Essays on the Microscope / Containing a Practical Description of the Most Improved Microscopes, a General History of Insects, etc., etc.

by George Adams

OF THE SAP VESSELS.

The most numerous and the largest apertures are generally to be found in the wood, which are perceived very distinctly in a transverse section, in which the ends of the vessels are seen as cut through by the knife. The scarlet oak of America is recommended as a proper object for exhibiting them. If a short cylinder of a three years branch of this oak, a little macerated, be hollowed away with a chissel, we shall see what a large portion of the wood is occupied by these vessels; they are thick and strong, and it is easy, with some care and attention, to loosen several of them.

If a number of these thus separated be put into a vial of rain water, and frequently shook for several days, some will at length be found perfectly clean; these are then to be put into spirit of wine, and when that has been two or three times changed, they will be in a condition to be viewed for understanding their structure; another method of preparation has already been shewn in [page 162].

These are the vessels which have been called by some writers air, by others, tracheal vessels. It is, however, to be remarked, that most of those who have considered them as air vessels, refer us to the tree while in a more herbaceous state; in this case they say, that we shall find these parts filled with a fine spiral filament. As these vessels are often to be found empty, they have been supposed to answer the purposes of lungs to the plant. Malpighi asserts, that if they be examined in winter, they often exhibit a vermicular motion, which astonishes the spectator.

Those who suppose the corona to contain the whole structure of the tree in miniature, and that it is the embryo of future shoots, suppose it to contain the vessels proper for each part, a subject that must be left to the decision of future observers.