the cauline bundles, which extend above to the punctum
vegetationis of the stem without bending outwards into leaves;
and leaf-bundles, which belong to the leaves only. He laid it
down as a general rule as regards the common bundles in
Dicotyledons and Conifers that they begin to form where their
ascending and descending halves meet, at the spot therefore
where they bend outwards into the leaf, and continue to form
as they descend into the stem and ascend into the leaf by
differentiation of suitable tissue. It follows from the nature of
these common bundles, that a more thorough understanding
of their course and origin presupposes a more accurate knowledge of the order of formation of the leaves at the end of the
stem and of the changes in the phyllotaxis during growth;
these relations Nägeli took into detailed consideration, and
even derived from them new points of view for the examination of the genetic arrangement of leaves, pointing out at the
same time the unsatisfactory nature of the principles of the
doctrine propounded by Schimper and Braun. Nägeli was
also the first who compared the anatomical structure of roots
with that of stems, and drew attention to the peculiar character
of the fibrovascular body in these organs. As his previous
discovery of the apical cell and its segmentation promoted
further research, so now his treatise on fibrovascular strands
called forth many others from various quarters; among them
that of Carl Sanio on the composition of the wood ('Botanische Zeitung,' 1863) must be mentioned as one of the
first and most important, and as serving in conjunction with
the works of Hanstein and Nägeli to throw light upon the
processes of growth in thickness of stems. It has been already
said that neither von Mohl nor Schleiden, neither Schacht nor
Unger succeeded in finding the true explanation of growth in
thickness. It was impossible that they should do so, for they
were insufficiently acquainted with the origin, true course, and
composition of the vascular bundles before growth in thickness
commences; the study of the subject was greatly perplexed by
Page:History of botany (Sachs; Garnsey).djvu/369
Chap. iv.]
Classification of Tissues.
349
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