1273. _Why are there curious markings in walnut, mahogany, rose-wood,
satin-wood, &c.?_
Because those markings are produced by the various _structure of the
vessels_ by which the wood is formed; and by successive zones of wood,
which indicate the periods of growth.
The inclosure of zone within zone is owing to the mode in which
the wood is produced, and the position in which it is deposited.
Wood is formed by the leaves during the growing season, and
passes down towards the root between the bark and the wood of the
previous year (if any), or in the position in which cambium is
effused; and, as the leaves more or less surround the whole stem,
the new layer at length completes a zone, and perfectly encloses
the wood of all former years. This is the explanation of the term
_exogenous_, which is derived from two words signifying to grow
outwardly, for the stem increases in thickness by successive
layers on the outer side of the previously-formed wood. That this
is the mode of growth has been abundantly proved by experiment,
and demonstrated by accidental discoveries. Thus, if a plate of
metal be inserted between the bark and wood, it will, in progress
of time, become inclosed by the new wood which has overlaid them.
So in like manner if letters be cut deeply through the bark and
into the wood, the spaces will not be filled up from the bottom,
but may be seen in subsequent years overlaid by new wood. A
statement appeared in a daily paper, during the past year, to
the effect that in cutting down a tree a cat had been discovered
inclosed in the wood of the trunk. These facts prove that the wood
is applied from without. Again, if a branch be stripped of its
leaves down to a certain point, it will not grow above that point;
and so, in like manner, if branches be stripped from one side of
a tree, the tree will not grow on that side. If a circle of bark
be removed from a branch above and also below a leaf, it will be
found that increase of size will occur below, but not above that
bud; and so, likewise, whenever a ring of bark is removed from a
tree, the new woody fibre will not proceed from the lower but from
the upper edge.--_Orr's Circle of the Sciences._
[Verse: "And when he saw a fig tree in the way, he came to it, and
found nothing thereon, but leaves only, and said unto it, Let no fruit
grow on thee henceforward for ever. And presently the tree withered
away."--MATTHEW XXI.]
1274. _Why have trees with large trunks a great number of leafy
branches?_
Because it is _by the leaves_ that the secretion is formed which
supplies the _woody fibre_. The number of leaves on a tree, therefore,
generally bears a relation to the size of its trunk, and the number of
its branches.
1275. _Why have poplar-trees comparatively few branches and leaves?_
Because their trunks are comparatively _small_, although they grow to a
great height.
1276. _Why had the mammoth-tree comparatively few leaves in relation to
the immense size of its bark?_
Because the woody texture of this tree (_Wellingtonea gigantea_) is
_exceedingly light and porous_. It is, in fact, lighter than cork, and,
therefore, requires less leaf-produce in its formation.
1277. _Why have oak-trees an abundance of leaves?_
Because their wood is _so dense_ that they require a larger amount of
the wood-forming secretion which is supplied by the leaves.
1278. _Why are the trunks of trees round?_
Because, generally speaking, the leaves are distributed upon branches
around the trees in every direction. They consequently send down the
wood-forming principle on all sides. When a trunk is unduly developed
on one side, it may generally be traced to the unequal distribution of
the branches.
1279. _What are exogenous stems?_
Exogenous stems are those that grow by the addition of wood _on their
outer surface_, underneath the bark.
1280. _What are endogenous stems?_
Endogenous stems are those that _grow inwardly_, from the centre. Trees
of this class, of which palms are the best example, are almost peculiar
to tropical climates.
1281. _Why do endogenous stems chiefly abound in tropical climates?_
Because, probably, the excessive heat of those climates would
interfere with the _formation of wood from the sap_ upon the outer
surface.
The vascular structure of endogenous stems lying more abundantly
towards their centre, tends to _conserve the juices_ which in hot
climates are so highly valued. Palm-wine is a delicious and cooling
beverage, and is procured from various kinds of palms, but especially
from the cocoa-nut palm. Even the fresh sap is very refreshing. The
juice is procured by cutting the tree in the upper part, and attaching
a vessel to the opening, to receive the sap. Its flow is increased by
cutting off a slice of the wood daily.
[Verse: "I have caused thee to multiply as the bud of the field, and
thou hast increased and waxen great, and thou art come to excellent
ornaments."--EZEKIEL XVI.]
1282. _Why have endogenous stems no bark?_
Because, one of the chief functions of the bark in exogenous trees,
is to _protect the sap_ from which the wood is formed on the outward
surface; and as there is no such external flow of sap in endogenous
trees, the bark is _unnecessary to them_, and is therefore withheld.
They are furnished instead with a thin cuticle.
1283. _Why do endogenous stems grow to a great height?_
Because, as the stem grows from the centre, it soon reaches that limit
of diameter _which its vascular structure is calculated to support_;
and, therefore, the wood-forming sap is deposited chiefly at the top of
the stem, causing it to grow to a considerable height.
1284. _Why do the various vegetable fruits ripen in succession?_
Because the Author of Nature has thus arranged its economy, _in order
that the wants of living creatures may be adequately provided for_.
Some vegetable productions arrive at their perfection in the spring;
others in summer; and others in autumn. Among the latter are many that
require to come slowly to maturity after they are gathered; by these
the winter season is provided for, and a surplus of the winter stock
goes to supply the natural deficiency of spring.
[Verse: "O sing unto the Lord a new made song; for he hath done
marvellous things."--PSALM XCVIII.]
1285. _Why, when seeds are sown, and germination begins, does the
leaf-germ seek the light, and the root-germ grow down into the earth?_
Because the Creator has endowed every single seed with a _vital
instinct which governs its development_. The rootlet could more easily
grow upward than downward, because of the looser earth, and of the
exciting influences of light and moisture. Yet it takes the contrary
course, leaving the leaf-germ to come up to meet the sun-light, and to
send down to the stem and roots, the matter needed for their growth.
Frequently, indeed, when seeds are thrown into the earth, their natural
position is reversed, and when the germs first start from the seed, the
_root-germ_ is directed _upward_ and the _leaf-germ downward_. What
then occurs? They each turn, and, in doing so, frequently cross each
other. Each goes to its particular duty--the duty that God appointed.