152 PLANT PHYSIOLOGY 



more rapidly than nitrogen, but these gases make their way 

 less rapidly through drying cellulose and lignified mem- 

 branes, though more rapidly through cuticula and cork, 

 than does water-vapor. We have seen that the wood, which 

 consists mainly of the walls of dead cells, is the path of the 

 water-currents from root to leaves, that only rarely if ever 

 are the wood elements fiUed with water, and that generally 

 they contain air and water in alternating columns ( Jamin's 

 chains ) . Whenever transpiration is more rapid than water- 

 transfer, the air-pressure within the plant will decrease ; there 

 will be the largest volume of air under the least pressure 

 when transpiration has most reduced the amount of water, 

 and when the vessels are fullest of water there will be the 

 least volume of air under a pressure equal or nearly equal to 

 that of the atmosphere. The constant changes in the rate 

 of transpiration, causing differences in the water content of 

 every cell, living and dead, and in the amount of water-vapor 

 in the intercellular spaces, will cause constant changes in the 

 volume and pressure of the gases within the plant. 



More strictly vital activities also affect the gas pressures. 

 As may most conveniently be demonstrated on submersed 

 aquatics, photosynthesis tends, other things being equal, to 

 produce a gas pressure within the plant greater than that 

 outside. Because the stomata of a land plant are usually 

 open while the plant is manufacturing food, the free ex- 

 change of gases between the plant and the outside air keeps 

 the pressures about equal. But in submersed aquatics — e. g: 

 Elodea, CeratopbjUum, Myriopbyllum, etc. — the gas press- 

 ure may be made evident in two ways. In the first place, 

 the buoyancy of the plant increases while it is illuminated, 

 indicating the accumulation of gas in its body ; and second, 

 the classical experiment of inverting and cutting or prick- 

 ing the stem* shows that a stream' of bubbles, often 

 countable and therefore offering an index of the photosyn- 

 thetic activity, issues from the cut surface or from the 

 wound. The explanation of this phenomenon of increased 

 pressure is this : the carbon-dioxide used in the elaboration 



* For description see Darwin and Acton's or Detmer-Moor's laboratory 

 manuals. 



