Plant: Plant Physiology and Sap Contains Solutes Essay

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he history of physiology shows many examples of the same debate on how biological systems function, some saying that this functioning is mainly the result of specific properties of the living systems, others affirming that physical laws still play a major role, at least in some aspects. In fact, as we now see more clearly, properties of living cells and physical laws which apply to all living and non living systems continuously interact and cannot be separated. The history of our understanding of sap ascent in plants, and especially in trees, is a beautiful example for this classical debate.

Period of Preparation (Before 1889)

Attempts to explain water transport in plants started with a search for analogies with blood circulation in vertebrates, a physiological process previously elucidated by William Harvey (1628) and Marcello Malpighi (1661). Malpighi, who was also a pioneer in plant anatomy, studied the construction of stem wood and became convinced that vessels (tracheae) served for air transport, while the fibres should transport water. Stephen Hales in 1727 was most likely, the first scientist to show that water in plants is transported unidirectionally from the soil to the transpiring leaves; he could only speculate on the tissues conducting water in the stem. In 1798, the Imperial Academy of Scientists in Erlangen (Germany) organized a prize competition to clear up two questions not yet fully understood at that time:

"1) Which one of the known main parts of a plant (cortex, sapwood, heartwood and pith) is conducting the ascending sap? 2) Is sap descending in the cortex towards the root and into it? And if so, which is the pathway from the interior parts into the cortex?"

Heinrich Cotta, who won the competition, gave the correct answer (sapwood) to the first question and, while not denying the descent of sap in the "cortex" (i.e., the phloem), emphasized the fact that this descending sap contains solutes in high concentration and is therefore different from the sap in the sapwood, so that plant saps do not truly circulate.

Although the transporting tissue became thus finally identified in the 18th and early 19th centuries, the pathway problem lingered on at the microscopic level well into the second half of the 19th century. There were lively disputes on whether the lumen of a conducting xylem element (a tracheid or a trachea) is filled with water, with air or with a sequence of air bubbles interspersed
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