Note 84 — IDEAS FOR A PHYSIOGNOMY OF PLANTS.
Henry G. (Henry George) Bohn translation
p. 222—“Whether the height of the aërial ocean and its pressure have always been the same.”
The pressure of the atmosphere has a decided influence on the form and life of plants. This life, owing to the fulness and abundance of the leafy organs provided with interstitial openings, is principally directed outwards. Plants mainly live in and through their surfaces, and hence their dependence on the surrounding medium. Animals are more dependant on internal stimuli; they generate and maintain their own temperature, deriving from muscular movements their electric currents, and the chemical vital processes which arise from and re-act upon those currents. A kind of cutaneous respiration constitutes an active vital function of plants, and depends, so far as it is an evaporation, inhalation, and exhalation of fluids, on atmospheric pressure. Hence Alpine plants are more aromatic and hirsute than others, and more amply provided with numerous exhalants.[NU] Zoonomic experiments teach us, as I have shown in another work, that organs are more abundant and more perfectly developed in proportion to the facility with which their functional requirements are fulfilled. The disturbance occasioned in the respiration of their external integuments, by increased barometric pressure, renders it, as I have elsewhere shewn, very difficult for Alpine plants to thrive in the plain.
296Whether the aërial ocean surrounding the earth has always exerted the same mean pressure is a question wholly undecided. We do not even know for certain whether the mean barometric height has remained the same during a hundred years at any one given spot. According to the observations of Poleni and Toaldo, this pressure appeared variable. Doubts were long entertained regarding the accuracy of these views, but the more recent investigations of the astronomer Carlini render it almost probable that in Milan the mean barometric pressure is on the decrease. Perhaps the phenomenon is very local, and dependent on periodic variations in descending currents of air.