192 Plant Tissue Culture 



Tropisms. Kotte (1922, 52) and Fiedler (1936, 

 45) have both made cursory observations on geo- 

 tropic response of roots grown in an agar sub- 

 stratum and report normal curvature. It has 

 been the experience of this laboratory that roots 

 floated on or near the surface of a liquid sub- 

 stratum show no such response. Two possible 

 explanations of this difference have suggested 

 themselves — either that the positive chemotropic 

 response to the oxygen gradient in a liquid me- 

 dium is more powerful than the geotropic response 

 and masks the latter (an explanation which should 

 be equally applicable to a semi-solid substratum) 

 or that the absence of a fixed support or fulcrum 

 from which to bend makes it impossible for a root 

 to retain a unilateral orientation. The positive 

 reaction on agar seems to support the latter ex- 

 planation, yet, once a root has grown long enough 

 in a liquid nutrient to curve around the flask some- 

 what, it has built up a bilateral symmetry which 

 should make a reaction to gravity possible. This 

 does not take place. Yet in experiments in which 

 roots were fixed in a constantly aerated medium 

 lacking an oxygen gradient, the response reap- 

 peared (White, unpublished). A clear explana- 

 tion is yet to be obtained. This example should 

 suffice to show that cultures of this sort are capa- 

 ble of serving as excellent material for certain 

 types of tropistic studies. 



