176 BOTANY part i 



is not so apparent as it is in chemical or physical processes. This 

 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. 



If the free end of a flexible rod is placed horizontall}^ and exposed 

 to the influence of gravity, it will bend doAvnwards 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, Avill remain in the 

 position it thus assumes. If, however, a living growing stem has been 

 used in the experiment it will exhibit an eftect of gravity Avhich 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 ex- 

 periment is made with a tap-root, this will curve vertically downwards 

 much further than its own weight would cause it to do. A rhizome, 

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

 has sunk by its own weight out of the hoi'izontal 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 difterent 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 Avhich neither quantitatively 

 nor qualitatively have 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 

 knov/n as phenomena of irritability, and the factor which starts them 

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

 piienomena 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 })reliminary to its physiological 



4 



