Geology and Natural History. 503 



which is enveloped by a thin and porous membrane, less per- 

 meable than itself. Sometimes this has no normal pores, for in- 

 stance, in the apple, the orange. 



(6) It is probable that a gas which ti-averses the envelope can 

 penetrate to the deepest part of the tissue. 



(7) The gas passes through the external envelope of solid 

 masses, sometimes in the free state, and sometimes dissolved. 



(8) The changes which are produced at the surface depend on 

 the permeability and the porosity of the peridermic membrane. 



(9) Oxygen tends to enter especially by the pores, while car- 

 bonic acid escapes from the whole surface of the membrane. 

 There exists a veritable circulation of these gases, but the circu- 

 lation is only partial in the majority of instances. 



(10) Humidity acts in a very variable manner on the composi- 

 tion of the internal atmosphere, sometimes increasing permeabil- 

 ity, sometimes diminishing it. 



(11) Slow or rapid drying diminishes permeability and causes 

 a greater and greater accumulation of carbonic acid. 



(12) It is to these variations in the proportions of oxygen and 

 carbonic acid in the internal atmosphere that we must attribute 

 the different values of pressure in the internal atmosphere. 



(13) According as the total pressure is stronger or weaker in 

 the internal atmosphere, there is produced across the superficial 

 pores an inward or an outward current. This gaseous current is 

 purely mechanical, and can be regarded as a third series of 

 exchanges. 



(14) Nitrogen is passively held in this gaseous current. 



(15) When the gas acquires a constant difference of pressure, 

 in spite of the constant sweeping hither and thither, we must 

 believe that the current is produced in some other way than by 

 diffusion. A constant circulation of nitrogen exists in the aerial 

 parts of plants, but the circulation is purely passive. 



(16) Temperature increases or diminishes the intensity of res- 

 piration, and this modifies the composition of the internal atmos- 

 phere. 



(17) Light acts wherever green matter exists, sometimes 

 through chlorophylline assimilation, and sometimes by the with- 

 drawal of water, thus influencing the permeability of the mem- 

 branes. 



The mechanism of exchange can be summed up in the fol- 

 lowing conclusion : Gaseous exchanges of all solid organs hitherto 

 studied are produced in three different ways, which ordinarily co- 

 exist, but which act with a variable intensity. These are, Effu- 

 sion ; Dialysis; and a purely mechanical passage of a gaseous 

 current. 



Effusion. Diffusion of free gas across the pores of an envelope 

 under the influence of differences of pressure proper to each gas. 



Dialysis. Diffusion of gas, dissolved, across the membrane 

 under the same influence. 



