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?
You have been told that it absorbs oxygen from the air, and also
carbonic acid gas--that is, carbon and oxygen. This carbonic acid
floating in the air is absorbed in the day-time, though the leaf
exhales, or breathes out, oxygen gas while the sun shines.
_Does it not do so at night?_
No; then the leaf throws out some carbonic acid gas undigested. There
is this use of the oxygen being taken in by the leaf, that it acts
beneficially upon the sap to thicken it, &c.
What is the true shape of the leaf?
The course of veins or branching vessels. The fleshy part of the leaf
is of cellular tissue.
I have heard that our leaves differ from English ones: how are they
different? I know they do not fall off in autumn.
English leaves are made to fall, as from the first they are nearly
divided from the _petiole_ or leaf stalk, the mark of division being to
be seen in the budding leaf.
Have our Australian leaves that mark?
No, they have not. The petiole expands or becomes leafy, and the
leaves, therefore, do not readily fall. There is yet another
difference. While the glands or tumours for imbibing moisture are on
the underside of the leaf in England, they are on both sides of it
here. Instead, too, of being horizontal, our leaves are set in edgewise
on their stems.