NO. 9 PHOTOTROPIC RESPONSE OF OAT SEEDLINGS KARRER 3 



been used in which the energy in shorter wave lengths was equal to 

 that of the longer, the steepness of slope would probably be greater 

 on the blueward side than the curve indicates. One might expect that 

 the seeds placed at these sharp wave-length boundaries would experi- 

 ence greatly diflferent forces at various parts of their structure. If the 

 differential action is at all translated into mechanical effects, phe- 

 nomena such as the divergence and convergence noted above or twist- 

 ing may be expected. 



j\nother photic effect was observed in seeds which, after having 

 been soaked several hours in nutrient solution, were exposed for a 

 short time to an elevated temperature. They germinate to different 

 extents and grow at different rates. Wet seeds exposed for 3 minutes 

 to 40° C. are accelerated in the germinational growth, but a smaller 

 percentage of seeds may germinate. The marked effect of exposure 

 to 40° C. may be conditioned by the hydrolysis of starch which is 

 promoted at that temperature (3). The effect of heating on dormancy 

 of seeds is well known. However, if in addition to heating at different 

 temperatures the seeds be irradiated in different parts of the spectrum, 

 they appear to be differently sensitized to different wave lengths. 

 Seeds subjected to the higher temperatures appeared to have greatest 

 percentage germination in the blue, as though they were more and 

 more dependent for germination on the blue. These are preliminary 

 observations, but, like the photic effect first mentioned, are suggestive 

 of a line of experimentation. 



All the observations were begun at room temperatures (variable 

 from 20° to 28° C). Some tests for optimum temperature were 

 initiated. Four batches of seeds, after soaking for several hours on 

 filter paper in closed aluminum cups, were exposed for 3 minutes to 

 40° C. Three batches were not exposed. One of each set was kept 

 at 25° (call these f25 and C25). Another of each was kept at 20° 

 (call these f20 and C20). One from each set was alternated between 

 25° and 20° C. (f 25/20, C25/20), and one (f 40/20) of the first set 

 was alternated between 20° and 40° C, being exposed twice every day 

 for 3 minutes to a temperature of 40° C. 



The longest roots and shoots, but most spindly, were in f25 (those 

 exposed 3 minutes to 40° and kept at 25°). Next longest were in 

 C25/20. These two also had lowest root and shoot ratio (1.2). The 

 lowest percentage (25 per cent) of germination was in C25/20. The 

 shortest roots and shoots were in f 20/40 ; but greatest root shoot ratio 

 (4.2), most hair roots, and thickest coleoptiles. 



