THE LIVING TREE 109
shoots are referred to as lammas shoots; it is safe to say that
we do not understand the underlying causes which promote the
formation of these lammas shoots: high rainfall, drought,
mutilation, previous defoliation and heavy manuring are all
said to favour their formation. Sometimes the leaves of lammas
shoots differ from those of the rest of the tree : thus in Oak their
leaves may be more deeply incised while in one variety (Quercus
petraea mespilifolia (Wallr.) Schwarz) in which the leaves are
entire and pointed at the tip, the leaves of the lammas shoots are of
the normal form for the species. It would also be of supreme
interest to know what stimulus in some trees causes the leaf
primordia to develop into scale leaves in a period of the year
which appears to be most favourable to active growth, and then,
more or less suddenly, to revert to the formation of normal
leaves ready for the following year. On this matter we are
ignorant.
So far we have tacitly assumed that the extension growth of
a shoot is continued by its terminal bud: this is sometimes true,
as in Spruce, where a terminal bud persists throughout life,
but in other trees like Lime and Elm the terminal bud dies and
its place is taken by the topmost lateral bud. In the former
case, of course, the branch is directly formed from a single bud,
increasing in length year after year by the activity of the meri-
stem of this bud : the branch forms a monopodium and this type
of growth is spoken of as monopodial. In the latter type of
growth the branch is different, for each successive increment is,
in fact, a lateral outgrowth of its predecessor : it is a sympodium
and growth of this type is termed sympodial. As we shall see,
these two types of growth directly affect the ultimate shape
of the tree. It has been found possible to prolong the life of
the terminal bud by removing the upper leaves of a shoot or by
removing the more distally placed buds, while abnormal rainfall
has a similar effect : but sooner or later the result is always the
same and we must regard this abbreviated life of the terminal
bud in some trees as an inherent factor in the make-up of the
tree.
It is well known that all the shoots in some trees are not
similar. Thus most varieties of Apples, and Pears have shoots
which bear the leaves and dwarf, short or spur shoots on which the
fruit is borne. Sometimes the difference between the two types is
striking. In the Pines the foliage leaves are borne in clusters of
two, three or five on a very short stem and these dwarf shoots arise
on long shoots which themselves produce no foliage leaves, but
merely scale leaves. In the Cedars, on the contrary, both the long
and short shoots bear needles. A short shoot may suddenly ex-
pand into a long shoot (regeneration shoot) or it may die after a