Is there any other sort of vessels?
Some are spiral, or twisted. The tracheae are such. Some are round the pith. It is supposed that these convey air from one part to another.
Are these spirals everywhere?
No, there are none in the bark nor root—nor have ferns any. Speaking of air tubes, I may tell you that aquatic or water plants have air cells divided from other parts by layers of cellular tissue.
What do the spirals want with air?
You have been told that oxygen of the air is as necessary to the life and nourishment of a tree as of a boy; sometimes the leaves do not absorb enough, and then the spirals get it, through the root, from the moistened soil.
I can now understand why so large a thing as a tree becomes so small when the really solid part is got together. If all the cells and vessels were jammed up tight, so that the air and juices were driven out, how thin our thirty-feet round gum tree would appear! But will you tell me if the regular sap runs through those vessels we have talked of?
It is considered that it runs rather between the interstices or openings between the cells.
Why do you say considered and supposed?
Simply because the things we talk of are so very minute or small that the most powerful microscope will scarcely exhibit them, and thus it is difficult to observe their nature and action.