90

HELICOID CYMES.

dichotomous, as in the Chickweed (Fig. 127), when the primary axis, a', is terminated by a flower, at the base of which are two bracts, each of which develops in its axil secondary axes, a", a', ending in single flowers; and at the base of each of these flowers there are also two other bracts, from which tertiary axes, a"", a"", are developed, also terminated by flowers, and so on; and, as the division in this case always takes place into two branches, the cyme is said to be dichotomous. The dichotomous cyme is also called a biparous cyme or dichasium. is not a true dichotomous branching (see page 59), but only apparently so, in consequence of the greater development of the lateral axes as compared with that of the primary axis.

This

Cymes are also frequently characterized as corymbose, umbellate, spiked, or racemose, from their resemblance, except in the order of the expansion of their flowers, to the true corymb, umbel, &c.

b. In Helicoid or Scorpioid Cymes the flowers are only developed on one side, and the upper extremity is more or less coiled up in a circinate manner, so as frequently to resemble the shell of a snail, or the tail of a scorpion; hence the names helicoid and scorpioid. In these plants the bracts are alternate; but such a cyme may also occur in plants with opposite bracts, and the manner in which it is most commonly believed to be formed in the two cases is as follows: Thus, in plants where the bracts are opposite, it arises by the regular non-development of the axes on one side, while those on the other side are as regularly produced. This will be readily explained by a reference to the diagram (Fig. 128). Here a represents the flower which terminates the primary axis; at the base of this flower are two bracts, only one of which develops a secondary axis, b, which is in like manner terminated by a flower, at the base of which are also two bracts, only one of which (i. e., that on the same side with the first)

HELICOID CYMES.

91

produces a tertiary axis, c, also terminated by a flower with two bracts at its base, one of which gives origin to another axis, d, placed in a similar manner, and so on. In consequence of this one-sided (or, as it is called, secund) manner in which the successive axes are produced, the direction of the inflorescence is constantly drawn to one side at the formation of each axis, and that in proportion to the size of the angle formed by it with the axis from which it springs, and thus, when the angle is large, and many flowers are produced in succession, the upper extremity becomes completely coiled up in a circinate manner. In plants with alternate bracts, the helicoid cyme arises from the primary axis (Fig. 129, 1), being terminated by a flower,

FIG. 128.-Diagram to illustrate the formation of a belicoid cyme in a plant with opposite bracts. a. Flower terminating the primary axis. b. Secondary axis. c. Tertiary axis. d. Quaternary

axis. Each axis is terminated by a flower. The dotted lines represent the position of the undeveloped axes.

and giving off below it from the uppermost bract a secondary axis, 2, which also terminates in a flower, and gives off below it in like manner from the same side as the former a third axis, 3, which likewise terminates in a flower, and so on as seen by the figures. But the nature of this kind of inflorescence is at present by no means well defined.

FIG. 129.-Diagram to illustrate the formation of a helicoid cyme in a

plant with alternate bracts. The fig

ures represent the respective axes, and

the dotted lines below the flowers the

position of the bracts.

C. The Fascicle or Contracted Cyme is a cyme which is rather crowded with flowers placed on short pedicels of nearly equal length, and arising from about the same

point, so that the whole forms a flattened top, as in the

92

MIXED INFLORESCENCE.

Sweet William and some other plants of the Pink order to which it belongs.

d. The Glomerule is a cyme which consists of a few sessile flowers, or of those where the pedicels are very short, collected into a rounded head or short spike. Examples may be seen in Labiate plants. (See Verticil laster.)

e. In the Verticillaster the flowers appear at first sight to be arranged in whorls around the axis, but upon examination it will be seen that, in each apparent whorl, there are two clusters or glomerules axillary to two leafy bracts, the central flowers of which open first, and hence the mode of expansion is centrifugal. To these false whorls thus formed of two axillary glomerules, the term verticillaster is frequently applied; but this variety of inflorescence is sometimes regarded as a contracted form of the dichasium.

Besides the above regular kinds of inflorescence, all of which are comprehended under the two divisions of indefinite and definite, there is a third division, which consists in a combination of these two forms, to which the term mixed inflorescence has been accordingly given.

3. Mixed Inflorescence.-This kind of inflorescence is formed by the general inflorescence developing in one way, and the partial or individual inflorescence in another. Thus in plants of the natural order Compositæ, the terminal capitulum is the first to expand, and the capitula, as a whole, are therefore developed in a centrifugal manner; while the individual capitula open their flowers from the circumference to the center, or centripetally; hence, here the general inflorescence is definite, and each partial inflorescence indefinite. In Labiate plants we have a directly reverse arrangement, for here the individual verticillasters open their flowers centrifugally, but the general inflorescence is centripetal; hence the general inflorescence is here indefinite, while each partial inflorescence is definite.

ESTIVATION.

SECTION 2. ESTIVATION OR PRÆFLORATION.

93

As the general arrangement of the rudimentary leaves of the leaf-bud is called vernation (the spring state), or præfoliation, so the mode in which the different parts of the flower are disposed in the flower-bud is termed their astivation (the summer state), or præfloration. The terms used in æstivation especially refer to the relative positions of the sepals and petals, because the stamens and carpels, from their peculiar forms, can give us no such arrangements of their parts as are exhibited by the more or less flattened parts of the floral envelopes.

With regard to the disposition of each of the component parts of the floral envelopes considered independently of the others, the same terms are used as in similar modifications of vernation, with the addition of the crumpled or corrugated form, which is not found in the parts of the leaf-bud, and it derives its name from the parts being irregularly contracted into wrinkled folds.

With respect to the relation of the several members of either of the floral envelopes taken as a whole to one another, various modifications occur, all of which may be arranged in two divisions; namely, the Circular, and the Imbricated or Spiral Æstivation. The former includes all those varieties in

different levels in a more or less spiral manner, and overlap each other.

I. Varieties of Circular Estivation.-We distin

94

CIRCULAR ÆSTIVATION.

guish three well-marked varieties of this, i. e., the valvate, induplicate, and reduplicate. In the valvate (Fig. 130) variety the parts are flat or nearly so, and in contact by their margins throughout their whole length without any overlapping. When the component sepals or petals, instead of being simply flattened, are folded inward at the points where they come in contact (Fig. 131), the æstivation is induplicate. When the margins are turned outward under the same circumstances (Fig. 132), the æstivation is reduplicate.

When the parts of a whorl are placed as in the ordinary forms of circular æstivation, and one margin of each is directed obliquely inward, and is overlapped by the part adjacent on that side, while the other margin covers the corresponding margin of the adjoining part on the other

in a spiral manner.

FIG. 133. FIG. 134. FIG. 133.-Diagram to illustrate contorted or twisted æstivation. FIG. 134. -Diagram to illustrate imbricate æstivation. The figures 1, 2, 3, 4, 5, show that the successive parts are arranged side, so that the whole presents a more or less twisted appearance (Fig. 133), the æstivation is contorted or twisted. This kind of æstivation occurs frequently in the corolla, but is very rare in the calyx. When in this variety of æstivation the component organs become united, they are frequently plaited, in which case the æstivation is termed plicate or plaited.

2. Varieties of Imbricated or Spiral Estivation. -We distinguish five varieties of this kind of æstivation, i. e., the imbricate, convolute or enveloping, quincuncial, cochlear, and vexillary. The true imbricate æstivation (Fig. 134) is formed by the parts being placed at different levels, and overlapping each other more or less by their margins, like the tiles on the roof of a house, the whole forming a spiral arrangement; this is a very common variety. When the parts, instead of merely overlapping,