456 SCIENCE PROGRESS 



In order to appreciate the general mode of working of these 

 living cells a glance at their essential structure will not be out 

 of place. A typical plant cell consists of an outer membrane, 

 commonly composed of cellulose, which is extensible and also 

 elastic within varying limits. Within this is the protoplasm, in 

 form like a hollow bag completely filled with a solution of 

 various substances. The cell wall and the protoplasm are freely 

 permeable by water but the protoplasm does not allow the 

 substances dissolved in the water contained within it to pass 

 outwards ; in fact, the protoplasm is only a partially permeable 

 membrane and the dissolved substances, which are contained in 

 the hollow (vacuole) of the bag, are the cause of the "osmotic 

 tension " which is so characteristic and important a feature of the 

 plant cell. By its means the protoplasm is tightly pressed up 

 against the cell wall, which in its turn is thus forcibly stretched 

 until the tension of the wall is in equilibrium with the osmotic 

 pressure excited by the contents of the protoplasmic vacuole. 

 This pressure may amount to as much as ten atmospheres and 

 even more. 



When living cells are freely supplied with water they absorb 

 it and assume a turgid condition ; the tissues built up of such 

 cells can exhibit a considerable degree of rigidity. 



Inasmuch as turgid cells are always stretched, it is evident 

 that any agent which tends to reduce turgescence will effect a 

 shortening of the tissues affected ; also that the amount of con- 

 traction will be related to the elasticity of the wall. Furthermore, 

 if the cells on one surface of a sheet of tissue are alone affected, 

 the tissue as a whole will curve and become concave on the 

 surface from which the water has been withdrawn ; a correspond- 

 ing change of form would result if the cells of one surface were 

 so altered that the osmotic pressure of their contents was 

 increased. A good example showing the influence of a one-sided 

 alteration of pressure is afforded by the Tulip flower. Most 

 people who have used these flowers for indoor decoration are 

 aware that if brought from a cold into a warm room the flowers 

 open widely — often much too widely — ,and that they will close 

 again if transferred to a cool place. These movements, which 

 formerly were attributed erroneously to uneven growth, are due 

 primarily to a difference between the cells of the upper and 

 lower surface in their relation to water and consequently in their 

 turgescence. If the tulip be watched while the expansion is 



