11. THE DUTIES OF STEMS.
1. =The shapes of stems.=—Cut across a deadnettle stem and a wallflower
stem and examine the shape of the sections. The former is square, the
latter is five-ribbed. Is there any relation between the form of the
stem and the arrangement of the leaves?
2. =The “bleeding” of stems.=—Cut through the lower part of a
scarlet-runner plant in spring. Can you see any water escaping from
that part of the stem still in the ground? Similarly, cut back a
sunflower stem when it is from ½ to 1 inch thick. Does the water
exude from all parts of the cut surface equally, or does it come from
definite channels? To see this better, dry the cut end with blotting
paper and examine the surface with a lens. “Bleeding” is best seen in
vine stems; if a young vine is available, cut it back in spring and
observe the large escape of water.
3. =The water-current travels along definite channels.=—Colour some
water with red ink and put in it the stalks of cut white flowers such
as snowdrops or narcissi. The stalks and flowers soon become veined
with red. Cut the stalks across and see that the strands in the
interior are also coloured. The coloured water has evidently travelled
along definite channels.
4. =The food-channels in a soft stem.=—(_a_) Take a stout piece of the
stem of a deadnettle, including three nodes (p. 45), and slit it down
in a line between the end pairs of leaves. Then boil the stem in water
until the internal tissue is soft. Take the stem out and carefully
scrape away all the soft material until the woody strands can be well
seen. These are the _food-channels_. Notice their arrangement in the
stem, and the manner in which they run out to the leaves.
(_b_) Similarly, examine the strands in a piece of sunflower stem, and
in an old cabbage stalk.
5. =The path of the water-current in a woody stem.=—Take a leafy twig
of elder or laurel and, from the part of the shoot below the leaves,
remove a ring of about an inch of the bark and the soft tissues which
lie beneath it, so as to expose the wood. Put the end of the twig
(below the ring) in water. The leaves remain fresh and crisp, showing
that the water travels either along the wood or the pith, or along both.
6. =The water travels along the wood.=—Put a similar leafy twig of
elder dipping in water which has been coloured by red ink. Expose
to sunlight, and after an hour or two cut open the twig and observe
which parts are coloured. The bark and the soft tissues between it and
the wood are not coloured. The wood is stained red. The pith is not
coloured.
7. =The path of the leaf-made food.=—(_a_) Take a leafy branch of a
tree—_e.g._ willow—and near the bottom remove a ring of the bark and
the soft tissues lying between bark and wood. Put the twig in water, so
that the ringed part and a few inches above shall be immersed. After a
time roots are produced _above_ the cut. If any arise from the stripped
part they are few in number and much shorter than those above.
(_b_) Place a similar but uninjured twig in water, and notice that
the new roots are produced _at the end_. What is the reason for the
difference?
=The duties of stems.=—No one can study leaves without being impressed
by the great importance to the plant of the work which they do. Even
the casual observation that year after year thousands of fresh leaves
make their appearance would indicate this. And when it is learned that
throughout the day every leaf is busily engaged in decomposing carbon
dioxide, and joining up the carbon with the water and mineral compounds
which have come up from the root—thus forming sugars, starch and a
host of other valuable substances—some general idea is obtained that
the trunk and spreading boughs of a tree may, after all, be of minor
importance, and may exist mainly to help the leaves to perform their
duties as perfectly as possible.
This is the true view to take of the stem and its branches; their
duties are (1) to bear the leaves and spread them out, so that these
will receive as much sunshine and fresh air as possible; (2) to supply
the leaves with the water and mineral substances which they require for
their work; and (3) to receive from the leaves and distribute to the
rest of the plant the food materials which the leaves have prepared.
A stem, together with the leaves and branches which it bears, is called
a =shoot=.
It will be found that almost all the variations in the structure and
habits of stems are connected with the arrangements of the leaves. For
example, the weight of the mass of leaves borne by a forest tree is
very great, and they offer a great resistance to the wind; the trunk
and its branches, therefore, are correspondingly strong and stout.
Again, when a stem is ribbed or ridged in any particular manner it will
generally be found that the ridges have a definite relation to the
leaves and to their points of insertion on the stem. This has been seen
(p. 44) to be the case in the stem of the wallflower.
The tip of a stem or branch is occupied by a =bud=. As the tip
elongates (Fig. 40), the outer leaves of the bud become separated by
internodes, and new leaves continually arise around the growing point.
=The path of the water in the stem.=—It may easily be shown that
water travels up the stem to the leaves. It is common knowledge that
slightly withered leaves or flowers become fresh and crisp again if
the twig or stalk is put in water. If the stalks of white flowers—say
snowdrops or narcissi—are put into water coloured with red ink, the
flowers become veined with red, and, if the stalks are cut into, red
strands may be seen. This experiment not only shows that water has
travelled up the stalk, but it also shows that the water has passed
along =definite channels=. When a vine stem is cut back in the spring,
the water wells out rapidly from the end of that part of the stalk
which is still in the ground. This =bleeding=, as it is called, may
also be seen, though to a smaller extent, in stems of sunflower,
scarlet-runner, and other plants. By drying the cut surface with
blotting paper, and then examining it with a lens, it may be seen
that the water escapes from the ends of certain little tubes; and it
is possible in a boiled or rotting stem (Expt. 11. 4) to follow the
distribution of the strands containing these tubes. The strands run
along the length of the stem, =outside the pith=. Cross strands connect
the main ones, chiefly at the leaf-levels (nodes); and other strands
run out into the leaves to form the veins.
The tubes which convey the water current are in that part of the strand
which is nearest the pith, and they become woody. Thus, if the current
year’s growth of, say, a horse chestnut or an elder twig is cut across,
a thin ring of =wood= is seen surrounding the soft pith. Outside the
wood are the softer tissues, surrounded by the bark. A ring of the bark
and soft tissues beneath it may be entirely removed from a growing
twig, leaving the wood exposed, but the leaves above remain crisp and
fresh, showing that this treatment has not interfered with their water
supply. The water, therefore, travels along either the wood or the
pith. That the wood and not the pith conducts the water may be shown by
putting a leafy twig in water coloured with red ink. Only the wood is
stained.
=The path of the food made by the leaves.=—The plant food which the
leaves make is drained off into the stem, and distributed to the
parts where growth is taking place. This food travels in the soft
tissue—called the =bast=—which lies below the bark. This fact can be
shown indirectly by removing a ring of bast from the lower part of
a branch, say of willow, and putting the branch into water. When at
length the cutting puts out roots, these spring from the top of the
ring. If any spring from the stripped part they are markedly smaller,
and fewer in number. In an uninjured cutting, which is of course
supplied with prepared food along all its length by the bast vessels,
such roots spring from the cut end. The supply of leaf-made food can
also be cut off by ligaturing a twig below the leaves, as by twisting
a wire or cord tightly round it. In such a case growth usually ceases
in the part of the twig below the strangled part, while the upper part
of the twig, to which the leaf-made food is now restricted, grows much
more luxuriantly than before. Gardeners often produce unusually fine
fruits by ligaturing the lower parts of the twigs on which the fruits
are ripening.
12. HOW STEMS ARE STRENGTHENED.
1. =The formation of wood.=—(i.) In summer take a horse chestnut twig
of three or four years’ growth. Cut through it with a sharp knife at
the following places, and trim the cut ends flat:
(_a_) Near the apex;
(_b_) at the middle of the current year’s growth;
(_c_) near the bottom of the current year’s growth;
(_d_) about the middle of last year’s growth;
(_e_) ” ” the previous year’s growth.
Make a drawing of what you see in each case:—In (_a_) the twig is
covered on the outside by a green _skin_. In the middle is the soft
_pith_. Between the two is a ring of separate _strands_. In (_b_) and
(_c_) the strands have joined up, and a distinct, though thin, layer of
_wood_ surrounds the pith. _Bast_ and other soft tissues lie between
the wood and the bark. (_d_) has two layers of wood. (_e_) has three
layers of wood.
(ii.) Split each length longitudinally. Why is it easier to do this
than to cut the twig across? In which direction does the grain of
the wood run? Make out in each piece the pith, strands, or layers
of wood, bast, etc., and skin or bark. You can tear off the bast in
ribbon-like shreds. See how the strands run out into the young leaves.
Cut lengthwise through the junction between the main twig and any side
twigs, and notice that corresponding parts are continuous.
2. =The strength of a grass stem.=—Notice the relatively enormous
strength of a straw and other grass stems.
Burn a straw and observe the tube of mineral matter which is left
behind. Examine a piece of bamboo; is it hollow or solid?
=Woody stems.=—To enable them to bear the weight of the leaves and
branches, and to withstand the force of the wind, the stems of plants
are strengthened in various ways. Most commonly this is effected by the
formation of =wood= in the walls of the water-vessels.
Even in succulent stems, such as that of the sunflower, the strands of
vessels are stiffened by the long and narrow wood pipes which run along
them; and when the strands join up to form a complete cylinder a very
strong column is the result. Engineers make use of the same device,
knowing that the same amount of material will bear a far greater stress
when made into a hollow cylinder than it will in any other form.
=The thickening of woody stems.=—In dicotyledons (p. 23) and
gymnosperms (p. 163) the cylinder, which is formed by the joining-up
of the conducting strands, consists of three layers. The innermost of
these is the =wood=, and the outermost is =bast=. Between them is a
very delicate layer called =cambium=, which is continually dividing
and forming more wood on its inner side and more bast on its outer
side. The wood is hard and resists pressure, while the bast is soft and
is squeezed against the inside of the bark by the expanding wood (Fig.
43).
The formation of new wood and bast takes place vigorously during the
summer, at the expense of the food which is manufactured by the leaves
and travels along the bast to the active cambium. As autumn comes,
the activity of the tree slows down, and the new wood is formed of
closer texture. In winter the process stops altogether; but with the
warmth and the plentiful food-supply of spring the formation of new,
open-textured wood is resumed.
[Illustration: FIG. 43.—Portion of a four-year-old stem of the Pine,
cut in winter. 1, 2, 3, 4, the four successive annual rings of the
wood; _b_, bast; _br_, bark; _c_, cambium; _f_, spring wood; _i_,
junction of wood of successive years; _m_, pith; _ms_, medullary rays;
_s_, autumn wood. (× 4.)]
This difference in texture between the autumn wood and the later spring
wood is quite visible to the naked eye (Fig. 44), and gives rise to a
series of =annual rings=, each of which represents a year’s growth.
Thus, a cross cut through the four year old part of a branch (Fig. 43)
shows four layers of wood, one of which was formed each year. The length
formed last year has two layers of wood (if we look in summer), and the
current year’s growth has one layer, all of which has been formed since
spring.
=The advantages of secondary thickening.=—The formation of secondary
wood and bast is very important. The new wood is required (1) to
provide additional water-vessels to supply the demands of an increasing
number of leaves, and (2) to give the necessary increase of mechanical
strength to the growing tree. Again, the increase in the quantity
of food manufactured by the leaves makes a larger quantity of bast
necessary for its distribution. Last of all, new tissues are required
to take up the duties of those which are old and worn-out.
[Illustration: FIG. 44.—Cross section of Larch stem, showing annual
rings.]
In forest trees, the central pith becomes almost obliterated and the
old stem practically consists of wood, bast, and bark. The various
annual rings, and the bast, of a woody stem are joined together by a
number of radiating horizontal spokes called =medullary rays= (Fig.
43, _ms_, _ms′_, _ms″_, _ms‴_). These conduct water and food materials
across the stem from layer to layer. The beautiful lines and patches
called “silver grain,” which may be seen in oak furniture, consist of
the medullary rays exposed in radial-longitudinal section.
=Bark.=—The very young stem is surrounded by a thin waterproof skin,
perforated by stomata (p. 53) as the skin of a leaf is; and the part
immediately below the skin possesses leaf-green, and can therefore
decompose carbon dioxide as a leaf does (p. 51). An old stem, on the
other hand, is covered by a layer of tough bark (_br_, Fig. 43), which
splits from time to time, owing to the stretching which is caused
by the increasing thickness of the wood. The bark begins as a layer
of =cork=, which forms on the outer side of the bast. The cork cuts
off the food supply from all the external tissues, which die. Bark,
therefore, consists of the cork and the dead layers outside it.
=Grass stems.=—The great strength of grass stems—so apparent when we
try to bend a straw—is largely due to _silica_, the substance of which
rock-crystal and ordinary sand are composed. When a straw is burnt,
this remains as a hollow cylinder of mineral matter. The great strength
of the cylindrical form has already been referred to. It is very well
seen also in a bamboo stem, and anyone who has blunted the edge of his
knife on a piece of bamboo will appreciate the additional hardness
which is given by the presence of mineral matter.
13. CLIMBING STEMS.
1. =Hooking stems.=—Examine a bramble or a wild rose plant in a hedge.
Why does it need to climb? How does it climb? Pull a branch and notice
by what means it resists the pull. What is the shape of the _prickles_?
Do they point upwards or downwards? On what parts of the plant are
the prickles found? Notice how easily a prickle may be pushed off,
sideways. Is it as easy to tear it off lengthways? Is the prickle a
little branch, or merely an extension of the rind?
Contrast a prickle with the _thorn_ of the hawthorn. The thorn does not
come off easily, and it contains a woody core which is continuous with
the wood of the branch. Cut lengthwise through the thorn and the branch
which bears it to see this. The thorn is a short pointed branch; it
arises in the axil of a leaf, and sometimes bears leaves itself.
2. =The ivy.=—Observe how the climbing stem of the ivy is attached to a
wall or tree. It puts out a line of _roots_ on its shaded side. These
roots give out a sticky fluid, which, on hardening, fixes them to the
wall.
3. =Twining stems.=—Watch the growth of a convolvulus seedling. At
first the young stem grows straight up, but soon the tip begins to
move round and round. Try to find out how long it takes to describe
one revolution. Put a long stick in the ground near the plant and
notice how, when the revolving stem touches the stick, the spiral is
henceforth described round the support and the stem consequently clings
to it. Lay your watch face-upward, and notice whether the stem moves in
the same direction as the hands (the “clockwise” direction), or in the
opposite (“counter-clockwise”) direction.
Make similar observations on the hop, honeysuckle, and scarlet runner,
and note the results.
4. =Leaf climbers.=—Examine a climbing tropœolum (often, though
wrongly, called “nasturtium”). Which parts of the plant clasp the
support? Watch a young plant coming up. Does the stem revolve before
the leaf stalks come in contact with the support? Compare the
_clematis_.
5. =Tendril climbers.=—Examine plants of sweet pea, bryony, vine,
passion flower, and cucumber. Try to find out in each case which part
has been modified to form the tendrils. Watch a plant day by day until
a free tendril grasps a support, and notice how it becomes spirally
coiled. Can you straighten the tendril by pulling, without leaving any
kinks? Why? Is the spiral of the tendril continuous, or does it change
its direction in the middle? Make a _continuous_ spiral with wire, and
notice the kinks formed when the wire is straightened by pulling the
ends. Which is better for the plant in a gale of wind, a continuous or
a reversed spiral? Why?
Notice the sucker-like tendrils of the Virginian creeper, and the way
in which they fix the plant to the wall.
Plants whose stems are not strong enough to stand erect without support
must adopt some special means of spreading out their leaves to the
light and air. One of the commonest devices of such plants is that of
climbing up other and stronger plants, walls, trellis-work, etc.
=Scramblers and climbers.=—In the simplest cases the plant simply
scrambles over other plants. Many brambles and roses are merely
scramblers, but more often they are true climbers, weaving themselves
among their neighbours by the help of hooked =prickles= (Fig. 76).
The prickles point backwards, and therefore anchor the twigs firmly,
as is very evident on trying to pull a branch out of the hedge. A
prickle may easily be broken off by a side push, for it is merely an
outgrowth of the rind, and does not contain any woody core. It is so
attached, however, that it resists much greater force in the lengthwise
direction—a manifest advantage to the plant.
[Illustration: FIG. 45.—Ivy climbing up a wall. _R_, aërial roots. (×
⅓.)]
The differences between such a prickle and a =thorn= like that of the
hawthorn should be carefully noticed. A thorn is really a little twig
which has remained short and become pointed at the end. It has a core
of wood which is continuous with the wood of the branch bearing it.
That a thorn is really a little branch is shown by its origin in the
axil of a leaf, and by its often giving rise to leaves and buds.
[Illustration: FIG. 46.—Climbing stem of Honeysuckle. (× ¼.)]
[Illustration: FIG. 47.—Climbing stem of Convolvulus. (× ¼.)]
The stem of the ivy climbs by means of little =roots=, which it puts
out on the side furthest from the light (Fig. 45). These give out a
sticky liquid which, on drying, cements them to the wall or tree.
=Twining= stems are much in advance of these. There seems to be
something approaching intelligence in the manner in which the young
stem of a hop, honeysuckle, or convolvulus, which at first grows
straight up, begins to wander round and round, tracing a spiral path
in the air until it touches a support. Then, however, as if the plant
could feel, the movement below the point of contact stops; but the
upper part of the stem still revolves and therefore twines round the
support. The stems of the honeysuckle (Fig. 46) and the hop turn in the
same direction as the hands of a clock. This is called the “clockwise”
direction. On the other hand the convolvulus (Fig. 47) and most other
twining stems are “counter-clockwise” climbers. The stem of the
bittersweet revolves indifferently in either direction.
=Sensitive clasping organs= in their simplest form are seen in the
_twining leaf stalk_ of the clematis and tropœolum; the stem itself
revolves as if to give its leaf stalks every opportunity of finding
suitable supports. The leaf stalks seize these and twine round them.
Most wonderful of all climbing organs are the =tendrils=. They are
well seen in Fig. 48. A part of the plant—sometimes a leaflet, as in
the pea (Fig. 28); sometimes a branch, as in the passion flower; or a
flower stalk, as in the vine—becomes modified into a thread, slight
but strong. When the end of the thread touches and then twines round
a support, the whole tendril forms itself into a spiral which, like a
wire spring, draws the plant up to the support, and can yet lengthen
and yield to the wind when necessary. In the middle of the tendril
the direction of the spiral is reversed, so that the tendril can be
straightened without being twisted.
The tendrils of the Virginian creeper do not twine, but on meeting a
wall they form round red _suckers_ at the end, and attach the plant
(Fig. 49).
[Illustration: FIG. 48.—How a tendril grasps a support. The spiral is
reversed at _x_.]
[Illustration: FIG. 49.—Virginian Creeper. _R_, _R_, stem tendrils. (×
½.)]
14. CREEPING AND UNDERGROUND STEMS.
1. =A creeping stem.=—Examine a plant of the ground ivy. Is the stem
strong enough to stand upright? How does it spread out its leaves to
the light and air? The stem grows along the ground, and at intervals
it gives off a pair of leaves which grow upwards, and a tuft of roots
which grow down to the ground.
2. =A runner.=—Is the “runner” of the strawberry of the same nature,
_i.e._ is it a continuous stem like that of the ground ivy? When
the plant is carefully examined, the creeping “stem” is seen to be
a _branch_ arising in the axil of a leaf. The branch runs along the
ground for a little distance, and then roots itself and gives off a
number of leaves. In the axil of one of these another branch arises and
runs on in the same direction. The same branch does _not_ run on and on.
3. =A stolon.=—Follow carefully the underground part of the couch grass
and make out its connection with the main shoot. It is a branch like
the runner of the strawberry, which arises in the axil of a leaf, and
extends only to the next shoot.
Compare the stolons of the cinquefoil.
4. =A potato tuber.=—Examine a potato tuber (the part which is eaten).
Notice the “eyes.” These are buds, with scale leaves. _Leaves never
occur on roots_, so that the potato must be an underground stem. Put a
pin into every eye, and wind a thread round the tuber along the bases
of the pins. It forms a spiral. Cut the potato into halves, and pour a
drop of iodine solution on the cut surface. What is the meaning of the
blue dots which at once make their appearance? Plant a potato in warm,
moist earth, and when it has sprouted notice that each bud (eye) has
given rise to a branch.
5. =Bulbs.=—Cut an onion or snowdrop bulb down the middle, and draw
what you see, marking on your drawing the outer scale leaves, the
swollen bases of last year’s leaves, the young leaves in the middle,
the short, thickened stem, and the roots. Also cut other bulbs across
and again draw. What is the similarity and what is the difference
between these bulbs and such a bud as a cabbage?
Also examine hyacinth, tulip and daffodil bulbs. Put them in glasses
with water touching their bases, and watch them grow. What do they live
upon?
6. =A crocus corm.=—Obtain a few crocus “bulbs” in the early winter.
Observe the tough outer tunic springing round the edge of a circular
scar on the base. If there are any roots they come off from the scar.
Take off the tunics from one “bulb” and observe the bud or buds at
the top of the white mass inside. Other tunics cover the buds. Cut
lengthwise through the mass of the “bulb” so as to bisect the largest
bud. Separate the parts of the bud with a needle and notice (_a_)
the thin outer leaves, (_b_) the young foliage leaves, (_c_) the
flower-sheath and flower. Pour a drop of iodine solution on the cut
white mass (the stem) below the bud. It turns blue. Why?
What is the principal difference between a crocus “bulb” and the bulb
of an onion, hyacinth, or tulip? A true bulb is mainly composed of
swollen leaves or leaf bases; in the crocus the thick, rounded stem
makes up most of the bulk. It is better, therefore, to speak of a
crocus _corm_, to indicate the difference.
Plant the remaining corms and examine them at intervals for a year.
Notice the formation of the roots, the lengthening of the buds, the
formation of the flowers, _the activity of the foliage leaves after
flowering_ (why?), the withering of the roots and leaves in summer, and
the growth of the enlarged base of the branch into next year’s corm.
=Creeping stems.=—Instead of climbing, many stems find that the best
method of spreading out their leaves is to creep along or under the
ground, and give off leaves and roots at intervals. Not only does this
device prevent the leaves of one node from interfering with the light
and air supply of those of the next, but the plant is continually
coming in contact with a fresh lot of soil. The ground ivy is an
instructive example of this method of growth. The stem creeps along
the ground, and at every node it gives off a pair of leaves which grow
upwards, and a tuft of roots which grow down into the ground.
[Illustration: FIG. 50.—Runner of Strawberry. (× ⅓.)]
The =runner= of the strawberry (Fig. 50) appears at the first glance to
grow in a similar manner. As a matter of fact, however, the apparent
stem is a _branch_ arising in the axil of one of the leaves of the last
node. The branch runs along the ground and gives rise to a new shoot,
and from this another branch, springing from the axil of a leaf, forms
another runner. The same branch does not run on from shoot to shoot.
The =stolon= of the couch grass (Fig. 51) is somewhat similar. The
erect stem of the plant is divided, as usual, into nodes or knots (from
which the narrow, sheathing leaves arise) and internodes. Branches
(stolons) spring in the axils of the lower leaves, turn downwards, and
run on underneath the soil, taking root again at some distance from the
parent plant.
[Illustration: FIG. 51.—Stolon of Couch Grass. (× ⅙.)]
[Illustration: FIG. 52.—Creeping underground stem of Solomon’s Seal.
_a_, bud of next year’s aërial growth; _b_, scar of this year’s growth;
_c_, _d_, _e_, scars of aërial growth of previous years; _w_, roots. (×
¾.)]
=Underground stems.=—Although the stem is usually that part of the
axis of a plant which is above ground, there are many exceptions. The
bracken fern, daisy, coltsfoot, Solomon’s seal (Fig. 52), and many
other plants have stems which are ordinarily buried in the ground,
giving off leaves above and roots below. In some cases such underground
stems become much swollen with stored food-material—manufactured by
the leaves in excess of immediate requirements. In the potato, for
example, certain underground branches of the stem store up starch to
such an extent that their ends become fleshy, ovoid masses some inches
in thickness (Fig. 53). The true nature of these =tubers= is revealed
by the buds or “eyes” which spring upon them. The buds are arranged
spirally—as may be seen by sticking a pin into each and joining up the
pins with thread—and when the tubers are kept in a warm, moist place,
each bud grows out into a new leafy shoot.
[Illustration: FIG. 53.—Part of a Potato plant, showing the old tuber
(dark) and several new ones. (× ⅙.)]
The structure of a =bulb= is easily made out in the onion, tulip (Fig.
54), hyacinth, or daffodil. When such a bulb is cut down the middle,
it is seen to be mainly composed of leaves or leaf-bases, swollen with
stored food. Inside these are the young leaves and the flower bud,
which would have expanded next season; and on the outside are a few
thin scale leaves. All these leaves spring from a fleshy button at the
base, which gives off roots below. The button is the flattened _stem_.
When a plant produces a bulb it will generally be found that it flowers
either very early or very late in the season; that is, at a period
which would not be very favourable for the work of the leaves. The
flower (Fig. 55) is produced at the expense of the stored food in the
bulb—made in excess of the requirements of the previous season. After
the plant has flowered, the new leaves work until they have made enough
food for next season’s flower, and then they also die.
[Illustration: FIG. 54.—Longitudinal section of Tulip bulb. _zk_,
modified stem; zs, scale leaves; _v_, terminal bud; _k_, young bud;
_w_, roots. (× 1.)]
[Illustration: FIG. 55.—Daffodil. (× ¹/₉.)]
The so-called bulb of the crocus is technically known as a =corm= (Fig.
56). It differs from a true bulb in consisting mainly of a fleshy,
rounded stem in which the surplus food made by last year’s leaves is
stored up. The plant is thus able to flower early, without waiting
for the new leaves to supply food. The swollen stem bears one or more
buds, and the whole is surrounded by tough tunics of scales. When the
corm begins to grow, roots are put out from the base, and the flowers
and—later—the leaves of the buds expand. The foliage leaves continue
their work after flowering, and the food which they make accumulates in
the base of the former bud, which becomes swollen to form the new corm
for next year’s flower. The leaves then die down, their bases becoming
the tunics of the new corm.
[Illustration: FIG. 56.—Crocus corm, seen from the side, from below,
and in longitudinal section. _c′_, base of bud, which will grow into
next year’s corm; _fd.ch._, food channel; _flr_, flower bud; _l_, young
leaves; _rts_, roots; _t₁_, _t₂_, _t₃_, _t₄_, tunics. (× ½.)]
EXERCISES ON CHAPTER V.
1. Mention experiments, which prove that organic
substances are formed in the leaves, and distributed to
other parts of a green flowering plant. By what channels are
they distributed? (1895)
2. Draw a cross section through the stem of a flowering
plant selected by yourself. Explain the uses of the chief
things seen in the section. (1897)
3. Mention experiments or observations which show by what
tissues water ascends to the leaves, and nutritive substance
descends from the leaves. (1897)
4. By what tissues does water pass along the stem of a
tree to the leaves? Give proofs of your statements. (1898)
5. Describe the effect of a tight ligature upon a growing
hazel stem. (1898)
6. What proofs can be given that the stem of a tree draws
nourishment from the leaves? (1898)
7. Show, by describing and drawing one example, that the
branch of a tree may preserve a record of past seasons in
its wood. (1901)
8. Mention an experiment which shows that organic
substance formed in the leaves travels down the stem outside
the cambium. (1901)
9. Obtain thin sections (_e.g._ plane-shavings) of as
many different kinds of wood as possible, and gum them into
a book, writing under each section the name of the wood
and the direction (transverse, radial-longitudinal, or
tangential-longitudinal) of the section.
10. Make yourself familiar with the appearance and
characters of the different kinds of timber used in
carpentry and joinery.
11. Notice the light-brown spots on the bark of twigs of
apple and horse chestnut. These are called _lenticels_; they
are breathing-pores. In how many other trees can you find
lenticels?
12. Observe whether an ivy stem puts out roots only where
they can become fixed to a support, or indiscriminately.
13. Make a list of plants which you have observed to climb
by twining stems, and note whether they are clockwise or
counter-clockwise climbers.
14. Make careful drawings of all the tendrils you can
find, and try to discover which part of the plant has been
modified to form the tendril.
15. Gently stroke a tendril of the passion flower and
write an account of any resulting movement.
16. Mention any three climbing plants which grow wild in
this country, and explain in each case how the plant climbs.
(1895)
17. What is the difference between (_a_) a thorn, (_b_) a
leaf-spine, and (_c_) a prickle? Give examples. (1896)
18. Enumerate and briefly describe the principal varieties
of tendrils, and explain how they act. (1897)
19. If a wire is fastened tightly about a growing
branch of a common tree, and left for two or three years,
what effect will be produced, and how can the effect be
explained? (1904)
20. What processes of vegetable growth are accompanied by
the presence of sugar? Give examples from plants within your
own experience. (King’s Scholarship, 1904)
21. What are the chief uses of the vessels of a herbaceous
stem? Mention observations and experiments in support of
your statements. (1905)
22. How can you demonstrate experimentally that food
substances, formed in the leaves of a tree, descend to the
branches below? (1905)