I 

 DECLINE OF GROWTH AND REST-PERIOD 107 



may, however, through the variations of offspring (18), 

 secure varieties in some cases that can endure an environ- 

 ment which the parents could not endure. 



176. Plant processes during the rest-period may not 

 entirely cease. Although food preparation is wholly 

 suspended, root growth and the callusing (72) of injured 

 root surfaces proceed to some extent during winter in 

 unfrozen layers of soil ; and in sufficiently mild weather, 

 the reserve food in the stem gradually moves in the 

 direction of the terminal buds. 



177. Cuttings (358) of woody plants are preferably 

 made in autumn in climates of severe winters and buried 

 in the ground below the limit of hard freezing, in order that 

 callusing (72) and the transfer of food may make some 

 progress before the final planting. 



178. The "turn of the year." Toward the close of 

 the dormant season, vegetation, as if benefited by the rest, 

 is prepared to start with renewed vigor, even at moderate 

 temperatures. Buds, that remained dormant during the 

 latter part of the previous summer, push into growth with 

 the first warm days of spring, and many seeds, that could 

 not be induced to germinate the preceding autumn, start 

 with vigor as soon as the soil is sufficiently warm. 



The cause for this energetic resumption of plant growth 

 after the rest period is not well understood, but exposure 

 to cold, in the case of temperate plants, and to prolonged 

 dryness in that of tropical ones, doubtless explains it in 

 part, for it is well known that potato tubers may be in- 

 duced to start their buds soon after maturity by exposing 

 them to the sun a few days, or by placing them for a like 

 time in a refrigerator containing ice. By these means, 

 the farmers of Tennessee grow a second crop of potatoes in 

 the latter part of summer and in autumn. 



