CHAPTER IV 

 THE AIR 



i. Atmospheric Pressure. Growth under decreased or increased atmospheric pres- 

 sure. Wieler's and Jaccard's experiments. Atmospheric pressure on high mountains. 



2. The Air contained in Water. Solubility, composition, and diffusion of air in water. 

 Devices for the absorption and transport of oxygen in aquatic plants. Aerenchyma and 

 other aerating tissues. Pneumatophores. G. Karsten's and Greshoffs experiments. 



3. The Wind. i. Wind and Tree-growth. Mechanical influences. Drying influences. 

 Injurious effects of the latter on tree-growth, ii. Wind and Reproduction. Anemo- 

 philous leaves. Their frequent presence in windy localities. Anemophilous devices 

 for dispersal of seeds. Importance in relation to distribution at great distances apart. 

 Treub's observations at Krakatoa. 



This chapter will not deal with the endless and manifold relations 

 between vegetation and the atmosphere, but will treat only of some 

 phenomena connected with the oecological character of certain formations 

 of plants or with the distribution of species. 



1. ATMOSPHERIC PRESSURE. 



As Wieler and Jaccard have shown, the pressure within the stratum of the 

 atmosphere in which plants grow does not at all correspond to the absolute 

 optimum pressure for the growth of plants. On the contrary, a diminu- 

 tion of the partial pressure of the oxygen — for the latter only, and not the 

 total atmospheric pressure, comes into question — occasions an acceleration 

 in growth until a certain low pressure is attained, which is constant for 

 each species, and beyond which any further diminution in pressure causes 

 a retardation in the rate of growth. We find this absolute optimum 

 atmospheric pressure for growth to be in the case of Helianthus annuus 

 about 100 mm., but for Vicia Faba about 200 mm. Again, an increase in 

 atmospheric pressure above 760 mm. (or the corresponding pressure of 

 oxygen) up to about i\ atmospheres occasions a retardation, but after that 

 an acceleration in growth. There are therefore for growth two absolute 

 optima of atmospheric pressure, both of which differ considerably from the 

 pressures that prevail in the inhabited stratum of air, the one being at a far 

 lower, and the other at a far higher oxygen-pressure. 



According to Jaccard a decrease in the pressure of oxygen occasions not 

 only more rapid growth, but also richer branching in the axes and roots, as 

 well as an increase in the size of the leaves. Jaccard's following tabular 



