They are thought to be the openings of the cells, through which various juices may pass. Some cells are round and regular, as in the leaf; while those of the wood, from being much crushed, are spindle shaped, like a tap root. Would they not be round in the fruit, then? They would, Willie, and are there filled with some nice secretions, that boys rather enjoy. In the pretty leaves of flower, they contain a coloured fluid. Some cells hold carbon, and others have crystals of flint, lime, iron, starch, &c. Sugar, gum, resins, turpentine, &c., are in cells. That is where the scratching stuff comes from, which I have felt in the cutting grass and wheat straw. But are there cells in roots? Yes. There are, also, long tubes in plants like our water pipes, which are open at each end, not closed like the cells. They are parallel to the line of the stem, and are extremely small. They may be called elongated or lengthened cells, and form what is called the _vascular system_, or system of vessels. 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. * * * * * Father, what are these silver lines through the solid wood, coming from the pith centre? They are the _medullary_ or _pith rays_, and reach the liber or inner bark. They convey nourishment. Through them there is also a communication between the root and the leaf. I want to ask a question about the leaf. Does it throw off anything as well as absorb?