NUTRITION 47 



must engage in the study of physical chemistry. For us, 

 however, who are studying only the elements of plant- 

 physiology, the mention of a few of the simplest principles 

 concerned, will sufficiently illuminate the subject. First, a 

 vacuum cannot be produced adjacent to an unconfined 

 volume of gas; the gas will flow, will be drawn, into the 

 space whence gas is being removed in the attempt to form a 

 vacuum. Second, the molecules of adjacent and unconfined 

 volumes of gases of different compositions will move spon- 

 taneously until the gases are perfectly mixed. Third, the 

 same molecular movement, which is called diffusion, will 

 take place when gases of different composition are separated 

 from one another by permeable substances. Most animal 

 and vegetable membranes are permeable. If one gas consist- 

 ing of oxygen and carbon-dioxide is separated only by a 

 permeable membrane from another gas consisting of oxygen 

 alone, carbon-dioxide will pass from the first through the 

 membrane into the second until there are equal proportions 

 of this gas on the two sides of the partition, and oxygen 

 will pass from the second into the first until the proportion 

 of oxygen to carbon-dioxide is the same in the two volumes. 

 If for any reason ( because of higher or lower temperature, 

 for instance) the pressures of the two gases are unequal, 

 diffusion of both gases will continue until the pressure be- 

 comes the same on both sides of the separating membrane. 

 Diffusion tends, then, to equalize the pressures and to pro- 

 duce uniform composition in adjacent volumes of gases either 

 unconfined or separated only by permeable substances. 



Turning now to plants, we find that the simplest plants, 

 consisting either of single cells or of cells in filaments or 

 small masses, enclose gases by the permeable membranes 

 bounding their cells. In higher plants, which consist of a 

 larger number of cells in larger masses, the gases are not 

 only enclosed within the cells by permeable membranes, but 

 the spaces between the cells also contain gases. These spaces, 

 and hence the volumes of gas which fill them, are continu- 

 ous with the unconfined mixture of gases which forms the 

 atmosphere. Any difference, therefore, between the compo- 

 sition of the mixture of gases filling the intercellular spaces 



