In which buds are the flower-clusters?
Are there flowers and leaves in the same buds?
What are the scales of the bud?
How are the leaves folded in the bud?
How do the axillary and terminal buds differ?
What are the dots on the leaf-scars?
Why is there no distinct band of rings as in Beech?
How old is your branch?
Where do you look for flower-cluster scars?
Which buds are the strongest?
How does this affect the appearance of the tree?
What makes the ends of the branches so rough?
Compare the arrangement of the twigs and branches with Beech and Elm, with
Horsechestnut and Lilac.
TULIP-TREE (_Liriodendron Tulipifera_).
The buds are small, flat, and rounded at the apex. They are sheathed by
scales, each leaf being covered by a pair, whose edges cohere. The outer
pair are brown and are the stipules of the last leaf of the preceding
year. The leaves are conduplicate, as in Magnolia, and have the blade bent
inwards on the petiole (_inflexed_). Their shape is very clearly to be
seen, and no bud is more interesting in the closeness of its packing.
Axillary buds are often found within. The flowers grow high upon the trees
and towards the ends of the branches.
The leaf-scars are round with many dots. The scar of the stipules is a
continuous line around the stem, as in Magnolia.
CHERRY _(Prunus Cerasus_).
The leaf-buds are terminal, or in the axils of the upper leaves of the
preceding year; the flower buds are axillary. There is but one bud in each
axil, and usually two or three flowers in each bud, but the leaves on
the twigs are crowded and the flowers therefore appear in clusters. The
blossom-buds are larger and more rounded than the leaf-buds.
The buds of the tree develop very easily in the house, and as they are
so small they can be better studied in watching them come out, than by
attempting to dissect them, unless the scholars are sufficiently advanced
to use the microscope easily. It is always bad for a pupil to attempt to
describe what he sees but imperfectly. He will be sure to jump at any
conclusions which he thinks ought to be correct.
The leaf-scars are semicircular, small and swollen.
The bud-rings are plain. The twigs make a very small growth in a season,
so that the leaf-scars and rings make them exceedingly rough.
The flower-cluster scars are small circles, with a dot in the centre, in
the leaf-axils. The flowers come before the leaves.
The leaf-arrangement is alternate on the 2/5 plan. The pupils may compare
the branching with that of their other specimens.
RED MAPLE (_Acer rubrum_).
This is a good specimen for the study of accessory buds. There is usually
a bud in the axil of each lower scale of the axillary buds, making three
side by side. We have already noticed this as occurring sometimes in
Lilac. It is habitually the case with the Red Maple. The middle bud, which
is smaller and develops later, is a leaf-bud. The others are flower-buds.
The leaf-scars are small, with three dots on each scar. The rings are very
plain. The flower-cluster leaves a round scar in the leaf-axil, as in
Cherry.
The leaves are opposite and the tree branches freely. The twigs seem to
be found just below the bud-rings, as the upper leaf-buds usually develop
best and the lower buds are single, containing flowers only.
NORWAY SPRUCE (_Picea excelsa_).
The buds are terminal, and axillary, from the axils of the leaves of the
preceding year, usually from those at the ends of the branchlets. They are
covered with brown scales and contain many leaves.
[Illustration: FIG. 18.--Branch of Cherry in winter state: _a_, leaf-scar;
_b_, bud-scar; _c_, flower-scar.]
[Illustration: FIG. 19.--Branch of Red Maple in winter state (reduced). 2.
Flower-buds]
The leaves are needle-shaped and short.[1] They are arranged densely on
the branches, alternately on the 8/21 plan (see section on phyllotaxy).
When they drop off they leave a hard, blunt projection which makes the
stem very rough. As the terminal bud always develops unless injured, the
tree is excurrent, forming a straight trunk, throwing out branches on
every side. The axillary buds develop near the ends of the branchlets,
forming apparent whorls of branches around the trunk. In the smaller
branches, as the tree grows older, the tendency is for only two buds to
develop nearly opposite each other, forming a symmetrical branch.
[Footnote 1: The pupils should observe how much more crowded the leaves
are than in the other trees they have studied. The leaves being smaller,
it is necessary to have more of them. Large-leaved trees have longer
internodes than those with small leaves.]
The bud-scales are persistent on the branches and the growth from year to
year can be traced a long way back.
The cones hang on the ends of the upper branches. They are much larger
than in our native species of Black and White Spruce.
The Evergreens are a very interesting study and an excellent exercise in
morphology for the older scholars.
2. _Vernation_. This term signifies the disposition of leaves in the bud,
either in respect to the way in which each leaf is folded, or to the
manner in which the leaves are arranged with reference to each other.
The pupils have described the folding of the leaves in some of their
specimens.
In the Beech, the leaf is _plicate_, or plaited on the veins. In the Elm,
Magnolia, and Tulip-tree, it is _conduplicate_, that is, folded on
the midrib with the inner face within. In the Tulip-tree, it is also
_inflexed_, the blade bent forwards on the petiole. In the Balm of Gilead,
the leaf is _involute_, rolled towards the midrib on the upper face.
Other kinds of vernation are _revolute_, the opposite of involute, where
the leaf is rolled backwards towards the midrib; _circinate_, rolled from
the apex downwards, as we see in ferns; and _corrugate_, when the leaf is
crumpled in the bud.
[Illustration: FIG. 20.--Branch of Norway Spruce.]
In all the trees we have studied, the leaves simply succeed each other,
each leaf, or pair of leaves, overlapping the next in order. The names of
the overlapping of the leaves among themselves, _imbricated, convolute,
etc_., will not be treated here, as they are not needed. They will come
under _æstivation_, the term used to describe the overlapping of the
modified leaves, which make up the flower.[1]
[Footnote 1: Reader in Botany. VIII. Young and Old Leaves.]
3. _Phyllotaxy_. The subject of leaf-arrangement is an extremely difficult
one, and it is best, even with the older pupils, to touch it lightly. The
point to be especially brought out is the disposition of the leaves so
that each can get the benefit of the light. This can be seen in any plant
and there are many ways in which the desired result is brought about. The
chief way is the distribution of the leaves about the stem, and this is
well studied from the leaf-scars.
The scholars should keep the branches they have studied. It is well to
have them marked with the respective names, that the teacher may examine
and return them without fear of mistakes.
In the various branches that the pupils have studied, they have seen that
the arrangement of the leaves differs greatly. The arrangement of leaves
is usually classed under three modes: the _alternate_, the _opposite_,
and the _whorled_; but the opposite is the simplest form of the whorled
arrangement, the leaves being in circles of two. In this arrangement, the
leaves of each whorl stand over the spaces of the whorl just below. The
pupils have observed and noted this in Horsechestnut and Lilac. In these
there are four vertical rows or ranks of leaves. In whorls of three leaves
there would be six ranks, in whorls of four, eight, and so on.
When the leaves are alternate, or single at each node of the stem, they
are arranged in many different ways. Ask the pupils to look at all the
branches with alternate leaves that they have studied, and determine in
each case what leaves stand directly over each other. That is, beginning
with any leaf, count the number of leaves passed on the stem, till one is
reached that stands directly over the first.[1] In the Beech and the Elm
the leaves are on opposite sides of the stem, so that the third stands
directly over the first. This makes two vertical ranks, or rows, of
leaves, dividing the circle into halves. It is, therefore, called the
1/2 arrangement. Another way of expressing it is to say that the angular
divergence between the leaves is 180°, or one-half the circumference.
[Footnote 1: The pupils must be careful not to pass the bud-rings when
they are counting the leaves.]
The 1/3 arrangement, with the leaves in three vertical ranks, is not very
common. It may be seen in Sedges, in the Orange-tree, and in Black Alder
_(Ilex verticillata)_. In this arrangement, there are three ranks of
leaves, and each leaf diverges from the next at an angle of 120°, or
one-third of the circumference.
By far the commonest arrangement is with the leaves in five vertical
ranks. The Cherry, the Poplar, the Larch, the Oak, and many other trees
exhibit this. In this arrangement there are five leaves necessary to
complete the circle. We might expect, then, that each leaf would occupy
one-fifth of the circle. This would be the case were it not for the fact
that we have to pass twice around the stem in counting them, so that each
leaf has twice as much room, or two-fifths of the circle, to itself. This
is, therefore, the 2/5 arrangement. This can be shown by winding a thread
around the stem, passing it over each leaf-scar. In the Beech we make one
turn of the stem before reaching the third leaf which stands over the
first. In the Apple the thread will wind twice about the stem, before
coming to the sixth leaf, which is over the first.
Another arrangement, not very common, is found in the Magnolia, the Holly,
and the radical leaves of the common Plantain and Tobacco. The thread
makes three turns of the stem before reaching the eighth leaf which stands
over the first. This is the 3/8 arrangement. It is well seen in the
Marguerite, a greenhouse plant which is very easily grown in the house.
Look now at these fractions, 1/2, 1/3, 2/5, and 3/8. The numerator of
the third is the sum of the numerators of the first and second, its
denominator, the sum of the two denominators. The same is true of the
fourth fraction and the two immediately preceding it. Continuing the
series, we get the fractions 5/13, 8/21, 13/34. These arrangements can
be found in nature in cones, the scales of which are modified leaves and
follow the laws of leaf-arrangement.[1]
[Footnote 1: See the uses and origin of the arrangement of leaves in
plants. By Chauncey Wright. Memoirs Amer. Acad., IX, p. 389. This essay
is an abstruse mathematical treatise on the theory of phyllotaxy. The
fractions are treated as successive approximations to a theoretical angle,
which represents the best possible exposure to air and light.
Modern authors, however, do not generally accept this mathematical view of
leaf-arrangement.]
[1]"It is to be noted that the distichous or 1/2 variety gives the maximum
divergence, namely 180°, and that the tristichous, or 1/3, gives the
least, or 120°; that the pentastichous, or 2/5, is nearly the mean between
the first two; that of the 3/8, nearly the mean between the two preceding,
etc. The disadvantage of the two-ranked arrangement is that the leaves are
soon superposed and so overshadow each other. This is commonly obviated by
the length of the internodes, which is apt to be much greater in this
than in the more complex arrangements, therefore placing them vertically
further apart; or else, as in Elms, Beeches, and the like, the branchlets
take a horizontal position and the petioles a quarter twist, which gives
full exposure of the upper face of all the leaves to the light. The 1/3
and 2/5, with diminished divergence, increase the number of ranks; the 3/8
and all beyond, with mean divergence of successive leaves, effect a more
thorough distribution, but with less and less angular distance between the
vertical ranks."
[Footnote 1: Gray's Structural Botany, Chap, iv, p. 126.]
For directions for finding the arrangement of cones, see Gray's Structural
Botany, Chap. IV, Sect. 1.
The subject appears easy when stated in a text-book, but, practically, it
is often exceedingly difficult to determine the arrangement. Stems often
twist so as to alter entirely the apparent disposition of the leaves. The
general principle, however, that the leaves are disposed so as to get the
best exposure to air and light is clear. This cannot be shown by the study
of the naked branches merely, because these do not show the beautiful
result of the distribution.[1] Many house plants can be found, which will
afford excellent illustrations (Fig. 21). The Marguerite and Tobacco, both
easily grown in the house, are on the 3/8 plan. The latter shows the eight
ranks most plainly in the rosette of its lower leaves. The distribution is
often brought about by differences in the lengths of the petioles, as in
a Horsechestnut branch (Fig. 22) where the lower, larger leaves stand
out further from the branch than the upper ones; or by a twist in the
petioles, so that the upper faces of the leaves are turned up to the
light, as in Beech (Fig. 23). If it is springtime when the lessons are
given, endless adaptations can be found.
[Footnote 1: Reader in Botany. IX. Leaf-Arrangement.]
[Illustration: FIG. 21. Branch of Geranium, viewed from above.]
[Illustration: FIG. 22.]
[Illustration: FIG. 23.]
_Gray's First Lessons_. Sect. IV. VII, §4. _How Plants Grow_. Chap. I,
51-62; I, 153.