103
THE LIVING TREE
BY FRANK W. JANE, PH.D., D.SC., F.L.S.
[Presidential Address delivered March 29th, 1947]
IT does not seem possible to give a precise definition of a
tree; that which is generally accepted is that a tree is a
perennial plant, growing to a considerable height and size and
having a single, self supporting, woody main stem. A shrub,
in contrast, is a perennial plant having several woody stems
growing from the same root. A rigid acceptance of these
definitions would mean the exclusion of many woody perennial
plants and for present purposes it will be convenient to use the
term "tree" quite loosely, including within its scope all the
larger plants with perennial woody stems, that is, both trees
and shrubs and such intermediate types as do not fit exactly
into one category or the other. Such a course seems justified,
for some plants, like the Common Elder, may be either trees
or shrubs. In fact, such sub-shrubs as the Heather and Ling
might, with equal propriety, lay claim to consideration.
These woody plants are characterised by certain common
features : their position is generally fixed—they cannot move;
they are perennials; their shoot system, at least in part, persists
from year to year above ground; the apices of their stems (and
roots) are permanently embryonic, adding new material each
year to that already existing. The possession of such features
creates special problems, and problems which must be solved if
plants of this type are to exist and propagate their kind. It will,
therefore, be worth-while to study these features rather more
fully and the consequences inherent in possessing them.
The herbaceous plant, be it a perennial, can pass periods
in which adverse conditions prevail below ground, thanks to
its perennating organs ; if its habit is annual, then the unfavourable
period can be spent in the compact, resistant form of a seed.
A tree, however, with its persistent aerial parts and its fixed
position must take what comes to it; it can only survive if it is
able to cope successfully with the most extreme conditions of its
aerial environment. It is to be expected, therefore, that it will
possess adequate means for protecting its exposed aerial members.
Further, its permanently embryonic apices bring about a
continued increase in length, and it is clear that some means must
be available for increasing the girth of its aerial members to keep
pace with their increasing length, otherwise they would rapidly
become too long to support themselves and would sprawl along
the ground. Increase in girth normally leads to rupture of the
outer protective skin or epidermis of the young stem and, in