DEVELOPMENT OF MIND IN ANIMALS 121 



photosynthesis and complete independence in food-getting. 

 The plant does not discriminate in sensation and movement, 

 but in growth. It spreads its leaves and turns them toward 

 the sun and sends its roots into the soil— always in a certain 

 parallelism with animal behavior. It grows in one direction 

 and not in another, avoiding this and selecting that, much 

 Hke animal movement; these movements are dramatically 

 apparent when the growth is speeded up by time-lapse 

 photography. In other words, the plant is sensitive to the 

 placement of sunlight, of water, of minerals and of obstacles, 

 and responds through reactions toward or away from these 

 growth factors, just as would an animal. 



Efforts have been made to show a high-order conduction 

 and sensitivity in plants, notably by Sir J. C. Bose; but the 

 fact of the matter seems to be that little more is involved 

 than the rude capacities of primitive cells. There is conduc- 

 tion of stimuli from cell to cell, and in some plants this is 

 exploited to an extent. In mimosa, the sensitive plant which 

 has already been mentioned, there is an animal-like reaction 

 when the leaves are touched. Contact at the leaf tip will 

 cause the leaflets to fold together, after which the whole 

 leaf bends away from the contact. H. S. Burr showed that 

 this reaction is much like a nervous response, even to its 

 electrical correlates. Certain structures in other plants like 

 the "fly-catcher" respond in a similar way. There is, then, 

 in plants an irritability to different kinds of stimuH, a gen- 

 eral response accomplished without the help of specialized 

 nerve cells. 



In the bodies of the higher animals there are many kinds 

 of cooperating cells, and none is more specialized than the 

 neuron. Conduction and sensitivity in this cell are exploited 

 to the highest possible degree, and it is exclusively through 

 the neuron that high order nervous reaction is possible. The 

 beginnings of this evolution are already apparent at low 

 animal levels. Even in the sponge, which is little more than 

 a loose aggregation of cells, there is a sort of "neuroid trans- 



