64 AGE OF 
fungi, commonly termed mould, live only a few 
hours, and seldom exist above a few days. All 
plants, in reference to their total period of exist- 
ence, are usually distributed into three classes,— 
annuals, biennials, and perennials. An annual, as 
generally understood, germinates, fructifies, and 
dies off at the roots in the course of a single sea- 
son; a dvennial germinates and grows in one sea- 
son, and fructifies and dies off at the roots in the 
second season; and a perenncal fructifies oftener 
than once, and may perform organic functions 
during any number of years from three to several 
thousands. Familiar examples of annuals are 
oats and corn-poppies; of biennials, cabbages, and 
winter-wheat; and of perennials, white clover, 
perennial rye-grass, docks, shrubs, and trees. 
Some plants, however, as stockgillyflowers, are 
either annual, biennial, or perennial, according 
to the culture they receive ; some, as many varie- 
ties of turnips, are, out of seeds of the same plant 
|| or specimen, prevailingly biennial but occasion- 
ally annual; some, as the common nasturtium or 
Indian cress, Tropeolum minus or major, are an- 
nuals in our country but perennials in lower lati- 
tudes; some, as wallflowers and hollyhocks, are 
nominally biennial, but really live and flourish 
during six or seven years; some, as many of the 
herbaceous species, are perennial only ina very low 
sense, living scarcely longer than some nominal 
| biennials; and some, as the oak, the yew, the 
cypress, the cedar, and the adansonia, are peren- 
nial in a very high sense, flourishing and main- 
taining great energy during many centuries. 
No rule is known for making more than a rough 
guess at the age of a herbaceous perennial plant ; 
and no infallible rule is known for determining 
the age of even long-lived and ligneous perennials. 
_ The well-known rule of counting the number of 
zones or circular layers of wood, and reckoning 
each zone for a year, applies only to exogenous 
or dicotyledonous shrubs and trees; and, though 
far from being uniformly correct, affords the best 
means, as far as concerns that great and impor- 
tant class of plants, of forming an approximate 
judgment. The practice of this rule requires the 
tree to be felled, and counts the zones or circular 
layers, from the pith of the tree to the circumfer- 
ence, as they appear on the transverse section of 
the base of the trunk. The zones are formed in 
all ligneous dicotyledonous plants—t(see article 
DicoryLteponous Puants|—by the formation of 
alburnum or soft wood during the plant’s periodi- 
cal activity, and by the consolidation of this into 
duramen or timber during its periodical repose ; 
and hence each zone is supposed to be completed 
exactly during a year or circle of the seasons. 
“This method,” says Decandolle, “is not liable to 
much error, and is a simple criterion to ascertain 
the age of a tree; but the inspection of these con- 
centric circles must be made with the greatest 
care. By their number they give the age, and 
the degree of their thickness gives also the rate 
of their increase; therefore they should be mea- 
PLANTS. 
sured as well as counted. My plan is as follows: 
When I have got a section of an old tree, on which 
I can see the circles, I place a sheet of paper upon 
it, extending from the centre to the circumfer- 
ence. On this paper I mark every circle, show- 
ing also the situation of the pith, the bark, the 
name of the tree, the country where it grew, and 
any other necessary observations. I also mark 
in a stronger manner, the lines which indicate 
every ten years, and thus I measure their growth 
at ten years’ intervals. Measuring from centre 
to circumference gives me the circles, doubling 
this I have the diameter, and multiplying by six 
I have the circumference.” The learned pro- 
fessor then presents a table of the periods of in- 
crease in the diameter of various trees; an in- 
spection of which proves that every tree, after 
having grown rapidly when young, seems at a 
certain age to take a regular march of growth, 
which may perhaps be accounted for by suppos- 
ing that young trees have more room to expand 
in, are less pressed by the roots and branches of 
their neighbours, and may not have penetrated 
down to a hard, arid, or otherwise unfavourable 
soil; and also, that as trees advance in age, they 
still continue to form layers as thick as they pre- 
viously did subsequently to the period of rapid 
growth. Jf such tables were multiplied to a 
sufficient extent, they would form data from | 
which, by ascertaining the circumference of a | 
tree, its age might be known without having re- 
course to the destructive process of cutting deep 
into the growing timber. “If,” says Decandolle, 
“one cannot get a transverse section of a trunk, | 
then one must seek for old specimens of each 
kind, the date of whose planting is known, mea- | 
sure their circumference, deduce their average | 
growth, and calculate from them the age of other — 
trees of the same kind, always keeping in mind 
that young trees grow faster than old ones.” 
But a recurrence of cold weather after a warm or 
forward spring may so check vegetation as to 
occasion the formation of two zones in one sea- 
son; and an opposite condition of the seasons, or 
the partial and temporary injury of the plant, 
may cause the omission of a zone or the fusing 
of two zones into one. 
turalist Duhamel has shown that a tree of ten 
years of age has sometimes upwards of twelve 
zones, and that a tree of twenty years of age has 
not always twenty distinct zones. The rule which | 
makes every zone represent a year, therefore, is 
essentially erroneous, and ought to be employed 
only as a good aid to conjecture. 
server know the exact age of any individual tree 
of a species, vegetating in what appears an aver- 
age situation for facilities of growth and develop- 
ment, he may count its layers and measure its 
diameter, and institute these and the known age 
as proportionals for determining the age of any 
other individual trees of the species. Yet even 
this method of attaining a nearer approximation 
is modified by limits in a tree’s progress of ex- 
Or if an ob- | 
Hence the celebrated na- | 
