The roots are put out of
their course by meeting with any obstacle in their way. Almost the only
exception to the rule in accordance with which roots descend under
natural circumstances, is that furnished by _Trapa natans_, the roots of
which in germination are directed upwards towards the surface of the
water. So in _Sechium edule_, the seed of which germinates while still
in the fruit, the roots are necessarily, owing to the inverted position
of the embryo, directed upwards in the first instance.

A downward direction of the stem or branches occurs in many weak-stemmed
plants growing upon rocks or walls, or in trees with very long slender
branches as in _Salix Babylonica_, and the condition may often be
produced artificially as in the weeping ash.

The opposite change occurs in what are termed fastigiate varieties,
where the branches, in place of assuming more or less of a horizontal
direction, become erect and nearly parallel with the main stem as in the
Lombardy poplar, which is supposed to be merely a form of the black
Italian poplar.

M. de Selys-Longchamps has described a similar occurrence in another
species of Poplar (_P. virginiana_ Desf.), and amongst a number of
seedling plants fastigiate varieties may frequently be found, which may
be perpetuated by cuttings or grafts, or sometimes even by seed; hence
the origin of fastigiate varieties of elms, oaks, thorns, chestnuts, and
other plants which may be met with in the nurseries.

Sometimes when the top of the main stem is destroyed by disease or
accident, one of the heretofore lateral shoots takes its place, and
continues the development of the tree in the original direction. It is
often an object with the gardener to restore the symmetry of an injured
tree so that its beauty may ultimately not be impaired.[210]

Climate appears sometimes to have some influence on the direction of
branches, thus Dr. Falconer, as quoted by Darwin,[211] relates that in
the hotter parts of India "the English Ribston-pippin apple, a Himalayan
oak, a Prunus and a Pyrus all assume a fastigiate or pyramidal habit,
and this fact is the more interesting as a Chinese tropical species of
_Pyrus_ naturally has this habit of growth. Nevertheless many of the
fastigiate varieties seen in gardens have originated in this country by
variation of seeds or buds."

M. Carriere has also recorded a curious circumstance with reference to
the fastigiate variety of the false acacia _Robinia pseud