afterwards there are large vessels, perfectly closed large cylindrical tubes with a transparent and very delicate membrane.'
He then shows how by degrees the sculpture peculiar to the
walls of vessels is formed on the inside of these tubes, and he
takes the opportunity of saying that a metamorphosis in time
from one form of vessel into another is entirely out of the
question, as Treviranus also and Bernhardi had maintained. 'The dividing walls (transverse septa),' he continues, 'are formed in a precisely similar manner to the side-walls of vessels; only the original tender membrane of the septa is usually lost in the meshes of the network of fibres.' From that time no phytoto-mist capable of an independent judgment has had any doubt with regard to this view of the formation of the vessels in wood. It is however striking enough that von Mohl, who thought it so important to show that the cell is the sole foundation of vegetable structure, never extended the proof to milk-vessels and
other secretion-canals in order to show whether and how these
also are formed from the cells. In his treatise on the vegetable
cell (1851) he still expressed doubt about Unger's assertion,
that the milk-vessels are also formed from rows of cells that
coalesce with one another, and held rather to the view of
an anonymous writer in the ' Botanische Zeitung ' of 1846,
page 833, that these vessels are membranous linings of gaps
in the cell-tissue. He might well lose his taste for the exam-
ination of these and similar organs after Schultz Schultzenstein had by his various treatises, written after 1824, on the so-called vital sap and the circulation which he attributed to it, made this part of phytotomy a very quagmire of error, and had not refrained from replying in an unbecoming manner to von Mohl, who repeatedly opposed his views; moreover Schultz's essay
'Ueber die Circulation des Lebenssafter' (1833), which teems with absurdities, had received a prize from the Academy of Paris.
2. The growth in thickness of the cell-membrane, and the sculpture caused by it was a subject that is more or less connected with most of von Mohl's writings. He developed the
