development before it is exposed
to these influences, extraordinary intermediate forms are obtained,
bearing the characters of both organs.

The study of regeneration following injury is of greater importance as
regards the problem of the development and place of origin of organs.
(Reference may be made to the full summary of results given by Goebel in
his "Experimentelle Morphologie", Leipzig and Berlin, 1908, Section IV.)
Only in relatively very rare cases is there a complete re-formation
of the injured organ itself, as e.g. in the growing-apex. Much more
commonly injury leads to the development of complementary formations, it
may be the rejuvenescence of a hitherto dormant rudiment, or it may be
the formation of such ab initio. In all organs, stems, roots, leaves,
as well as inflorescences, this kind of regeneration, which occurs in
a great variety of ways according to the species, may be observed on
detached pieces of the plant. Cases are also known, such, for example,
as the leaves of many plants which readily form roots but not shoots,
where a complete regeneration does not occur.

The widely spread power of reacting to wounding affords a very valuable
means of inducing a fresh development of buds and roots on places
where they do not occur in normal circumstances. Injury creates special
conditions, but little is known as yet in regard to alterations directly
produced in this way. Where the injury consists in the separation of
an organ from its normal connections, the factors concerned are more
comprehensible. A detached leaf, e.g., is at once cut off from a supply
of water and salts, and is deprived of the means of getting rid of
organic substances which it produces; the result is a considerable
alteration in the degree of concentration. No experimental investigation
on these lines has yet been made. Our ignorance has often led to the
view that we are dealing with a force whose specific quality is the
restitution of the parts lost by operation; the proof, therefore,
that in certain cases a similar production of new roots or buds may
be induced without previous injury and simply by a change in external
conditions assumes an importance. (Klebs, "Willkurliche Entwickelung",
page 100; also, "Probleme der Entwickelung", "Biol. Centralbl." 1904,
page 610.)

A specially striking phenomenon of regeneration, exhibited also by
uninjured plants, is afforded by polarity, which was discovered by
Vochting. (See the classic