and branch structure, I gave introductory account long since in the fifth
volume of 'Modern Painters.'[45] to which I would now refer the reader; but
both then, and to-day, after twenty years' further time allowed me, I am
unable to give the least explanation of the mode in which the wood is
really added to the interior stem. I cannot find, even, whether this is
mainly done in springtime, or in the summer and autumn, when the young
suckers form on the wood; but my impression is that though all the several
substances are added annually, a little more pith going to the edges of the
pith-plates, and a little more bark to the bark, with a great deal more
wood to the wood,--there is a different or at least successive period for
each deposit, the carrying all these elements to their places involving a
fineness of basket work or web work in the vessels, which neither
microscope nor dissecting tool can disentangle. The result on the whole,
however, is practically that we have, outside the wood, always a mysterious
'cambium layer,' and then some distinctions in the bark itself, of which we
must take separate notice.

21. Of Cambium, Dr. Gray's 220th article gives the following account. "It
is not a distinct substance, but a layer of delicate new cells full of sap.
The inner portion of the cambium layer is, therefore, nascent wood, and the
outer nascent bark. As the cells of this layer multiply, the greater number
lengthen vertically into _prosenchyma_, or woody tissue, while some are
transformed into ducts" (wood vessels?) "and others remaining as
_parenchyma_, continue the medullary rays, or commence new ones." Nothing
is said here of the part of the cambium which becomes bark: but at page
128, the thin walled cells of the bark are said to be those of ordinary
'parenchyma,' and in the next page a very important passage occurs, which
must have a paragraph to itself. I close the present one with one more
protest against the entirely absurd terms 'par-enchyma,' for common
cellular tissue, 'pros-enchyma,' for cellular tissue with longer