PERIODICITY IN VEGETATION. 129 
ing. This class, though composed of individuals whose grand 
periodicity or duration might extend from half a day to a hun- 
dred years, might be rigidly or precisely defined. Its character- 
istic would be that all its members flowered but once in the whole 
course of their lives. Their duration as a class would indeed be 
very indefinite, but they would all have the common property of 
flowering only once. Many successive generations or races might 
be produced in the terms of one year or of two years (annual and 
biennial periods), or they might in many instances require three 
years to reach maturity, or they might, as in the case of the 
American Aloe, only flower .after an existence of many years. 
The second class is capable of exact definition, though the periods 
of the existence of the individual members have a range as exten- 
sive as that of the first class. All plants that spring up again 
from the same root, or do not need to be sown afresh every time 
they are to produce flowers and seeds, are deemed perennials, al- 
though several of these do not live more than two or three years. 
The Crucifers supply us with examples of these short-lived peren- 
nial plants. Under very favourable circumstances these plants 
flower from the same root several times in succession. Sometimes 
they flower only once, becoming biennial, or even annual. Some of 
them, on the other hand, exist hundreds, yea thousands of years. 
The Yew-trees of England and Wales are examples of a vegeta- 
tion that probably existed since the Flood. Let the sceptic look 
at the Yews at Merrow Downs, near Guildford : these trees are re- 
duced to a mere film of wood and bark—the interior has been gone 
for ages. The gipsies have made their fires on the ground within 
their hollow stems ; yet they exist, without exhibiting the slightest 
appearance of decay. . They have been exposed to all atmospheric 
accidents, and have been mutilated and scorched, yet they bid 
fair to double their previous period of existence. The Baobab- 
trees of Senegal are computed (many of them) to be about five 
or six thousand years old. But these periods, great as they are, 
dwindle into imsignificance when they are contrasted with the 
humble crustaceous Lichens. Many of these are coeval with the 
rocks on which they grow: we leave the determination of their 
age to the geologists. The stems of herbaceous plants that flower 
oftener than once from the same root, annually die down to the 
root; but the vitality of the species exists in a bud, which being 
formed the previous year, is expanded the next, and becomes a new 
Ne SEO Le de 8 
