SENSITIVITY IN PLANTS AND ANIMALS 63 



the same stimulus applied in different intensities, may 

 excite very varied responses on the part of the same 

 protoplasm. A few illustrations will make this 

 clear. 



First, the same stimulus may induce very different 

 reactions in different varieties of protoplasm. Select 

 a young seedling whose shoot and root have attained 

 a certain development and lay or suspend it, horizon- 

 tally in a moist chamber, so that root and shoot are 

 free to move (Fig. 24, a-a'). 

 Both root and shoot are 

 affected equally by the 

 stimulus of gravity, yet 

 after a few hours it will 

 be found that the root has 

 begun to bend downwards 

 towards the earth's centre, f J.' 



while the shoot has begun F IG . 24. Geotropic curva- 

 to bend upwards and away ture in root and shoot 

 from the earth's centre of mustard. (Natural 

 (Fig. 24, b-V). The proto- 

 plasms of the root and of the shoot have thus 

 responded differently to the same stimulus. 



Secondly, the same stimulus may induce different 

 reactions in the same protoplasm at different stages in 

 its (jroivth. It will be remembered that in Chapter I. 

 we referred to a very lowly organism (Fig. 3), known 

 as a Myxomycete or Slime Fungus. If exposed to 

 light in its young state the protoplasmic mass creeps 

 slowly away from the source of light, attempting to 

 hide itself, so to speak, in a crevice in, or in the shade 

 'of, say, a piece of bark. When fully ripe and ready 

 to produce reproductive organs, however, it seeks 

 the light, which it previously made every effort to 

 avoid. 



