213 



with the tongue, and so on for the other senses, seems like a matter of 

 course. But to explain why the nerves of the eye are only sensitive to 

 light, of the ear to sound, of the fingers to impact and temperature, and 

 so on, there being no structural differences detectable between the various 

 sets of nerves that bring about such diverse results, we are content to say 

 that it is due to a specialization of sensibility. The nerves at the tips of 

 the fingers are more sensitive to touch than those at the back of the hand. 

 The fingers have nerves that respond when stimulated bjf heat, but in the 

 eye the nerves will not respond to heat but will respond to light. We do 

 not marvel at this, it is everyday knowledge. We put it in scientific lan- 

 guage by saying that irritability, a universal property of living matter, 

 has been developed and specialized in different organs so as to respond 

 diflerently to difierent stimulation. Fundamentally there is agreement, 

 but the results of specialization are diverse. In the plant the root has 

 a special sensitiveness to gravity, which is manifested by causing it to 

 bend earthward, the stem possesses a sensitiveness which causes it to bend 

 skyward. To meet its conditions of existence the plant has developed a 

 special sense, that of geotropism, by which it is enabled to take advantage 

 of the directive influence of gravity to place and keep itself upright in the 

 world. It has a sense which animals, with their freedom of movement, 

 appear to be nearly or entirely without. Animals assume an upright po- 

 sition, not in response to a direct gravity sense, but to secure the most 

 comfortable adjustment of the weight of the parts of the body. Upright- 

 ness is a question of weight in the animal, a question of special sense in 

 the plant. 



It may be objected to this designation of the gravity sensQ in plants as 

 a special rather than a general sense, that it is difi"used throughout the plant 

 and not confined to particular, specialized organs. This objection has 

 some show of validity, but is not formidable. The apparent difference is 

 not fundamental, but necessitated by certain structural features. Animals 

 have a jointed, or wholly mobile body. In the jointed forms, and often in 

 the others, there is an arrangement of muscles, with a communication of 

 nerves with which to bring about movement as a response to stimulation. 

 Plants, on the contrary, have a rigid body; the sensitive protoplasm 

 being divided into innumerable minute particles, each little mass separ- 

 ated from its neighbors by thick, nearly rigid walls of wood or cellulose. 

 It used to be a favorite illustration to say that a plant was like a great 

 prison, with innumerable cells separated by thick walls, each cell occupied 



