n] HABITS OF CONVOLUTA 69 



to cause horizontally growing lateral roots to take up 

 vertical positions, it suffices merely to remove the 

 main root. As a result of the operation, the physio- 

 logical state of the whole root-system is so changed 

 that members formerly transversely geotropic become 

 positively geotropic, and tertiary roots which previous 

 to the operation were ageotropic (non-geotropic) and 

 hence grew indifferently in any direction, become 

 transversely geotropic. 



Similar changes in sign of tropistic response 

 may be induced by definite changes in the environ- 

 ment. For example, as Loeb has pointed out, fresh- 

 water Copepods, (small Crustacea) taken from the 

 same pond at the same time, may exhibit, some a 

 positive, some a negative, phototropic response and 

 others may be non-phototropic. If, however, a little 

 carbon-dioxide is added to the water they all become 

 positively phototropic. It is not improbable that this 

 uniform migration of the animals in the direction of 

 the light which follows on the addition of carbon- 

 dioxide is an example of response to associated 

 stimuli. Copepods feed no doubt on algse, which 

 can only live and grow in the light. In the course of 

 their nutrition, algse decompose carbon-dioxide and 

 liberate oxygen, so that the amount of carbon-dioxide 

 contained in the water in their immediate neigh- 

 bourhood is less than that contained in the darker 

 regions of the pond. Much carbon-dioxide will be 



