Page 10 - Tài liệu Ebook cây cảnh Bonsai Basics

12 HOW TREES GROW

Flowers and fruits
Almost all trees propagate themselves
sexually from seeds, which develop as
the result of the union of a male and a
female cell. This union takes place within
the flower, which is simply a leaf spe-
cially adapted for reproduction pur-
poses. Seeds vary greatly in type and
shape from tree to tree. For instance,
they may take the form of a fleshy fruit
whose pulp contains the seeds, or a
protective nut, a berry, or an achcnc
(dry, one-seeded fruit), or held within a
cone in the case of a conifer. Reproduc-
Elm, ash and maple produce winged fruits l.ikc this, tive cells are produced in the flower by
called samaras. specialized organs: the stamens produce
male cells (pollen), while the pistil pro-
duces the female cells (ovules). Where
stamens and pistils are present in the
same flower, the plant is said to be
hermaphrodite; otherwise it is unisexual.
In hermaphrodite plants, pollen falls
directly on to the pistil sheltered by the
petals, thereby fertilizing the ovules. In
unisexual plants, pollen must be con-
veyed from flower to flower by the wind
or honey-gathering insects.
The most beautiful flowers are fre-
quently found on monoecious plants,
since the bright colours of the petals are
designed to attract honey-gathering in-
sects. The flowers of forest trees (as
The acorn, fruit and seed of the oak, is attached to
opposed to ornamental shrubs) are often
the twig by a cupula.
lacking in colour, to the point of insig-
nificance, hardly standing out from the
leaves, since they are mainly adapted to
wind pollination. Some plants bear
Oowers of one sex o nly, male or female
(dioecious), others flowers of both sexes
(monoecious), while some kinds have
male, female and hermaphrodite Oowers
(polygamous).
Pollination produces seeds, which,
after germinating in the earth will
produce a new plant like its parent
(unless the plant has been grafted). For
successful germination, conditions, par-
ticularly humidity, heat and light must
be favourable. The percentage of seeds
which germinate in nature is very low,
Some conifers, like the false cypress
( Cltamaecyparis) and juniper do not produce typical particularly in those trees whose seeds
cones but round, scaly fruits. are small and without any food reserves.