SENSITIVITY IN PLANTS AND ANIMALS 69 



this accomplished ? In the higher animal, as every 

 one knows, transmission is effected by specialised pro- 

 cesses from cells which are elongated very much in 

 one direction, and known as nerves but in plants the 

 transference of the impulse is not so easily explained. 



Microscopic research has shown that the cells in 

 many plants are in communication with each other 

 through their walls by very fine threads of proto- 

 plasm, so that there may be direct protoplasmic 

 communication from one part of the plant to another. 

 Indeed a recent investigator, Nemec, has gone so far 

 as to affirm the existence of special tracts in the cells 

 themselves, along which impulses may be carried. 



This general survey of the phenomena of sensitivity, 

 so far as we have as yet carried it, has thus taught us 

 one important principle, viz., that, in so far as animals 

 and plants respond to stimuli from without, develop- Fixed and 

 ment of sensitivity proceeds along two divergent 

 lines, the one corresponding to the needs of free 

 organisms, the other corresponding to the needs of 

 fixed organisms. 



Let us look a little more in detail in the first place 

 at fixed organisms. 



It will be at once obvious that to a fixed organism 

 orientation is all-important, for the root must pene- 

 trate the soil, and the shoot must expand in the air. 

 Now if a seedling be laid on its side, and its shoot and 

 root in consequence be horizontal, how are these 

 two parts to ascertain which is the way up and which 

 the way down ? 



In the beginning of the last century Knight dis- 

 covered that gravity acted as a stimulus to the plant, 

 and that the root and shoot responded differently to 

 this stimulus, so that the root, no matter what its ori- Gravity, 

 ginal position, bent towards the soil and the shoot, no 



