216 BOTANY PART i 



depends on the part always taken by the protoplasm, so that the 

 reaction observed is not the direct effect of an external cause, but a 

 very indirect result. Further, according to the condition of the proto- 

 plasm, the same factor may produce different effects. An example 

 will make this clear. 



If the free end of a flexible rod is placed horizontally, it will bend downwards 

 to a definite point as the result of its weight. A part of a plant will behave 

 similarly, and if dead, as for instance a withered stem, will remain in the position 

 it thus assumes. If, however, a living growing stem has been used in the experi- 

 ment it will exhibit an effect of gravity which is very surprising in comparison 

 with the purely physical effect. The growing portion of the stem curves, and by 

 its own activity becomes erect again ; it thus moves against the force of gravity. 

 If the experiment is made with a tap-root, this will curve vertically downwards 

 much further than its own weight would cause it to do. A rhizome (e.g. of Scirpus], 

 on the other hand, will place its growing tip horizontally when it has sunk by 

 its own weight out of the horizontal plane. In these three experiments the 

 physical conditions are the same. The weight of the earth acts on a horizontally- 

 placed portion of a plant. The results in the three cases are as different as 

 possible. 



The explanation of this remarkable behaviour of the plant is to 

 be sought in the fact that, while to begin with gravity influences it as 

 it would influence an inorganic structure giving weight to the mass 

 this primary physical change then acts as what is called a stimulus. 

 This liberates inner activities of the plant which have neither quantita- 

 tively nor qualitatively a recognisable connection with the force of 

 gravity. Such relations become clearer if the organism is compared 

 with a mechanism. The connection between the light pressure of the 

 finger on the trigger of a gun and the flight of the bullet is not a 

 simple one. The pressure first liberates a trigger ; the energy thus 

 obtained drives the hammer on to the percussion-cap ; this explodes 

 and causes the powder to explode ; the gases liberated by the explosion 

 force the projectile from the barrel. It is clear that the force of the 

 hammer bears no relation to that of the pressure of the finger of the 

 marksman, and there is just as little connection between the amount of 

 force generated by the expansion of the powder and that exerted by 

 the hammer of the gun. There are energies present, those of the 

 trigger and powder, which are set free. Such liberations of energy, 

 especially when they follow in order and constitute a chain of 

 processes, are of very frequent occurrence in the organism. They are 

 known as phenomena of irritability, and the factor which starts them 

 is termed the stimulus. They are always found when the specific 

 phenomena of life are concerned. 



Just as the action of a machine is only comprehensible when its 

 construction is known, a knowledge of the external form and internal 

 structure of the plant is a necessary preliminary to its physiological 

 study. It has been seen, however, that it is not possible to under- 

 stand the function from the structure to the same degree in the case 



