Chapter V 



INFLUENCE OF CHANGES IN BAROMET- 

 RIC PRESSURE ON PLANT LIFE 



It was impossible not to wonder whether barometric pressure 

 has some direct or indirect effect upon the phenomena of vegeta- 

 tion. 



Everyone knows that as one ascends mountains, the vegetation 

 is modified. Certain species disappear, others appear which do not 

 grow in the plains. At great heights, vegetation becomes scanty, 

 and finally disappears. 



These changes in the flora have been carefully studied by 

 botanists who realize that not only altitude, but also latitude in- 

 fluence this geographical distribution of a new type. The habitats 

 of certain species or certain groups vary in altitude according to 

 the nearness to or the distance from the equator of the mountain 

 under consideration. 



These observations added to the fact that as one ascends the 

 temperature drops and the other fact that certain plants called 

 alpine are found at sea level in cold regions have led botanists to 

 think that the influence of altitude is only the influence of tempera- 

 ture; so that temperature alone is considered by the classic authors 

 as the determining factor of the characteristics of the flora of high 

 altitudes. 



There is no proof, however, that diminished pressure in itself 

 is not a factor in these differences; there is no proof that plants 

 of the plain would live at a very low barometric pressure, even if 

 the temperature there satisfied their needs. For plants, in fact, 

 to the absorption of oxygen is added the daily intake of carbonic 

 acid, and the effect of pressure upon these gases is not negligible. 



On the other hand, when we investigate the depths of the ocean, 

 we find that plant life ceases at depths which are not very great, 



780 



