1167. _Why do some plants droop, and turn to the earth after sunset?_
Because, when the warmth of the son's rays is withdrawn, they turn
downwards, and _receive the warmth of the earth by radiation_.
1167. _Why does the young ear of corn first appear enfolded in two green
leaves?_
Because the light and air would _act too powerfully for the young ear_;
two leaves therefore join, and embrace the ear, and protect it until it
has acquired strength, when they divide, and leave the ear to swell and
ripen.
1168. _Why are the seeds of plants usually formed within the corollas
of flowers?_
Because the petals of the flowers, surrounding the seeds, _afford them
protection until they are ripened_, when the flower dies, and the
petals fall to the ground.
1169. _Why does the flower of the poppy turn down during the early
formation of seed?_
Because the heat would probably be too great for the seed in its
early stage. The plant is therefore provided with a curious _curve in
its stalk_, which turns the flower downward. But when the seeds are
prepared for ripening, _the stalk erects itself_, and the seeds are
then presented to the ripening influences of the sun.
1170. _Why have plants of the pea tribe, a folding blossom called the
"boat," or "keel?"_
Because, within that blossom the pea is formed, and the shape of the
blossom is exactly suited to that of the pea which is formed therein.
The blossom is itself protected by external petals; and when the wind
blows, and threatens to destroy the parts upon which the seeds depend,
the plants _turn their backs to the wind_, and shelter the seed.
[Verse: "The fruit of the righteous is a tree of life; and he that
winneth souls is wise."--PROVERBS XI.]
1171. _Why are the leaf buds enclosed in scales which fall off as the
leaf opens?_
Because the scales _serve as a shelter_ to the tender structure of the
young leaf. The scales are rudimentary leaves, formed at the end of the
previous season, and which, being undeveloped then, serve to guard the
young leaves of the future year.
In trees, especially those which are natives of colder climates,
this point is taken up earlier. Many of these trees (observe
in particular the _ash_ and the _horse-chestnut_) produce the
embryos of the leaves and flowers in one year, and bring them to
perfection the following. There is a winter therefore to be gotten
over. Now what we are to remark is, how nature has prepared for
the trials and severities of that season. These tender embryos
are, in the first place wrapped up with a compactness, which
no art can imitate; in which state they compose what we call
the bud. This is not all. The bud itself is enclosed in scales;
which scales are formed from the remains of past leaves, and
the rudiments of future ones. Neither is this the whole. In the
coldest climates, a third preservative is added, by the bud
having a _coat_ of gum or resin, which, being congealed, resists
the strongest frosts. On the approach of warm weather this gum
is softened, and ceases to be an hindrance to the expansion of
the leaves and flowers. All this care is part of that system
of provisions which has for its object and consummation, the
production and perfecting of the seeds.--_Paley._
1172. _Why are the seeds of many plants enclosed in a rich juice, or
pulp?_
Because the matter by which the seed is surrounded, as well as being
intended for the _nourishment and care of the seed_, is designed for
the use of man and of animals, by whom the seed is set free to take its
place in the earth.
By virtue of this process, so necessary, but so diversified, we
have the seed, at length, in stone-fruits and nuts, incased in
a strong shell, the shell itself enclosed in a pulp or husk, by
which the seed within is, or hath been, fed; or, more generally
(as in grapes, oranges, and the numerous kinds of berries),
plunged overhead in a glutinous syrup, contained within a skin or
bladder; at other times (as in apples and pears) embedded in the
heart of a firm fleshy substance; or (as in strawberries) pricked
into the surface of a soft pulp.
These and many more varieties exist in what we call _fruits_.
In pulse, and grain, and grasses; seeds (as in the pea tribe)
regularly disposed in parchment pods, which, though soft and
membranous, completely exclude the wet even in the heaviest
rains; the pod also, not seldom, (as in the bean), lined with
a fine down; at other times (as in the senna) distended like a
blown bladder; or we have the seed enveloped in wool (as in the
cotton-plant), lodged (as in pines) between the hard and compact
scales of a cone, or barricadoed (as in the artichoke and thistle)
with spikes and prickles; in mushrooms, placed under a pent-house;
in ferns, within slits in the back part of the leaf; or (which
is the most general organisation of all) we find them covered by
strong, close tunicles, and attached to the stem according to an
order appropriated to each plant, as is seen in the several kinds
of grains and of grasses.
[Verse: "And I will send grass in thy fields for thy cattle, that
thou mayest eat, and be full."--DEUTERONOMY XI.]
In which enumeration, what we have first to notice is, unity of
purpose under variety of expedients. Nothing can be more _single_
than the design; more _diversified_ than the means. Pellicles,
shells, pulps, pods, husks, skin, scales armed with thorns, are
all employed in prosecuting the same intention. Secondly; we may
observe, that in all these cases, the purpose is fulfilled within
a just and _limited_ degree. We can perceive, that if the seeds
of plants were more strongly guarded than they are, their greater
security would interfere with other uses. Many species of animals
would suffer, and many perish, if they could not obtain access
to them. The plant would overrun the soil; or the seed be wasted
for want of room to sow itself. It is, sometimes, as necessary to
destroy particular species of plants, as it is, at other times,
to encourage their growth. Here, as in many cases, a balance is
to be maintained between opposite uses. The provisions for the
presentation of seeds appear to be directed, chiefly against
the inconstancy of the elements, or the sweeping destruction of
inclement seasons. The depredation of animals, and the injuries
of accidental violence, are allowed for in the abundance of the
increase. The result is, that out of the many thousand different
plants which cover the earth, not a single species, perhaps, has
been lost since the creation.
When nature has perfected her seeds, her next care is to disperse
them. The seed cannot answer its purpose, while it remains
confined in the capsule. After the seeds therefore are ripened,
the pericarpium opens to let them out, and the opening is not
like an accidental bursting, but for the most part, is according
to a certain rule in each plant. What I have always thought very
extraordinary; nuts and shells, which we can hardly crack with
our teeth, divide and make way for the little tender sprout
which proceeds from the kernel. Handling the nut, I could hardly
conceive how the plantule was ever to get out of it. There are
cases, it is said, in which the seed-vessel, by an elastic jerk,
at the moment of its explosion, casts the seeds to a distance.
We all, however, know, that many seeds (those of most composite
flowers, as of the thistle, dandelion, &c.) are endowed with what
are not improperly called _wings_; that is, downy appendages,
by which they are enabled to float in the air, and are carried
oftentimes by the wind to great distances from the plant which
produces them. It is the swelling also of this downy tuft within
the seed-vessel that seems to overcome the resistance of its
coats, and to open a passage for the seed to escape.
But the _constitution_ of seeds is still more admirable than
either their preservation or their dispersion. In the body of the
seed of every species of plant, or nearly of every one, provision
is made for two grand purposes: first, for the safety of the
_germ_; secondly, for the temporary support of the future plant.
The sprout, as folded up in the seed, is delicate and brittle
beyond any other substance. It cannot be touched without being
broken.
Yet in beans, peas, grass-seeds, grain, fruits, it is so fenced
on all sides, so shut up and protected, that whilst the seed
itself is rudely handled, tossed into sacks, shovelled into heaps,
the sacred particle, the miniature plant remains unhurt. It is
wonderful, also, how long many kinds of seeds, by the help of
their integuments, and perhaps of their oils, stand out against
decay. A grain of mustard-seed has been known to lie in the earth
for a hundred years; and as soon as it had acquired a favourable
situation, to shoot as vigorously as if just gathered from the
plant. Then, as to the second point, the temporary support of
the future plant, the matter stands thus. In grain, and pulse,
and kernels, and pipins, the germ composes a very small part
of the seed. The rest consists of a nutritious substance, from
which the sprout draws its aliment for some considerable time
after it is put forth; viz., until the fibres, shot out from the
other end of the seed, are able to imbibe juices from the earth,
in a sufficient quantity for its demand. It is owing to this
constitution that we see seeds sprout, and the sprouts make a
considerable progress, without any earth at all.
[Verse: "Say not ye, There are four months, and then cometh harvest?
behold, I say unto you, Lift up your eyes, and look on the fields; for
they are white already to harvest."--JOHN IV.]
From the conformation of fruits alone, one might be led, even
without experience, to suppose, that part of this provision
was destined for the utilities of animals. As limited to the
plant, the provision itself seems to go beyond its object. The
flesh of an apple, the pulp of an orange, the meat of a plum,
the fatness of the olive, appear to be _more_ than sufficient
for the nourishing of the seed or kernel. The event shows, that
this redundancy, if it be one, ministers to the support and
gratification of animal natures; and when we observe a provision
to be more than sufficient for one purpose, yet wanted for another
purpose, it is not unfair to conclude that both purposes were
contemplated together.--_Paley._
1173. _Why have climbing plants tough curly tendrils?_
Because, _having no woody stalks of their own_ to support them, they
require to take hold of surrounding objects, and raise themselves from
the ground by climbing. Their spiral tendrils are, therefore, so many
hands, assisting them to rise from the earth.
1174. _Why does the pea put forth tendrils, and the bean not?_
Because the bean has in its stalk _sufficient woody fibre to support
itself_, but the pea has not. We do not know a single tree or shrub
having a firm strong stem sufficient for its support which is _also_
supplied with tendrils.
1175. _Why do the ears of wheat stand up by day, and turn down by
night?_
Because, when the ear is becoming ripe, the cold dew falling into the
ear, might _induce blight_; the ears therefore turn down to the earth,
and _receive warmth by radiation_.
1176. _Why have grasses, corn, canes, &c., joints, or knots in their
stalks?_
Because a long hollow stem would be liable to bend and break. But the
joints are so many points where the fibres are bound together, and the
structure _greatly strengthened_.
[Verse: "Then shall the earth yield her increase; and God, even our
own God, shall bless us."--PSALM XLVII.]
1177. _Why have the berries of the mistletoe a thick viscid juice?_
Because the mistletoe is a _parasitical_ plant, growing upon the bark
of other trees. It will not grow in the ground; its seeds are therefore
filled with an exceedingly sticky substance, which serves to attach
them to the bark of trees, to which the berries attach themselves at
once, by throwing out tough fibres; and the next year the plant grows.
[Illustration: Fig. 75.--THE MISTLETOE.]
1178. _How are the seeds of the mistletoe transferred from its own stem
to the bark of trees?_
Various birds, and particularly the _missel thrush_, feed upon the
berries. As the bird moves in pursuit of its food, the viscid berries
attach themselves to its feathers, and in this way the thrush is the
instrument which conveys the seed to the spot to which it adheres, and
from which the tree ultimately grows.
1179. _What is the circulation of the sap in plants?_
The circulation of the sap is the movement of the nutritive juices by
which the plant is sustained. There is a slow uninterrupted movement
of the sap from the root through the stems to the leaves, and downwards
from the leaves through the bark to the root.
[Verse: "For the sun is no sooner arisen with a burning heat, but it
withereth the grass, and the flower thereof falleth, and the grace of
the fashion of it perisheth: so also shall the rich man fade away in
his ways."--JAMES I.]
1180. _Why does the sap of plants thus ascend and descend?_
Because it _conveys upward_ from the ground some of the matter by which
the plant is to be nourished, and which must undergo digestion in the
leaves; and it _brings downward_ from the leaves the matters absorbed,
for the nourishment of the plant, and discharges through the root the
substances which the plant cannot use.
The movement of the sap is most active in the spring; but in the depths
of the winter it almost ceases.
There are other motions of the sap in plants, which are called
_special_, in distinction from the ascending and descending of
the sap, which is called _general_, or common to all plants. The
special movements of the sap are peculiar to certain plants, in
some of which a fluid, full of little green cells, is found to
have a rotatory motion; in other plants, a milky fluid is found to
move through particular tissues of the vegetable structure.
1181. _Why are the leaves of plants green?_
Because they secrete a carbonaceous matter, named _chlorophyll_, from
which they derive their green colour.
1182. _Why are the hearts of cabbages, lettuces, &c., of a pale yellow
colour?_
Because the action of _light_ is necessary to the formation of
_chlorophyll_; and as the leaves are folded upon each other, they
exclude the light, and the green matter is not formed.
1183. _Why do leaves turn brown in the autumn?_
Because, when their power of decomposing the air declines, the _oxygen_
absorbed in the carbonic acid gas, _lodges in the leaf_, imparting to
it a red or brown colour.
1184. _Why do succulent fruits, such as gooseberries, plums, &c., taste
acid?_
Because, in the formation of juices, a considerable amount of _oxygen_
is absorbed, and the oxygen imparts acidity to the taste.
[Verse: "The earth is full of the goodness of the Lord."--PSALM
XXXIII.]
1185. _Why do ripe fruits taste sweet, and unripe fruits taste sour?_
Because the juices of the ripe fruit contain a large proportion of
_sugar_, which in the unripe fruit has not been formed.
1186. _Why do some leaves turn yellow?_
Because they retain an excess of _nitrogen_. Leaves undergoing decay
turn either yellow, red, crimson, or violet. Yellow is due to the
excess of _nitrogen_; red and crimson to various proportions of
_oxygen_; violet to a mixture of _carbon_; and green to _chlorophyll_.
1187. _Why do leaves fall off in the autumn?_
Because they have supplied for a season the natural wants of the tree.
Every part has received nutrition through the spring and summer months;
and the wants of the tree being supplied, the chief use of the leaf
ceases, and it falls to the ground to decay, and enrich the soil.
1188. _Why do plants suffer from the smoke of cities?_
Because the smoke _injures the porous structure of the leaves_, and
interferes with their free respiration.