portance in phytotomy, because it is the only way to the understanding of secondary growth in thickness in true woody plants.
It was noticed above, that von Mohl had proved in 1831
the separate character of the bundles which begin in the stem
and bend outwards into the leaves where they end, so that the
entire system of bundles in a plant consists of single bundles
isolated when formed and subsequently brought into connection
with one another. Nägeli had already examined the corresponding circumstances in the vascular Cryptogams in 1846, when
Schacht took the retrograde step of making the vascular
system in the plant originate in repeated branching, instead of
in subsequent blending of isolated strands ; Mohl declared
unhesitatingly against this mistake in 1858, but it was refuted
at greater length and still more clearly by Johannes Hanstein
in 1857, and by Nägeli in 1858. Hanstein in a treatise on the
structure of the ring of wood in Dicotyledons confirmed
Nägeli's previous statements, and proved in the case of
Dicotyledons and Conifers that the first woody circle in the
stem is formed from a number of vascular bundles, which are
identical with those of the leaves and originate in the primary
meristem of the bud. These primordial bundles pass downwards through a certain number of internodes in the stem
independent and separate, and either retain their isolation to
the point where they end below or unite with adjacent bundles
which originated lower down. Hanstein happily termed the
portions of the vascular bundles, which enter the stem from
the base of the leaf and traverse a certain portion of it in a
downward direction, leaf-traces, so that it may be stated
briefly, that the primary wood-cylinder in Dicotyledons and
Conifers consists of the sum of the leaf-traces. Nägeli's observations were of a more comprehensive character, and supplied,
as we have seen, a terminology for tissues. He distinguished
three kinds of vascular bundles according to their course; the
common bundles, which represent Hanstein's leaf-traces in the
stem, and whose upper ends bend outwards into the leaves
Page:History of botany (Sachs; Garnsey).djvu/368
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348
Development and
[BOOK II.
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