9 2 



RELATION OF PLANTS TO TEMPERATURE 



One of the most interesting manifestations of such irritability 

 is that shown by many seeds and propagating bodies, tubers, and 

 spores of plants living in high latitudes. Many of these forma- 

 tions may not be induced to emerge from the resting period until 

 they have undergone a period of low temperature, in imitation 

 of the winter through which they naturally pass. The shock of 

 change from a law to a high temperature seems to be necessary 

 to start the protoplasmic machinery in action, and may perhaps 

 serve as an indirect signal stimulus. 



143. Resistance and Acclimatization of Seeds to Heat. Secure 

 two or three hundred seeds of pea, radish or corn, by selecting 



only those apparently capable 

 of germination. Place a dozen 

 in a Zurich germinating dish 

 and note time necessary for 

 germination, and proportion of 

 active seeds. Place ioo seeds 

 in an incubator the tempera- 

 ture of which is under accurate 

 control and gradually raise the 

 temperature to 40 C. at which 

 point it should be maintained 

 for 1 2 hours. Take a dozen 

 seeds from the incubator and 

 germinate as above. Slowly 

 raise the temperature of the 

 incubator to 50" C. and place 

 in it an additional lot of seeds 

 not previously heated. Main- 

 tain the temperature for 12 hours. Put the freshly treated lot, 

 and a dozen of the others in a germinator. Note results as be- 

 fore. Raise the temperature of the incubator to 6o° C, again 

 placing in it a fresh lot of untreated seeds, with the old lot. After 

 1 2 hours take out the fresh lot and a dozen others and germinate 

 as before. Repeat at 65° C, 70 C, 75° C. and 8o° C. Tabu- 



FlG. 40. Form of incubator suitable for 

 tests of endurance of temperature by seeds 



