THE LIVING TREE 115
of tropical trees, while sometimes trees have adventitious roots
which leave the trunk above ground level and grow obliquely
downwards into the soil: the common Umbrella tree (Musanga
Smithii R.Br.) of tropical West Africa shows beautiful roots
of this type (flying buttress roots) ; exactly similar roots are
the prop roots so well shown in a maize plant, although here,
of course, they do not attain large dimensions.
Nor in this account of the root must mention be omitted
of the knee roots or pneumatophores characteristic of some trees
which grow in swamps. These roots are beautifully shown
by some Mangroves and vary in form, although all are charac-
terised by an abundance of aerating tissue which permits of
gaseous interchange : in Avicennia they are erect and rise a few
inches above the mud or water surface and in Sonneratia a regular
forest of erect knee roots appears, and these are sometimes several
feet long. In Britain good examples can be seen in Swamp
Cypress trees growing in wet places.
So much for the growth of the primary meristems of woody
plants. By their continued, if periodic growth, the extremities
of the tree continually increase in length: thus precision of form,
comparable to that of an animal, can never be obtained; never-
theless, by the growth of some meristems and the suppression
of others a characteristic shape is attained. To compensate
for the extension growth, however, there must be an increase in
girth in the stems if, with their increasing length, they are to remain
erect. Moreover, as the tree becomes bigger the need for a larger
conducting system becomes imperative, hence the need for an
increase in girth to accommodate it. Increase in diameter is
brought about by a cylinder of secondary meristem, the vascular
cambium, which lies between the xylem and phloem and which
produces new wood internally and new phloem externally. The
process need not be described, as it is well known and is dealt with
in the elementary text books. Sometimes, especially in young
trees, growth is rapid and in the wood an increment of half an inch,
that is. an increase in diameter of one inch in a season, is not
unusual. In many trees in this country this seasonal growth does
not start until the leaves begin to open and finishes by about
August. In some fast-growing tropical trees, like the Balsa
already mentioned, an increase in diameter of from two to four
inches is normal.
In temperate trees cambial activity starts at the buds and works
downwards, very rapidly in coniferous trees, but perhaps more
slowly in some broad-leaved trees. It seems that cambial
activity can only take place from the bud downward : thus if
a twig is broken just below a bud there is no activity from the
break down to the next bud, and that part of the stem dies.