PERIODICITY IN ‘VEGETATION. 131 
shrub flowers in March, but its leaves do not appear for several 
weeks after the flowers. Most of our common fruit-trees expand 
their blossoms just before their leaves, as the Pear and Apple: 
or they unfold both leaves and flowers together, as the Cherry. 
Some shrubs and trees retain the greater portion of their leaves 
longer than one year, and hence are called Evergreens. These 
latter produce new leaves every season, but only a portion of the 
then existing leaves fall off, and consequently they are always 
leafy. The permanence of leaves, either for a few months or for 
a few years, is probably owing to the state of the sap in the tree. 
We observe that, in most trees, when the sap is less abundant in 
the extremities, the leaves begin to turn pale, or become of some 
hue very different from green, and subsequently fall off. Such 
trees are called deciduous, and the period of leaf-fallmg is termed 
the fall of the leaf. When the sap abounds in the leafstalk, and 
at the junction of the leaf and the stem or branch, the leaf is not 
easily detached. When, on the other hand, the sap is evaporated 
or condensed, the leaf falls off by its own weight, or by the agi- 
tation of the tree. Deficiency of sap therefore appears to be the 
cause of the fall of the leaf. Is sap more abundant in Evergreens, 
or is the longer permanency of their leaves due to the structure 
of the leaf? Do Evergreens abound more in sap at all periods 
than deciduous trees? After an herbaceous plant has done flow- 
ering, and before its stem entirely decays, a bud is formed at its 
root, in which all the vitality of the plant resides during its dor- 
mant state. Before the leaves begin to decay, and some consi- 
derable time before they fall off, buds are formed in their axils 
(angles which they make with the stem or branch), and these 
buds remain unexpanded, but in a vital state, till next season. 
Though provision is thus made, both in roots and leaf-buds, in 
both herbaceous and ligneous plants, for the continuation of the 
growth or of the existence of the species, neither can be accom- 
plished without some increase of temperature. This is shown by 
the fact that the Oak, which in early seasons expands its leaves 
in the first week of May, in late or backward seasons is not in 
full leaf before midsummer. This is also proved by the leafing 
of all trees about a week earlier in England than in Scotland. If 
the Oak were removed to the extreme north of Europe it would 
not expand its leaves at all, and consequently would soon perish. 
Diurnal Periodic Phenomena in Plants.—In many, probably 
