FLOWER-CLUSTERS
=Origin of the Flower-cluster.=—We have seen that branches arise from
the axils of leaves. Sometimes the leaves may be reduced to bracts
and yet branches are borne in their axils. Some of the branches grow
into long limbs; others become short spurs; _others bear flowers_. In
fact, a flower is itself a specialized branch.
[Illustration: FIG. 211.—TERMINAL FLOWERS OF THE WHITEWEED (in some
places called ox-eye daisy).]
Flowers are usually borne near the top of the plant. Often they
are produced in great numbers. It results, therefore, that flower
branches usually stand close together, forming a =cluster=. The shape
and the arrangement of the flower-cluster _differ with the kind of
plant_, since each plant has its own mode of branching.
Certain definite or well-marked types of flower-clusters have
received names. Some of these names we shall discuss, but the
flower-clusters that perfectly match the definitions are the
exception rather than the rule. The determining of the kinds of
flower-clusters is one of the most perplexing subjects in descriptive
botany. We may classify the subject around three ideas: =solitary
flowers=, =centrifugal= or =determinate clusters=, =centripetal= or
=indeterminate clusters=.
[Illustration: FIG. 212.—LATERAL FLOWER OF AN ABUTILON. A greenhouse
plant.]
=Solitary Flowers.=—In many cases flowers are borne singly; they are
separated from other flowers by leaves. They are then said to be
=solitary=. The solitary flower may be either at the end of the main
shoot or axis (Fig. 211), when it is said to be =terminal=; or from
the side of the shoot (Fig. 212), when it is said to be =lateral= or
=axillary=.
=Centripetal Clusters.=—If the flower-bearing axils were rather
close together, an open or leafy flower-cluster might result. If the
plant continues to grow from the tip, the older flowers are left
farther and farther behind. If the cluster were so short as to be
flat or convex on top, the outermost flowers would be the older. A
flower-cluster in which the lower or outer flowers open first is
said to be a =centripetal cluster=. It is sometimes said to be an
=indeterminate= cluster, since it is the result of a type of growth
which may go on more or less continuously from the apex.
The simplest form of a definite centripetal cluster is a =raceme=,
which is an open elongated cluster in which the _flowers are borne
singly on very short branches_ and open from below (that is, from
the older part of the shoot) upwards (Fig. 213). The raceme may be
_terminal_ to the main branch; or it may be _lateral_ to it, as in
Fig. 214. Racemes often bear the flowers on one side of the stem,
thus forming a single row.
[Illustration: FIG. 213.—RACEME OF CURRANT. Terminal or lateral?]
When a centripetal flower-cluster is long and dense and the flowers
are sessile or nearly so, it is called a =spike= (Fig. 215). Common
examples of spikes are plantain, mignonette, mullein.
[Illustration: FIG. 214.—LATERAL RACEMES (in fruit) of BARBERRY.]
[Illustration: FIG. 215.—SPIKE OF PLANTAIN.]
A very _short and dense spike_ is a =head=. Clover (Fig. 216) is
a good example. The sunflower and related plants bear many small
flowers in a very dense and often flat head. Note that in the
sunflower (Fig. 189) the outside or exterior flowers open first.
Another special form of spike is the =catkin=, which usually has
scaly bracts, the whole cluster being deciduous after flowering or
fruiting, and the flowers (in typical cases) having only stamens
or pistils. Examples are the “pussies” of willows (Fig. 182) and
flower-clusters of oak (Fig. 180), walnuts (Fig. 204), poplars.
[Illustration: FIG. 216.—HEAD OF CLOVER BLOSSOMS.]
[Illustration: FIG. 217.—CORYMB OF CANDY-TUFT.]
When a loose, elongated centripetal flower-cluster has some primary
branches simple, and others irregularly branched, it is called a
=panicle=. It is a branching raceme. Because of the earlier growth of
the lower branches, the panicle is usually broadest at the base or
conical in outline. True panicles are not very common.
When an indeterminate flower-cluster is short, so that the _top is
convex or flat_, it is a =corymb= (Fig. 217). The outermost flowers
open first. Centripetal flower-clusters are sometimes said to be
corymbose in mode.
When the branches of an indeterminate cluster _arise from a common
point_, like the frame of an umbrella, the cluster is an =umbel=
(Fig. 218). Typical umbels occur in carrot, parsnip, caraway, and
other plants of the parsley family: the family is known as the
Umbelliferæ, or umbel-bearing family. In the carrot and many other
Umbelliferæ, there are small or secondary umbels, called =umbellets=,
at the end of each of the main branches. (In the centre of the wild
carrot umbel one often finds a single, blackish, often aborted
flower, comprising a 1-flowered umbellet.)
[Illustration: FIG. 218.—REMAINS OF A LAST YEAR’S UMBEL OF WILD
CARROT.]
=Centrifugal or Determinate Clusters.=—When the terminal or central
flower opens first, the cluster is said to be =centrifugal=. The
growth of the shoot or cluster is =determinate=, since the length is
definitely determined or stopped by the terminal flower. Fig. 219
shows a determinate or centrifugal mode of flower bearing.
[Illustration: FIG. 219.—DETERMINATE OR CYMOSE ARRANGEMENT.—Wild
geranium. ]
Dense centrifugal clusters are usually flattish on top because of
the cessation of growth in the main or central axis. These compact
flower-clusters are known as =cymes=. Centrifugal clusters are
sometimes said to be cymose in mode. Apples, pears (Fig. 220), and
elders bear flowers in cymes. Some cyme-forms are like umbels in
general appearance. A head-like cymose cluster is a =glomerule=; it
blooms from the top downwards rather than from the base upwards.
=Mixed Clusters.=—Often the cluster is mixed, being determinate in
one part and indeterminate in another part of the same cluster. The
main cluster may be indeterminate, but the branches determinate. The
cluster has the appearance of a panicle, and is usually so called,
but it is really a =thyrse=. Lilac is a familiar example of a thyrse.
In some cases the main cluster is determinate and the branches are
indeterminate, as in hydrangea and elder.
[Illustration: FIG. 220.—CYME OF PEAR. Often imperfect.]
=Inflorescence.=—The _mode_ or _method_ of flower arrangement is
known as the =inflorescence=. That is, the inflorescence is cymose,
corymbose, paniculate, spicate, solitary, determinate, indeterminate.
By custom, however, the word “inflorescence” has come to be used
in works on descriptive botany for the _flower-cluster itself_. Thus
a cyme or a panicle may be called an inflorescence. It will be seen
that even solitary flowers follow either indeterminate or determinate
methods of branching.
[Illustration: FIG. 221.—FORMS OF CENTRIPETAL FLOWER-CLUSTERS.
1, raceme; 2, spike; 3, umbel; 4, head or anthodium; 5, corymb.]
[Illustration: FIG. 222.—CENTRIPETAL INFLORESCENCE, _continued_.
6, spadix; 7, compound umbel; 8, catkin.]
[Illustration: FIG. 223.—CENTRIFUGAL INFLORESCENCE.
1, cyme; 2, scirpioid raceme (or half cyme).]
=The flower-stem.=—The stem of a solitary flower is known as a
=peduncle=; also the general stem of a _flower-cluster_. The stem of
the individual flower in a cluster is a =pedicel=. In the so-called
stemless plants the peduncle may arise directly from the ground, or
crown of the plant, as in dandelion, hyacinth, garden daisy; this
kind of peduncle is called a =scape=. A scape may bear one or many
flowers. It has no foliage leaves, but it may have bracts.
SUGGESTIONS.—=166.= Name six columns in your notebook as follows:
spike, raceme, corymb, umbel, cyme, solitary. Write each of the
following in its appropriate column: larkspur, grape, rose,
wistaria, onion, bridal wreath, banana, hydrangea, phlox, China
berry, lily-of-the-valley, Spanish dagger (or yucca), sorghum,
tuberose, hyacinth, mustard, goldenrod, peach, hollyhock, mullein,
crêpe myrtle, locust, narcissus, snapdragon, peppergrass,
shepherd’s purse, coxcomb, wheat, hawthorn, geranium, carrot,
elder, millet, dogwood, castor bean; substitute others for
plants that do not grow in your region. =167.= In the study
of flower-clusters, it is well to choose first those that are
fairly typical of the various classes discussed in the preceding
paragraphs. As soon as the main types are well fixed in the
mind, random clusters should be examined, for the pupil must
never receive the impression that all flower-clusters follow the
definitions in books. Clusters of some of the commonest plants are
very puzzling, but the pupil should at least be able to discover
whether the inflorescence is determinate or indeterminate. Figures
221 to 223 illustrate the theoretical modes of inflorescence. The
numerals indicate the order of opening.