217 



far as twenty inches or more in very vigorous sensitive plants, that is, in 

 Mimosa. 



To complete the comparison I should say there are no muscles in plants, 

 although they execute movements of very considerable amplitude. The 

 real mechanism by which the movements are accomplished, is not well 

 understood. There is agreement, however, in assuming it to be due to the 

 movement of water. Herbs, and the soft parts of all plants are kept dis- 

 tended and firm by internal water pressure, just as a rubber bag would be 

 filled and made tense if tied to an open faucet of the city water works. 

 Each cell acts like a separate distended bag. By stimulation the water is 

 made to flow from the various cells in one side of the organ into the empty 

 spaces surrounding the same and contiguous cells; the pressure is released 

 on one side and the organ bends over in that direction. But this process 

 is much complicated by growth, and other conditions too recondite to be 

 explained here. 



There are some peculiarities of plant senses which need special empha- 

 sis. All the senses, except that of contact, have for their end the adjust- 

 ment of the plant as a whole, and of each of its organs, in a suitable posi- 

 tion for best development. The contact sense has been more variedly de- 

 veloped, aiding the plant to climb, to catch insects for food, and if we are 

 to accept Darwin's suggestion, to enable the sensitive plant in particular 

 to escape the injury of hail storms. All the movements are very slow, 

 except a few like the insect- catching and hail-avoiding movements, and 

 their wonderful diversity and extent are only realized by instituting care- 

 fully devised experiments, and the use of delicate instruments. 



It is also to be noted that the same organ always responds to the same 

 stimulus with the same corresponding movemei^jt. If, for instance, the 

 light strikes a shoot from the east, it bends toward the east, if possessed of 

 positive direct irritability. There is no opportunity for choice. The plant 

 secures a diversity of movement by having each set of organs endowed 

 with their own specific form of irritability. As there is no choice in the 

 character of the response, so there can be no volition, and consequently no 

 mental activity, no psychic life, even of ever so humble and rudimentary 

 nature. 



This brings us back to our starting point. When we trace the develop- 

 ment of irritability as a universal property of protoplasm into its various 

 phases of sensibility, and mental activity, the first and fundamental divi- 

 sion of organic life is into fixed and motile organisms, without regard to 



