nduce the coalition of organs primarily distinct.

CHAPTER I.

COHESION.

Following Augustin Pyranius De Candolle, botanists have applied the term
cohesion to the coalescence of parts of the same organ or of members of
the same whorl; for instance, to the union of the sepals in a
gamosepalous calyx, or of the petals in a gamopetalous corolla. It may
arise either from a union between organs originally distinct, or more
frequently from a want of separation between parts, which under general
circumstances become divided during their development. Nothing is more
common as a normal occurrence, while viewed as a teratological
phenomenon it is also very frequent. For the purposes of convenience it
admits of subdivision into those cases wherein the union takes place
between the branches of the same plant, or between the margins of the
same leaf-organ, or between those of different members of the same
whorl.

=Cohesion between the axes of the same plant.=--This cohesion may occur
in various manners. Firstly. The branches of the main stem may become
united one to the other. Secondly. Two or more stems become joined
together. Thirdly. The branches become united to the stem; or, lastly,
the roots may become fused one with another.

[Illustration: FIG. 2--Cohesion of two branches in _Dipsacus
sylvestris_.]

The first of these is most commonly met with, doubtless owing to the
number of the branches and the facilities for their union. An
illustration of it is afforded by the figure (fig. 2), showing cohesion
affecting the branches of a teazle (_Dipsacus sylvestris_). Union of the
branches may be the result of an original cohesion of the buds, while in
other cases the fusion does not take place until after development has
proceeded to some extent. Of this latter kind illustrations are common
where the branches are in close approximation; if the bark be removed by
friction the two surfaces are very likely to become united (natural
grafting). Such a union of the branches is very common in the ivy, the
elder, the beech, and other plants. It may take place in various
directions, lengthwise, obliquely, or transversely, according to
circumstances. This mode of union belongs, perhaps, rather to the domain
of pathology than of teratology. Some of the instances that have been
recorded of very large trees, such as the chestnut of Mount AEtna, are
really cases where fusion has taken place between several of the
branches, or sucker