CHARACTER OF THE CELL MEMBRANE AFTER DEATH 119 



amount of solution within until some of the solution is forced 

 up the tube. The solution rises in the tube because the increase 

 in the amount of solution within the bladder requires more space, 

 and is, therefore, accompanied by an increase in pressure against 

 the wall of the bladder. This pressure, which in this case de- 

 pends upon the concentration of the sugar solution in the bladder, 

 might become so great as to burst the bladder, if no tube for an 

 outlet were provided. This pressure, known as osmotic pressure, 

 has been found to follow quite well the laws governing gas pres- 

 sure. Consequently, if the number of molecules of the dissolved 

 substance contained in a certain volume of the solution is known, 

 the osmotic pressure can be calculated. Thus 342 grams of Cane 

 sugar in 1 Uter of solution (called a gram-molecular solution) will 

 exert a pressure of about 22.3 atmospheres or 336 lbs. and in 

 whatever proportion the number of grams is increased or de- 

 creased, the pressure is altered in a similar proportion. Osmotic 

 pressure in cells, called turgor pressure, is usually not less than 50 

 lbs. and often more than 100 lbs. per sq. inch. The rigidity of 

 organs such as leaves, soft stems, and roots is largely due to turgor 

 pressure, as can be easily shown by immersing strips of a fresh 

 Beet or Radish in a strong solution where they lose water and 

 become flaccid. The wilting in leaves when exposed to excessive 

 evaporation is due to the loss of turgor pressure, which occurs 

 whenever cells lose water more rapidly than they absorb it. The 

 preservative value of such substances as salts and sugars when 

 apphed to meats and fruits depends largely upon the withdrawal 

 of water from the micro-organism, so that they can not become 

 active. Wilted cells, if not dead, will also draw in water and again 

 become turgid when put in contact with moisture. In this way 

 flowers are revived by placing their stems in water, and Cucum- 

 bers, Lettuce, and Celery are made crisp by putting them in cold 

 water. Sometimes, as the pollen of some plants illustrates when 

 immersed in water, the pressure becomes so great that the cells 

 burst. Even fruits, such as Plums, sometimes burst on the trees 

 from this cause when the weather is warm and moist. 



The Character of the Cell Membrane After Death. — With the 

 death of the cell, the cell membrane ceases to be an osmotic mem- 

 brane and thus becomes permeable to all substances in solution. 

 After the cell membrane is dead substances pass through it, 

 either into or out of the cell, almost as easily as through a piece of 



