CONTROLLED ENVIRONMENTAL CONDITIONS 



333 



Table 37. Summary: Degree of Development Listed in Order with the Highest First 



* Exception in sunflower. 



house show better development in most cases than those in the visible spec- 

 trum, although the illumiiijation and the total radiant energy are somewhat 

 lower in the former than in the latter. This indicates that ultraviolet has 

 minor importance in plant differentiation. Shirley's work reported earlier 

 also indicated this. This work also emphasizes the fact that plants gro^vn 

 under long rays alone lack differentiation of tissues, have thin leaves and 

 stems, and are spindling. 



Some Low-Temperature Effects 



In Chapter 3 there is a full discussion of the importance of keeping cer- 

 tain seeds at a low temperature with proper moisture and oxygen supply 

 in order to after-ripen them. In some seeds that respond to this treatment 

 the coats rather than the embryos seem to cause the delayed germination, 

 but even in these see_ds it is probable that the low temperature is effective, 

 at least in part, through modifying the embryo. In other seeds of this group 

 there is a sluggislmess in the growth of the radical, as well as a dormancy 

 in the epicotyl that produces a dwarfish plant. Both the sluggishness in 

 the radical and the dormancy of the epicotyl are overcome by the low- 

 temperature treatment. In Chapter 7 there is a discussion of the effect of 

 low temperature exposures on the after-ripening of buds of trees, bulbs, 

 and tubers. In most, but not all, cases mentioned in this paragraph the 

 low-temperature exposures overcame dormancy in vegetative organs and 

 increased the vigor of the resulting vegetative growth. 



Periods at low temperatures in many cases also shorten the vegetative 

 period and hasten the appearance of the reproductive phase of plants 

 (yarovization or vernalization) . The treatment may be applied to imbibed 

 and slowly germinating seeds, to seedlings ^^' ^^ such as celery, beet, and 

 cabbage, or to the well-developed plants, ^^ such as cabbage and onion. 



