4 MISSOURI AGR. EXP. STA. RESEARCH BULLETIN NO. 17 



medium, as damp soil or sand. This fact has given rise to the prac- 

 tice of stratification, so extensively followed by gardeners and nursey- 

 men. 



Seeds like the cocklebur (Xanthium canadense, Mill.), often fail 

 to germinate even when planted under the most favorable growing 

 conditions. It is also a matter of common observation that a large 

 number of common weeds do not begin growing from the seed until 

 settled summer weather has come. These seeds will lie in the warm 

 soil for weeks during April, May, June, and often into July or 

 August before beginning to grow. 



If a viable seed with no hindering seed coat refuses to grow 

 when planted in a warm, moist soil, it may be said to have a pro- 

 nounced rest period. If a seed fails to grow when planted under 

 favorable growing conditions, immediately after ripening, it has a 

 rest period. Also if seeds fail to grow after being in dry storage for 

 a month, they probably have a rest period. 



The rest period of seeds, then, is a physiological condition or 

 state which inhibits germination in certain seeds immediately after 

 maturity, or possibly later, even tho they be planted in an ideal en- 

 vironment for growth. In other words, it is that period during which 

 the resting embryo refuses to resume growth activity when all hin- 

 dering external factors are excluded. 



It is thus seen that the resting phase is due to internal causes. 

 External factors may and do prevent germination, but failure to 

 sprout under such circumstances must not be always attributed to 

 a rest period. In the case of seeds that have been in storage for 

 a time, germination may be slow and imperfect as a result of both 

 internal and external hindrances; that is, the rest period may be 

 just setting in or may not be quite over and also the seed coats 

 may have become dry and hard in storage. 



The fact of a rest period in plants has been fully established by 

 Johannsen 1 and others. Nearly 300 species of woody plants were 

 studied at the Missouri Agricultural Experiment Station 2 and it 

 \vas found that practically all have a rest period. It was also found 

 that the period of rest in woody plants is of varying length ; in fact 

 the length of rest ranges all the way from a few days or weeks to 

 several months. The intensity of the resting state varies greatly 

 among the different species. Some species can be easily forced into 



1. Johannsen, W., Das Aetherverfahren beim Fruhtreiben mit Besonderet 

 Berucksichtigung der Fliedertreiberi. Jena, 1906. 



2. Howard, W. L. f Experimental Study of the Rest Period of Plants. The 

 Winter Rest. Research Bui. No. 1, Mo. Agr. Exp. Sta. 1910. 



