20.  Wood, whether vascular or fibrous, is however formed, in outlaid plants, first outside of the pith, and then, in shoots of the second year, outside of the wood of the first, and in the third year, outside of the wood of the second; so that supposing the quantity of wood sent down from the growing shoot distributed on a flat plane, the structure in the third year would be as in Figure 27.  But since the new wood is distributed all round the stem, (in successive cords or threads, if not at once), the increase of substance after a year or two would be untraceable, unless more shoots than one were formed at the extremity of the branch.  Of actual bud 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 cells;—­’cambium’ for an early state of both, and ‘diachyma’ for a peculiar position of one![46] while the chemistry of all these substances is wholly neglected, and we have no idea given us of any difference in pith, wood, and bark, than that they are made of short or long—­young or old—­cells!