MORPHOLOGY OF THE ANGIOSPERMS: TRILLIUM; DENTARIA.
Trillium.
=639. General appearance.=—As one of the plants to illustrate
this group we may take the wake-robin, as it is sometimes called,
or trillium. There are several species of this genus in the United
States; the commonest one in the eastern part is the “white wake-robin”
(Trillium grandiflorum). This occurs in or near the woods. A picture of
the plant is shown in fig. 378. There is a thick, fleshy, underground
stem, or rhizome as it is usually called. This rhizome is perennial,
and is marked by ridges and scars. The roots are quite stout and
possess coarse wrinkles. From the growing end of the rhizome each year
the leafy, flowering stem arises. This is 20-30_cm_ (8-12 inches) in
height. Near the upper end is a whorl of three ovate leaves, and from
the center of this rosette rises the flower stalk, bearing the flower
at its summit.
=640. Parts of the flower. Calyx.=—Now if we examine the flower
we see that there are several leaf-like structures. These are arranged
also in threes just as are the leaves. First there is a whorl of three,
pointed, lanceolate, green, leaf-like members, which make up the
_calyx_ in the higher plants, and the parts of the calyx are _sepals_,
that is, each leaf-like member is a _sepal_. But while the sepals are
part of the flower, so called, we easily recognize them as belonging to
the _leaf series_.
=641. Corolla.=—Next above the calyx is a whorl of white or
pinkish members, in Trillium grandiflorum, which are also leaf-like in
form, and broader than the sepals, being usually somewhat broader at
the free end. These make up what is the _corolla_ in the higher plants,
and each member of the corolla is a _petal_. But while they are parts
of the flower, and are not green, their form and position would suggest
that they also belong to the leaf series.
[Illustration: Fig. 378. Trillium grandiflorum.]
=642. Andrœcium.=—Within and above the insertion of the corolla
is found another tier, or whorl, of members which do not at first
sight resemble leaves in form. They are known in the higher plants
as _stamens_. As seen in fig. 379 each stamen possesses a stalk (=
filament), and extending along on either side for the greater part of
the length are four ridges, two on each side. This part of the stamen
is the _anther_, and the ridges form the anther sacs, or lobes. Soon
after the flower is opened, these anther sacs open also by a split in
the wall along the edge of the ridge. At this time we see quantities of
yellowish powder or dust escaping from the ruptured anther locules. If
we place some of this under the microscope we see that it is made up of
minute bodies which resemble spores; they are rounded in form, and the
outer wall is spiny. They are in fact spores, the microspores of the
trillium, and here, as in the gymnosperms, are better known as _pollen_.
[Illustration: Fig. 379. Sepal, petal, stamen, and pistil of Trillium
grandiflorum.]
[Illustration: Fig. 380.
Trillium grandiflorum, with the compound pistil expanded into three
leaf-like members. At the right these three are shown in detail.]
=643. The stamen a sporophyll.=—Since these pollen grains are
the spores, we would infer, from what we have learned of the ferns
and gymnosperms, that this member of the flower which bears them is
a sporophyll; and this is the case. It is in fact what is called the
_microsporophyll_. Then we see also that the anther sacs, since they
enclose the spores, would be the sporangia (microsporangia). From this
it is now quite clear that the stamens belong also to the leaf series.
They are just six in number, twice the number found in a whorl of
leaves, or sepals, or corolla. It is believed, therefore, that there
are two whorls of stamens in the flower of trillium.
=644. Gynœcium.=—Next above the stamens and at the center of the
flower is a stout, angular, ovate body which terminates in three long,
slender, curved points. This is the pistil, and at present the only
suggestion which it gives of belonging to the leaf series is the fact
that the end is divided into three parts, the number of parts in each
successive whorl of members of the flower. If we cut across the body of
this pistil and examine it with a low power we see that there are three
chambers or cavities, and at the junction of each the walls suggest to
us that this body may have been formed by the infolding of the margins
of three leaf-like members, the places of contact having then become
grown together. We see also that from the incurved margins of each
division of the pistil there stand out in the cavity oval bodies. These
are the _ovules_. Now the ovules we have learned from our study of the
gymnosperms are the _sporangia_ (here the macrosporangia). It is now
more evident that this curious body, the pistil, is made up of three
leaf-like members which have fused together, each member being the
equivalent of a sporophyll (here the macrosporophyll). This must be a
fascinating observation, that plants of such widely different groups
and of such different grades of complexity should have members formed
on the same plan and belonging to the same series of members, devoted
to similar functions, and yet carried out with such great modifications
that at first we do not see this common meeting ground which a
comparative study brings out so clearly.
[Illustration: Fig. 381. Abnormal trillium. The nine parts of the
perianth are green, and the outer whorls of stamens are expanded into
petal-like members.]
[Illustration: Fig. 382. Transformed stamen of trillium showing anther
locules on the margin.]
=645. Transformations of the flower of trillium.=—If anything
more were needed to make it clear that the parts of the flower of
trillium belong to the leaf series we could obtain evidence from the
transformations which the flower of trillium sometimes presents. In
fig. 381 is a sketch of a flower of trillium, made from a photograph.
One set of the stamens has expanded into petal-like organs, with the
anther sacs on the margin. In fig. 380 is shown a plant of Trillium
grandiflorum in which the pistil has separated into three distinct and
expanded leaf-like structures, all green except portions of the margin.
Dentaria.
=646. General appearance.=—For another study we may take a plant which
belongs to another division of the higher plants, the common “pepper
root,” or “toothwort” (Dentaria diphylla) as it is sometimes called.
This plant occurs in moist woods during the month of May, and is well
distributed in the northeastern United States. A plant is shown in fig.
383. It has a creeping underground rhizome, whitish in color, fleshy,
and with a few scales. Each spring the annual flower-bearing stem rises
from one of the buds of the rhizome, and after the ripening of the
seeds, dies down.
The leaves are situated a little above the middle point of the stem.
They are opposite and the number is two, each one being divided into
three dentate lobes, making what is called a compound leaf.
[Illustration: Fig. 383. Toothwort (Dentaria diphylla).]
[Illustration: Fig. 384. Flower of the toothwort (Dentaria diphylla).]
=647. Parts of the flower.=—The flowers are several, and they
are borne on quite long stalks (pedicels) scattered over the terminal
portion of the stem. We should now examine the parts of the flower
beginning with the calyx. This we can see, looking at the under side
of some of the flowers, possesses four scale-like sepals, which easily
fall away after the opening of the flower. They do not resemble leaves
so much as the sepals of trillium, but they belong to the leaf series,
and there are two pairs in the set of four. The corolla also possesses
four petals, which are more expanded than the sepals and are whitish in
color. The stamens are six in number, one pair lower than the others,
and also shorter. The filament is long in proportion to the anther, the
latter consisting of two lobes or sacs, instead of four as in trillium.
The pistil is composed of two carpels, or leaves fused together. So we
find in the case of the pepper root that the parts of the flower are
in twos, or multiples of two. Thus they agree in this respect with the
leaves; and while we do not see such a strong resemblance between the
parts of the flower here and the leaves, yet from the presence of the
pollen (microspores) in the anther sacs (microsporangia) and of ovules
(macrosporangia) on the margins of each half of the pistil, we are,
from our previous studies, able to recognize here that all the members
of the flower belong to the leaf series.
=648.= In trillium and in the pepper root we have seen that the
parts of the flower in each apparent whorl are either of the same
number as the leaves in a whorl, or some multiple of that number. This
is true of a large number of other plants, but it is not true of all. A
glance at the spring-beauty (Claytonia virginiana), and at the anemone
(or Isopyrum biternatum, fig. 563) will serve to show that the number
of the different members of the flower may vary. The trillium and the
dentaria were selected as being good examples to study first, to make
it very clear that the members of the flower are fundamentally leaf
structures, or rather that they belong to the same series of members as
do the leaves of the plant.
=649. Synopsis of members of the sporophyte in angiosperms.=
Higher plant.
Sporophyte phase {Root. {Foliage leaves.
(or modern {Shoot. {Stem. {Perianth leaves. }
phase). { Leaf. {Spore-bearing leaves }
{ with sporangia. } Flower.
{(Sporangia sometimes }
{ on shoot.) }