262 CONTRIBUTIONS TO
we find that it consists throughout of gelatinous prosenchymatous cells which perfectly resemble one another. Shortly afterwards, some separate longitudinal rows of these cells appear to have increased somewhat in breadth, which is the only circumstance that distinguishes them from the adjacent mass. As development advances, we observe that some dark spots appear upon the walls of some of these expanded cells, which we soon recognise to be small, flat air-bubbles, that have been formed between their walls and those of the neighbouring cell. Gradually all the expanded cells which are so disposed one upon the other are changed in this way: the air-bubble gradually appears more sharply defined, assuming the circular or oval figure; and there appears in its centre a smaller circle which constantly becomes more distinct, and which originates in the following manner: when the deposition of new masses takes place upon the inner wall of the cell, the parts corresponding to the outer air bubble remain free from the deposit, thus forming a small canal which traverses the newly-deposited mass. We now recognise the fully-developed porous vessel, the partition walls between each two superimcumbent cells appearing at the same time to be more or less absorbed. This history of the formation of the porous vessels, which may readily be observed in limes and willows, greatly contradicts the general notion that the porous canals serve to facilitate the communication of the sap. As the air-bubble is first formed on the outside of the wall, it renders the passage of the sap at that spot impossible, and for this reason the origin of the porous canal might be most readily and naturally explained as a local atrophy of the cell-wall. At the same time the above shows that the distinction between fir-wood and that of trees which bear leaves, in respect to anatomical structure, cannot be of such vast physiological importance; since, with similar elements and development, the distinction is really based on the larger or smaller number of cells that are converted into porous vessels.
There are still, however, a great many gaps to fill up. In particular the origin of the medullary rays, and their relation to the wood; the formation of the new bark; and, lastly, the origin of the buds in the body of the wood, are so many questions for extended researches, to the execution of which, however, we
