continuously cleansed the atmosphere. Ingenhousz, however, demonstrated through rigorous empirical trials that atmospheric restoration was neither continuous nor universal to all plant organs.

Ingenhousz's critical breakthrough lay in identifying sunlight as the indispensable catalyst for gas emission. By submerging fresh leaves of various terrestrial and aquatic species in clear glass jars filled with spring water, he observed the immediate accumulation of tiny gas bubbles on the submerged surfaces. When these glass jars were exposed to direct solar radiation, the bubbles formed rapidly and detached, rising to the top of the vessel. Upon collection and testing with a glowing wood splinter, this trapped gas proved to be highly enriched oxygen—or 'dephlogisticated air,' as it was known within the framework of eighteenth-century phlogiston theory. Conversely, when the exact same apparatus was placed in total darkness or shielded by opaque wooden cabinets, the production of oxygen ceased entirely. In fact, under dark conditions, the leaves actively degraded the surrounding air, producing carbon dioxide ('fixed air') in a manner analogous to animal respiration.

Furthermore, Ingenhousz systematically isolated different plant structures to determine whether the restorative property belonged to the organism as a whole or to specialized tissues. He noted that only the green foliage and green herbaceous stems emitted oxygen when illuminated. Non-green anatomical structures—such as petals, ripe fruits, woody bark, and roots—failed to generate oxygen under any lighting conditions. Instead, these non-green components consistently absorbed oxygen and released carbon dioxide, even when subjected to intense, focused sunlight. Ingenhousz further established that the rate and volume of oxygen production were governed strictly by the intensity of light falling upon the green surfaces, rather than by the ambient temperature of the water medium or the total mass of the submerged plant tissue.

Despite the clarity of his findings, Ingenhousz’s conclusions were initially met with skepticism by contemporary chemists who struggled to reconcile his results with prevailing theories of plant nutrition. At the time, standard botanical theory held that plants derived their substance exclusively from soil minerals and humus absorbed through root systems. Ingenhousz’s demonstration that leaves actively exchanged gases with the surrounding air suggested an atmospheric contribution to plant growth, laying the conceptual foundation for modern photosynthetic science. His meticulous documentation of light intensity, tissue color, and gas volumes established a new standard for quantitative rigor in experimental biology.

Based on the passage, Ingenhousz explicitly determined that the volume of oxygen generated by illuminated green plant tissue was directly governed by which of the following factors?