106 PHYSIOLOGY OF NUTRITION 



migration of gases through different kinds of plant septa has been investigated 

 by many authors. The most recent and extensive studies on the molar or 

 streaming movement and the diffusion of gases through plant cell walls are due 

 to Wiesner and Molisch. 1 In these experiments a piece of dry plant tissue was 

 fastened over one end of a straight glass tube (6 mm. in internal diameter and 

 50 to 100 cm. long) with sealing wax, and the joint was then covered with a 

 mixture of equal parts of resin and beeswax. When soft, succulent tissues 

 were employed, the tissue was kept in place by a perforated metal cap, and was 

 kept from being crushed by rubber rings, the openings of which just fitted the 

 end of the glass tube. The tube was partly or entirely filled with mercury and 

 the open end was closed with the finger while the tube was inverted, the open 

 end being then placed in a vessel of mercury. The tube was finally ar- 

 ranged in an upright position, with the mercury below. After a number of 

 days the height of the mercury column in the tube was measured. 



An experiment with birch bark may serve as an example. A piece of white 

 periderm, 0.09 mm. thick, was used. The height of the mercury column in the 

 tube was 400 mm. at the beginning of the experiment and remained the same, 

 after fourteen days, the usual corrections for temperature and pressure having 

 been applied. Wiesner and Molisch came to the following conclusions from 

 the result of these experiments. 



1. Plant cell walls, either wet or dry, whether the cells are alive or dead, are 

 impermeable to the molar movement of gases under ordinary pressures. 



2. Protoplasm and cell sap are likewise impermeable to this kind of gas 

 movement, so that there is no movement of air as such through tissue without 

 intercellular passages. This experiment explains how negative gas pressure 

 (i.e., pressure less than that of the surrounding atmosphere) in wood may 

 be maintained, which will be discussed later. 



Similar tubes filled partly with mercury and partly with various gases were 

 employed in experiments upon the diffusion of gases through dry and moist 

 plant membranes. The velocity of outward diffusion was indicated by the rate 

 of rise of the mercury column in the tube. An experiment with periderm of the 

 potato tuber may be taken as an example. Two tubes were filled with carbon 

 dioxide, one being closed with a dry, the other with a moist piece of periderm. 

 In the tube with the dry membrane the mercury rose only 5 mm. during a period 

 of thirty days, while the tube with moist membrane showed a corresponding rise 

 of about 40 mm. This experiment shows that the interchange of gases through 

 the wet membrane occurred according to the principles of diffusion of dissolved 

 gases; carbon dioxide passed outward through the membrane more rapidly than 

 air passed inward, thus causing the mercury to rise in the tube. If the septa 

 to be studied were permeable to, but did not dissolve the gas (as in the case of 

 a dry porous clay plate), then, according to the law of gas diffusion, the 

 mercury column should fall. From a series of experiments similar to this, these 

 authors, came to the following conclusions: 



1 Wiesner, J., and Molisch, H., Untersuchungen iiber die Gasbewegung in der Pflanze. Sitzungsber. 

 (math.-naturw. Kl.) K. Akad. Wiss. Wien. o8 7 : 670-713. 1890. 



