8 THE ACTION OF THE LIVING CELL 



tropic response is to be found in the ingenious experi- 

 ment of Paal (1918), who found that if the tip of a 

 seedling was severed from its stalk and then replaced 

 after the insertion of a sheet of collodion between the 

 stalk and the tip, a more or less normal heliotropic re- 

 action was obtained. When, however, tin foil was used 

 in place of the collodion no such response could be 

 obtained. Obviously the former is permeable and the 

 latter impermeable to the photochemical product 

 produced in the plant tip. According to Blaauw's 

 measurements, an intensity of light equal to seventeen 

 hundred thousands candle-meter is sufficient to elicit 

 a heliotropic response in the oat seedlings examined 

 by him. This is equivalent to saying that the seed- 

 lings in a darkened room would bend towards a 

 lighted candle some two hundred and fifty feet dis- 

 tant. These surprising figures well illustrate the 

 sensitivity of plants to changes in their physical en- 

 vironment. 



A similar reaction is observable in the stereotropic 

 •response of the young roots of plants. The applica- 

 tion of pressure to the apex of such a root causes it 

 to grow away from the source of pressure. A unilateral 

 trauma, bruise, or application of lunar caustic elicits 

 a similar growth response. Hence it is highly prob- 

 able that each of these agencies causes the elaboration 

 of a substance similar to that formed in the illumi- 

 nated plant tips. 



Such responses to changes in the physical environ- 

 ment are by no means limited to plants. Jacques Loeb 

 as well as numerous other investigators have observed 

 similar phototropic and stereotropic responses in ani- 

 mals; and in consequence he was for a long time an 



