106 Principles of Plant Culture. 
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 foli- 
age 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. 
173. Most Plants Under Glass Require Rest from 
time to time, or they do not thrive. This rest is provided 
either by placing them in a lower temperature than is favor- 
able to growth, or by submitting them to a degree of dry- 
ness that prevents growth. The latter is preferable for 
plants native in the tropics, where they naturally lie dor- 
mant during the dry season. 
174. The Time of Leaf Fall is an Index of Wood 
Maturity in deciduous trees and shrubs. In these, the color- 
ing and fall of the leaves in autumn is not necessarily due 
to frost, but, in hardy plants, results from the dormant con- 
dition that accompanies maturity. As a rule, the more 
mature leaves are precipitated by the first autumn frosts. 
Those less mature usually remain until the more severe 
frosts. In trees with well-ripened wood, the leaves at the 
tip of the shoots usually fall before, or not later than, those 
on the older parts of the tree. With poorly-matured wood, 
the reverse is the case. In afew deciduous trees, as the 
beech, and some oaks, many of the mature leaves remain 
on through the winter. 
175. Hardiness Depends upon -the Degree to which 
the Dormant State is Assumed. Since the most severe 
climatic extremes come during the natural rest period of 
