IRRITABILITY 201 



should be set up. "If now a specimen," prepared as shown 

 in the accompanying figure, "is placed so that the terminal 

 part points vertically downwards, whilst the rest of the root 

 is horizontal, no geotropic curvature takes place. This, 

 however, always took place, and with about the same 

 promptness as in straight roots, when the terminal portion 

 was placed horizontally, or in general at an acute angle 

 with the normal position. From these experiments it fol- 

 lows that the root thus treated is perfectly capable of 

 reaction. . . . By this means therefore it is proved with 

 the most perfect certainty, that in an uninjured root only 

 the root-tip is geotropically sensitive." With this is also 

 proved that from the 

 part which is sensitive 

 the part of the root 



^j\ " 



with the largest amount (^ 



of living protoplasm in 



proportion to its vol- 



Fierure 14. Root-tip in bent glass tube, 

 ume-the stimulus is (From Czapek . } 



transmitted to cells cap- 

 able of responding to the stimulus. These are the cells 

 at that time increasing in volume, growing. The posi- 

 tion of the tip is determined by its own sensitiveness, but 

 its position can be changed, except by artificial means, 

 only by the action of the responding part. The stimulus is 

 transmitted from living cell to living cell. The transmission 

 takes time, but the interval, known as the latent period, 

 between exposure and response to the stimulus, is necessarily 

 employed in preparing to execute the response as well as in 

 transmitting the stimulus. The latent period may mean 

 still more, but at least it means these two things. 



The transmission of the stimulus cannot be understood 

 until it is known in what the stimulus exerted by gravity 

 consists and what it does in the sensitive cells. All the 

 ponderable parts of an organism and of a cell are subject 

 to the physical pull of gravity. From this it follows that 

 while these parts are in place their position is maintained 

 as the result of the activity of living protoplasm. When the 

 position of these parts is changed, either by the living pro- 



