262 CONTRIBUTIONS TO 
we find that it consists throughout of gelatinous prosenchy- 
matous cells which perfectly resemble one another. Shortly 
afterwards, some separate longitudinal rows of these cells ap- 
pear 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 gra- 
dually 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 corre- 
sponding 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 superincumbent cells ap- 
pearing 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 ele- 
ments 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 
