176 PHYSIOLOGY OF NUTRITION. 



produced collects in the intercellular spaces ; it does not pass out 

 into the water by dialysis at the same rate as it is formed, and 

 consequently the gas in the intercellular spaces soon stands at a 

 positive pressure. In fact, if we prick the stem of one of the 

 plants with a needle, a very rapid, but it is true instantly 

 diminishing, stream of bubbles at once springs from the wound. 

 If we repeat this experiment with plants which have been kept 

 in darkness for a time under water, the injury does not lead to 

 this copious liberation of gas, since the Carbon dioxide formed in 

 the process of respiration cannot much affect the pressure of the 

 intercellular gases. It is easily soluble in water, and can readily 

 pass over by dialysis from the plants into the medium surrounding 

 them. To acquaint ourselves more minutely with the intercellular 

 system of Elodea canadensis, we subject transverse sections of the 

 stem to microscopic examination. Under the feebly differentiated 

 epidermis lies a comparatively well-developed cortex, which in turn 

 surrounds the axile bundle. The cells of the cortical tissue have 

 small intercellular spaces between them, but besides these there is 

 present in the cortex a circle of large air-canals. 



The air in the intercellular spaces may, however, under certain 

 conditions have a slight negative pressure, being more rarefied 

 than the atmospheric air. This may be due to a variety of con- 

 ditions, but I will consider only one case. If branches of trees 

 are exposed to direct sunlight, their cortical tissue especially will 

 frequently attain a higher temperature than the surrounding air. 

 The gases in the intercellular spaces expand, and if lenticels are 

 present some of it escapes through them. The air remaining in 

 the intercellular spaces is now more rarefied than that of the 

 atmosphere. 



The fact that the gases in the elements of the wood, especially 

 at the time of active transpiration, stand at a considerable negative 

 pressure is very important, and is of great significance in the 

 explanation of many phenomena of plant life. We shall later 

 return to this remarkable phenomenon of the negative pressure 

 of the air in the wood, but will first fix our attention on some 

 conditions which stand in relation to. it. 



It is first to be noticed that the cork tissue, if we leave out of 

 consideration lenticels, does not allow gas to pass through it, even 

 under considerable pressure. 1 By means of sealing-wax I fixed a 

 thin transverse section of cork (which had been prepared from a 

 small cork by means of a razor) air-tight over the opening of the 



