SEED GERMINATION 531 



Sensibilization and desensibilization are very much dependent upon the 

 presence of gases during pretreatment. There is no difference between 

 pure oxygen and air. Nitrogen suppresses the desensibiUzation entirely, 

 but nitrogen and hydrogen stimulate the sensibihzation, whereas carbon 

 dioxide retards it. 



The effect of temperature pretreatments of short duration (20-60 min) 

 was tried out by Siegel (1950). When Digitaria and Sporobolus seeds of 

 different water content were exposed to a temperature of 10°C, the seeds 

 became negatively photoblastic. After a pretreatment at 10°-50°C they 

 were indifferent to light. A pretreatment at 50°C and more produced a 

 positive photoblastic response. Brief exposures to 100°C of seeds of flax 

 and radish induced a pronounced negative photoblastic response. 



3-3. PRESOAKING AND DRYING 



Presoaking has a strong effect on photoblastism, since the photosensi- 

 bility changes with the time of presoaking preceding the illumination. 

 With Lythrum, photosensitivity in certain strains starts after 30 min of 

 presoaking, but with other strains only after some hours (Wieser, 1927). 

 The peak of photosensitivity is reached after about 12 hr for Lythrum 

 {ibid.) and 16-24 hr for Eschscholzia and Epilobium (Bihlmeier, 1927) . For 

 tobacco seed, according to Kincaid (1935), the time of presoaking needed 

 to initiate photosensitivity is 1 day, and the peak is reached only after 

 about 4 days — a statement that stands in contrast to Bihlmeier's data 

 (1927) for the same plant, indicating much shorter times. For lettuce 

 the photosensitivity is measurable after 4-8 min of presoaking (Evenari, 

 unpublished observations). The curve of its photosensitivity with 

 increasing time of presoaking is given in Fig. 11-4. The existence of 

 a parallelism between photosensitivity, soaking time, and amounts of 

 water taken up was shown for Phleum pratense (Maier, 1933b). Tem- 

 perature is again a modifying factor. 



Since light has no effect on dry photoblastic seeds, photosensitivity is 

 clearly a function of the amount of ^vater the seeds imbibe when soaked 

 in Avater. But the minimum amounts of water needed for the beginning 

 of photosensitivity are different for different seeds. This explains the 

 difference in behavior of different seeds when not presoaked but put in 

 different relative humidities before being illuminated. Tobacco seeds 

 even in a relative humidity of 96 per cent do not become photosensitive 

 as they take up only 20.1 per cent water (Kincaid, 1935). Lettuce 

 seeds, which become photosensitive after only 4-8 min (Evenari, unpub- 

 lished observations; see Fig. 11-4) of presoaking, develop photosensitivity 

 when taking up 10.1 per cent water from a humid atmosphere (Nutile, 

 1943-1944). 



The light effect is not reversed when, after a sufficient presoaking time, 

 the seeds are illuminated and then dried (Fhnt, 1934; Kincaid, 1935; 



