THE LIVING TREE 105
props which support the ever-lengthening branches ; in time the
tree may cover an enormous area—one such tree was stated to
have a bole 14 feet in diameter and a crown 300 feet across :
eventually the trunk dies, but the branches with their roots
continue to grow as separate thickets. We do not know for how
long this growth may continue. In the Banyans, incidentally,
we have an example of a tree which is not completely confined
to one position throughout life. Apart from the movement just
mentioned, in which the individual tree breaks up into several
discrete units, the Banyan seed commonly germinates in the
crown of a palm tree, from which it sends down roots to the
ground, eventually killing the palm and becoming established in
the soil.
In the past, there have been many exaggerated statements
concerning the height of trees. Mueller mentions a specimen
of the Black Peppermint (Eucalyptus salicifolia Car.) 471 feet high
and claims that Karri (Eucalyptus diversicolor F. v. M.)
exceptionally attains 400 feet. According to reliable estimates,
however, the tallest known tree appears to be a Redwood
(Sequoia sempervirens) 364 feet high : this species may reach a
diameter of 25 feet. Such figures really mean little to us, unless
they can be compared with something with which we are familiar:
364 feet is about twice the height of the Nelson column in Trafalgar
Square ; 25 feet is about the width of the average small villa
garden. The Tulip tree (Liriodendron tulipifera L.) may top
200 feet and have a bole 10 feet through, but for girth some of
the world's largest trees are Baobabs (Adansonia digitata L.)
of tropical Africa, with a diameter of as much as 30 feet, although
this species rarely exceeds 70 feet in height. There is on record
a Swamp Cypress (Taxodium mucronatum Ten.) growing in
Mexico, with the colossal diameter of 52 feet.
Size, however, is no indication of age. The Tulip tree grows
fast, and on a good site may attain a diameter of two feet in
40 years. The very soft-wooded Balsas (Ochroma spp.) of
Central and tropical South America normally grow to a height
of 70 feet and a diameter of from 18 inches to two feet in seven
years ; they may even top 80 feet and a diameter of 21/2 feet in five
years. Many trees, however, grow slowly : such among our
native trees are the Yew (Taxus baccata L.) and the Box (Buxus
sempervirens L.). Moreover, the size to which a tree attains
depends much upon its environment : dwarf conifers and other
trees, a few inches high and many years old, are well-known
horticultural curiosities and are no more than trees which have
grown under exceptionally rigorous conditions. MacMillan has
recorded naturally dwarfed conifers growing in rock crevices on
Vancouver Island: among them two Silver Spruces (Picea