PHYSIOLOGICAL PRECONDITIONING 6ii 



plants in the greenhouse or room to be tested for several days. If traces of 

 ethylene are present epinasty of the leaves will soon become apparent (Fig. 

 136). 



Physiological Preconditioning. — The growth performance of a plant at 

 any stage of its development is not only continuously influenced by its heredi- 

 tary makeup and by the prevailing environmental conditions, but often shows 

 a lingering effect of the environmental conditions to which it has been exposed 



Fig. 136. Epinasty in tomato due to exposure to ethylene. Plant on right exposed 

 to 0.1 parts per million of ethylene for 48 hours; plant on left kept under same condi- 

 tions in an ethylene-free atmosphere. Photograph from Crocker, et al. (1932). 



during some previous stage in its life histor>^ The induction within a plant 

 of certain internal conditions which carry over into a later stage of its life 

 cycle is often designated by the term of physiological preconditioning. Un- 

 doubtedly such effects are often due to certain compounds which are synthe- 

 sized during an earlier stage in the development of a plant. In some phys- 

 iological preconditioning phenomena these compounds may be foods, in others 

 they are probably compounds of the general nature of hormones. Our dis- 



