104 THE ESSEX NATURALIST
order that the internal tissues may be effectively protected from
adverse external conditions, a new protective layer, the bark,
must be provided. With the continually expanding crown,
more and more leaves are produced and since these must receive
adequate supplies of water and mineral salts from the soil, an
increasing amount of water conducting tissue—xylem or wood—
is called for, as well as additional phloem or bast to conduct
away the food materials which the leaves have produced. In
fact, an indefinite increase in length without a corresponding
increase in girth may not be possible, for as length increases so
do the demands on the conducting system, which must likewise
increase in size.
These are some of the problems which beset a tree, problems
for which a satisfactory solution must be found if the tree is to
persist. There are others. There must be a firm anchoring
system to withstand the enormous pull exerted by the crown,
not only under summer breezes, but in equinoctial gales. The
form of the tree must be such that at least some of the water which
falls on the crown shall reach the roots and not merely evaporate
or drain off beyond the area in which the roots lie. There is the
difficulty that, under the most favourable conditions, the tree
is not living in a uniform environment, for its roots may lie in
cool, moist soil, its leaves in an atmosphere which often subjects
them to drought conditions. There is, further, the problem of
dealing with a large amount of wood which is not wanted, for
any particular area of wood in the tree has a relatively short life ;
when its period of usefulness is past it cannot, in general, be
discarded: it must remain, an attraction to certain wood-destroying
fungi, the entry of which, if possible, must be prevented.
Here we shall attempt to examine the ways in which some
of these problems are solved.
First as to the span of life and the size to which trees attain.
The Black Poplar (Populus nigra L.) lives for about 150 years,
the English Oak about 10 times as long, while the Wellingtonia
(Sequoia gigantea Decne.) may attain 4,000 years, as has been
determined from ring counts of old trunks. The Giant Redwood
(Sequoia sempervirens Endl.) is likewise Very long lived : when the
trunk falls sucker growths spring up from its base and in course
of time some may develop into trees ; it is known that such second-
growth trees occur naturally, but it is not known how often this
cycle may be repeated : it does appear, however, that a single
Redwood tree may produce a plant, or rather trees, whose life,
in the aggregate, is to be measured in terms of thousands of years.
Again, the Banyan trees of India (Ficus indica L. and F. ben-
ghalensis L.) produce aerial roots which grow down from the
spreading branches and enter the ground, forming stout vertical