390 PLANT PHYSIOLOGY 



bascum), Rumex, and mistletoe (Phoradendron) . About 30% 

 are hindered by light, including clover, jimson weed, Phacelia 

 tanacetifolium, Solarium ly coper sicum, and Amaranthus atropur- 

 purea. The remaining 10% are neutral to light, but with increas- 

 ing age sensitivity to light decreases and hence more are neutral 

 (Niethammer, 1927). Aquilegia atrata seeds may lie in the dark 

 for ten years without germinating, but when exposed to light 

 will germinate in fifty days. In Spiraea tomentosa, 50% of the 

 seeds germinated in the light after 41 days, while only 2% ger- 

 minated in the dark in the same period. On the other hand, 32% 

 of the seeds of Polygonum biflorum germinated after 146 days in 

 the dark and only 13% in the light (Mitchell). The light effect 

 seems to vary directly with the wave length and is also propor- 

 tional to the quanta of energy acting on the seed contents (Kom- 

 merell, 1927). 



The light effect, however, is not always so simple, as shown by 

 the fact that its inhibiting effect may be neutralized by placing 

 in a 0.1M mineral acid for a short time or in a solution of potas- 

 sium nitrate. Also in some cases, e. g., Chloris ciliata, the tem- 

 perature plays a part. Above 20° C. these seeds are favored by 

 light and below that are hindered (Gassner). Similarly Ama- 

 ranthus caudatus seeds at 15-20° C. germinate only in the dark, 

 but at 43-45° germination occurs only in the light. Between these 

 points (at about 30° C.) germination is equally good in light or 

 darkness, but the inhibitory effect of light can be removed by 

 various nitrates (Van den Bos, 1920) . 



In many cases the light inhibiting effect seems to be related to 

 the oxygen pressure. Light inhibited seeds are not inhibited in 

 many cases if the oxygen pressure is increased 4-5 times normal. 

 Likewise, light favored seeds may be inhibited with increases in 

 oxygen pressure; and light-indifferent seeds are also commonly 

 oxygen-indifferent over a wide range (Bohmer, 1928). 



Many experiments have been carried out in order to hasten 

 the germination of seeds by the same substances used in forcing 

 buds, bulbs, etc. (Chap. XXVI), and since germination is a phase 

 of growth it is not surprising to learn that, in general, the same 

 stimulants found suitable for hastening normal growth processes 

 are also of value here. 



Dormancy. — Failure to germinate when water, air, the proper 

 temperature, and all favorable external conditions are supplied 



