310 Biology in America 



are not strictly symmetrical they may be assumed to be so 

 for the purposes of the theory. Thus if a Paramoecium be 

 acted upon by an electric current whose direction is oblique 

 to the long axis of its body, the cilia on the side toward the 

 negative pole beat more vigorously than do those on the 

 positive side, and in the opposite direction, causing the animal 

 to turn until it is in line with the current when it swims 

 ahead, toward the negative pole. The stem of a plant turns 

 toward the light, or bends upward, because of a difference 

 in amount of chemical substances on the two sides, and ' ' this 

 causes a difference in the velocity of chemical reactions be- 

 tween (the two sides). " The organism has no control over 

 its behavior but is so to speak blown about "by every wind 

 that blows" as helplessly as a derelict ship upon the sea. 



SAGGING IN A STEM 



Due to unequal growth on the two sides. From Loeb, ' ' Forced Move- 

 ments, Tropisms and Animal Conduct." 



By permission of J. B. Lipplncott Company. 



But what proof have we that such chemical changes as Loeb 

 assumes do occur in the organism? If we suspend a stem 

 of a plant in a horizontal position, it soon bends downward, 

 taking the form of a U. This bending is not due to sagging 

 of the stem as a rope sags, but rather to unequal growth of 

 the two sides, which can be proven by marking equal dis- 

 tances on upper and lower sides by lines of India ink and 

 later measuring the amount of growth occurring between 

 the marks. If the amount of bending in such a stem with 

 leaves attached be compared with that in a stem lacking 

 leaves, it will be found to be much greater in the former 

 due to the greater amount of growth material available, and 

 similarly there is greater bending in a stem furnished with a 



