MOVEMENT 36i 



away to the opposite side of the container. Blue light is more 

 effective than red in producing these reactions. 



By topotaxy is meant the orientation and movement as affected 

 by differences in the concentration of a substance. This may 

 have no relation to the nature of the substance, i. e., it is not a 

 question of quality but only of quantity. It is thus hard to dis- 

 tinguish from osmotaxy, where the movement is due to changes in 

 osmotic pressure, which is also a quantity relationship. 



In chemotaxy we are dealing with a qualitative difference which 

 the organism is able to detect. Some substances produce positive 

 chemotaxy, others negative, and to others the organism is in- 

 different. The responses may occur to substances which the 

 plant would never meet in nature and in many cases they have 

 no apparent survival value. On the other hand, the positive 

 chemotaxy exhibited may be extremely helpful in some cases. 

 The sperms of ferns are attracted by minute quantities of malic 

 acid, which the archegonia excrete, and thus fertilization is in- 

 sured. The amount of a substance required to produce the re- 

 sponse is very small. A 0.001% solution of malic acid attracts 

 the fern spermatozoa, and, while it is estimated that the amount 

 which stimulates any one sperm is about one-millionth of the 

 weight of the sperm, this is still ten times as much as the relative 

 amount of morphine which is required as the minimum effec- 

 tive dose for man. Aerotaxy and hydrotaxy are special forms of 

 chemotaxy. 



Rheotaxy is the response made to a current of water. Many 

 free-swimming organisms swim against the current, i. e., they are 

 negatively rheotactic. 



By thigmotaxy is meant the tendency to remain attached to the 

 object around which the organism is swimming. When sperms 

 of Fucus come in contact with an egg they swim around and 

 around in close proximity to the egg until one has penetrated and 

 fertilized it. This may be due to some substance secreted by the 

 egg, in which case it would be a form of chemotaxy. 



Some forms prefer to stay at a certain level in the water. If 

 too near the surface they swim down, and if too near the bottom 

 they rise until they reach their normal level. Such movements 



are geotadic. 



The response to heat or thermotaxy is also very easily observed. 

 If one side of a vessel of water which is just above the freezing 



