130 PERIODICITY IN VEGETATION. 
stem, which in its turn perishes, after having performed its func- 
tions in the economy of the plant. This is one of the periodic 
changes of plants, but it is not the most remarkable. The period 
when a living being, whether vegetable or animal, begins to exist, 
and the period when it perishes, or, in other words, the space 
which elapses between its birth and its death, is to that individual 
of the greatest importance. Plants of greater permanency than 
the short-lived individuals which only eudure for a few months, or 
for shorter spaces, undergo certain annual changes, which are de- 
nominated ascent or circulation or motion of sap, expansion and 
falling of the leaf, production of flowers, and ripening of seeds ; 
or, im herbaceous plants, viz. those which have not permanent 
stems, as trees and shrubs have, there is an analogous periodic 
change in the decay of the herbaceous stem, and in the preser- 
vation of the life of the plant im the root-bud, which expands the 
following year, the year after the decay of the stem, and becomes 
a renewed stem, which in its turn perishes, after having fulfilled 
the functions peculiar to it. 
Annual Periodic Changes.—The most important annual peri- 
odic changes of plants, next to their germination, maturity, de- 
cay, and death, are the generally annual phenomena of leafing 
and flowermg. Some of the lower orders of plants germinate, 
bear flowers, and die in short periods; but the great bulk of 
plants only leaf and flower once a year, and are regulated partly 
by laws peculiar to themselves, and partly by the temperature, 
ete. All perennial plants, whether herbaceous or ligneous, have 
1 period of cessation from growth. During this dormant state 
the vitality of herbaceous plants is resident in the root, or m the 
stem if the plant be bulbous, like the Onion, or tuberous, like 
the Potato. In the British and European forest and other trees, 
during at least six months in the year, there is no apparent 
growth ; during this season leaves, young branches, and shoots 
are not produced. Some trees are clothed with leaves at an ear- 
ler period of the spring or summer than others are ; for example, 
the Horse Chestnut, though an exotic, is in full leaf at least ten 
days earlier than the Oak. The Hawthorn is in leaf before the 
Blackthorn is, though the latter blossoms two or three weeks ear- 
lier than the former. Several of the Willows exhibit their golden 
catkins long ere their leaves appear. Some of the same tribe 
bear both leaves and blossoms at the same time. The Mezereon 
