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Show ANNOTATIONS A D ADDITIONS. 311 ( 14 ) p. 23 .-"If the height of the aerial ocean and its p1·ess1tre have not always been the same." The pressure of the atmosphere has a decided influence on the form and life of plants. From the abundance and importance of their leafy organs provided with porous openings, plants live principally in and through their surfaces; and hence their dependence on the surrounding medium. Animals are dependent rather on internal impulses and stimuli ; they originate and maintain their own temperature, and, by means of muscular movement, their own electric currents, and the chemical vital processes which depend on and react upon those currents. A species of skin-respiration is an active and important vital function in plants, and this respiration, in so far as it consists in evaporation, inhalation, and exhalation of fluids, is dependent on the pressure of atmosphere. Therefore it is that alpine plants are more aromatic, and are hairy and covered with numerous pores. (See my work, iiber die gereizte Muskel- und Nervenfaser, bd. ii. s. 142-145.) For, according to Zoonomic experience, organs become more abundant and more perfect in proportion to the facility with which the conditions necessary for the exercise of their functions are fulfilled-as I have elsewhere shown. In alpine plants, the disturbance of their skin-respiration, occasioned by increased atmospheric pressure, makes it very difficult for such plants to flourish in the low grounds. The question, whether the mean pressure of the aerial ocean which surrounds our globe has always been the same, is quite undecided : we do not even know accurately whether the mean height of the barometer has continued the same at the same place for a century past. According to Poleni' s and Toaldo' s observations, the pressure would have seemed to vary. The correctness of these observations has long been doubted, but the recent researches- of Carlini render it almost probable that the mean height of the barometer is diminishing in Milan. Perhaps the phenomenon is a very local one, and dependent on variations in descending atmospheric currents. |
