Decline of Growth and the Best Period. 113 



171. The Rest Period is Not Peculiar to the Temper- 

 ate Zones, but occurs in the tropics as well. It can be 

 ascribed in part to the change of seasons, as a few 

 familiar examples will indicate. Tubers of the earlier 

 varieties of the potato, that ripen in northern countries 

 by the beginning of August, do not sprout if left in 

 the ground till October, but if stored in a cellar during 

 winter at a temperature little above freezing, they 

 often begin to sprout in March. Bulbs of the crocus, 

 tulip, narcissus, crown imperial, etc., that form in 

 spring, lie dormant in the warm soil during summer 

 and early autumn, but start vigorously in the colder 

 soil of the late autumn or the succeeding spring. The 

 buds of many trees, that form in summer for the next 

 year's foliage and flowers, remain dormant during the 

 hot weather of August and September, to push vigor- 

 ously in the first warm days of spring. The rest period 

 is to be regarded as a normal, if not a necessary factor 

 of plant life. 



172. Most Plants Under Glass Require Rest from 

 time to time, or they do not thrive. The rest is pro- 

 vided either by keeping them at a lower temperature 

 than is favorable to growth, or by submitting them to a 

 degree of dryness that prevents growth. The latter is 

 preferable for plants native in the tropics, where they 

 naturally lie donnant during the dry season. 



173. The Time of. Leaf Fall is an Index of Wood 

 Maturity in healthy deciduous trees and shrubs. In 

 these, the coloring and fall of the leaves in autumn is 

 not necessarily due to frost, but results from the dor- 

 mant condition that accompanies maturity. As a rule. 



