106 THE ESSEX NATURALIST
sitchensis Carr.) one less than 30 centimetres high and with a trunk
1.8 centimetre in diameter, the other about 30 centimetres high
and two centimetres in diameter, had reached an age of about 98
years. Assuming that the growth rings of the wood were of
equal thickness, the latter tree increased in diameter about 0.2
millimetre annually. In contrast, English-grown Sitka Spruce
may have growth rings approaching half an inch in width—the
annual increment is in the region of 100 times as much. Sitka
Spruce trees 120 feet high and five feet in diameter are growing in
Britain, and the tree was not introduced here until 1831 ; thus
the height increments in these trees must have been something
like 100 times that of the Vancouver Island dwarfs.
Trees appear to grow throughout the year in a few favoured
tropical regions; but even in the tropics there is frequently,
probably usually, a regular periodicity in the unfolding of the
new foliage and in the shedding of the old. Although tropical
rain forest is evergreen, bare trees are to be found in it at any time
of the year, for while there is no simultaneous shedding of leaves
or appearance of new foliage, at least some of the trees are
deciduous. In temperate climates the leaves of the deciduous
trees are ephemeral structures, lasting but a few months and
replaced by a new crop in the following year : the evergreens
likewise produce new leaves each year, but these leaves are longer
lived and such trees are never devoid of leaves. In some, referred
to as semi-evergreens, the old leaves fall at about the time the
new ones appear. For temperate trees cold and lack of water are
the principal adverse conditions which have to be met in winter,
while in some tropical regions it is shortage of water during the
dry season which constitutes the problem to be overcome—cold
does not enter into the picture.
The weakest parts of the tree are its stem apices, where lie the
delicate meristems (or growing points) which are responsible for
extension growth, and the cambia of the aerial, members. The
growing points, during a certain period of the year, form new
tissue, which is added to the twigs and which, as a consequence,
pushes the meristems ever farther from the base of the twig
—the growing points are responsible for the extension growth
of the branches. The new tissue which the growing point
produces forms little flaps, the primordia of the leaves, which
wrap around the growing point, the whole forming a bud.
In the angle between each leaf primordium and the stem a
little bit of the meristematic tissue remains, behaving in its
turn like the apical growing point from which it was formed, i.e.,
it produces another bud. This position of the bud in the angle
between the leaf and stem is known as axillary, and is the normal
one for a bud except, of course, for the terminal bud, which is