DECLINE OF GROWTH AND EEST-PEEIOD 105 



after midsummer in the temperate zones. Cultivation, 

 mulching, manuring and wet weather tend to prolong the 

 growth period (199). 



171. Occurrence of the rest-period. The rest-period 

 is not peculiar to the temperate 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 and the like, 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 vigorously 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. Greenhouse plants. Most plants under glass 

 require rest from time to time, or they do not thrive. 

 The rest is provided 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 dormant during the dry season. 



173. Maturity. 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 dormant 

 condition that accompanies maturity. As a rule, the 



