336 PHYSIOLOGY 



coenocyte from which it escapes. This may also be the case in guttation 

 among seed plants. The epithem of the water chamber, receiving an 

 adequate supply of water from the adjacent vein, may develop turgor 

 sufficient to cause water to pass the cytoplasm and the wall. It is ob- 

 vious that to issue from the free surface it will encounter less resistance 

 than elsewhere; consequently it takes this direction. The chamber 

 fills and water soon oozes from the water pore. But the epithem can- 

 not develop an adequate turgor unless the water supply is sufficient. 

 That may be made sufficient either by checking the transpiration, or by 

 forcing water up to these cells so that they may get enough, even though 

 transpiration is unchanged. Water may be supplied thus artificially by 

 cutting the stem and attaching it to a water tap; or the same end would 

 be accomplished in nature if the root cortex had a supply adequate to 

 enable it to become fully turgid and exude water under pressure into the 

 conducting system. 



" Root pressure." — The condition just mentioned often exists in the 

 root cortex, and perhaps always when plants are not flaccid. The loca- 

 tion of this turgor has suggested for it the name " root pressure." This 

 is unfortunate, because it tends to obscure the fact that any live thin- 

 walled cells with like conditions may develop a turgor which will cause 

 water to exude. Thus, bleeding was found to occur in the inflorescence 

 of some tall palms, but the root cortex had no part in so distant an exuda- 

 tion; the pressure originated near the base of the flower stalk. The 

 " root pressure " being a frequent cause of bleeding, the phrase " bleed- 

 ing pressure " has been suggested as a substitute; but this is little better, 

 since whether or not bleeding results is purely incidental. No special 

 term is needed other than turgor pressure; that is general and is specific 

 enough. (See also p. 349.) 



Amount and pressure. — Experiments on bleeding are often con- 

 ducted with potted plants, which are decapitated, and to the stump is 

 affixed apparatus for measuring the amount of water exuded, or the 

 pressure with which it is forced out. With trees, the trunk is bored 

 and the receptacles or gages attached. A few examples will give an 

 idea of the maximum quantity of the sap and the pressures involved. 



A calla lily bled 39 cc. in 24 hours. A vigorous European grape some- 

 times exudes nearly a liter per day. The Mexican agaves, cultivated 

 for this purpose, are said to give out 5-6 liters daily for several months. 

 Under favorable conditions, the sugar maple yields 5-8 liters in the 

 course of a day, and the birches give out about as much. 



