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 



