The Living Machine 



311 



complete leaf than in one with a leaf which has been partly 

 cut away. ' ' What has been demonstrated in this case explains 

 probably also why the apex of many plants when put into 

 a horizontal position grows upward, and why certain roots 

 under similar conditions grow downward. It disposes also 

 in all probability of the suggestion that the apex of a posi- 

 tively geotropic root has 'brain functions/ It is chemical 

 mass action and not 'brain functions' which are needed to 

 produce the changes in growth underlying geotropic curva- 

 ture." 3 



Such an explanation however is difficult to apply to many 



EELATIVE AMOUNT OF BENDING 



Due to unequal growth in stems with and those without leaves. From 

 Loeb, " Forced Movements, Tropisms and Animal Conduct." 

 By permission of J. B. Lippincott Company. 



of the reactions of a Stentor or a Paramrecium. While the 

 latter animal reacts to an electric current by a difference in 

 the beat of the cilia on the two sides, and the animal is thus 

 turned so as to swim with the current, by a process seem- 

 ingly as mechanical as that of turning a boat; in other 

 cases, as when running into a salt solution, the behavior of 

 Paramoecium is not so simply explained, for in this circum- 

 stance it always turns in the same direction, regardless of 

 the angle at which it meets the salt current, and even though 



9 Loeb, ' ' Forced Movements, Tropisms and Animal Conduct, ' ' pp. 

 121-2. By permission of J. B. Lippincott Company. 



