THE SYNFLORESCENCE OF
A. B. STOUT
The New York Botanical Garden
An examination of a considerable number of horticultural Amaryllis hybrids grown at The New York Botanical Garden revealed that the highest number of flowers on any scape was four. Two of these were primary flowers of the main axis of the scape and for them the disposition and sequence of development were undoubtedly those of a raceme. But of the two secondary flowers one was lateral to each of the primary flowers in a cymose relation. Thus there are two distinct branching systems in such a synflorescence. The primary inflorescence of two flowers is a monopodial determinate raceme. But each of its flowers and its own lateral form a sympodial helicoid cyme or bostryx. The entire group of flower branches is not only compound but heterogeneous in regard to the branching. The external features of these conditions are fully evident in a typical but relatively simple four-flowered synflorescence, a term applied by Goebel (2), such as that shown in Plate 268. In it, and in the diagram of it that is shown in 1 of Plate 269, the bracts, bracteoles, and flowers or pedicels are numbered in respect to their relative positions and the flowers are also numbered in sequence of anthesis. A description of this synflorescence follows.
There are two primary flowers (nos. 1 and 2) and for each there is a large bract that arises from the main axis. One of these flowers is the terminal of the main axis of the scape; the other is terminal for a lateral on this axis. On flower (1) matures first and it is larger than the other (2). Its bracteole is larger than that of the other flower (compare b1 with b2).
In various monocots which have synflorescences less compacted than those of Amaryllis hybrids the positions of the bracts indicate beyond any doubt that the primary branching is monopodial, determinate and racemose. When there are only two bracts one is entirely below the other and the flower in its axil is the first to mature (see diagram 6 in Plate 269). When there are more than two primary bracts the racemose development is often evident both in compacted synflorescences as in Clivia (5 in Plate 269) and in loosely branched compound heterogeneous inflorescences as in most species of Hemerocallis (4). But in Amaryllis hybrids the two primary bracts are much modified in growth, in position, and in symmetry which is associated with the development of the laterals to time primary flowers.
In the four-flowered synflorescence here illustrated (Plate 268 and no. 1 in Plate 269) the flower 1-1 is lateral on the axis of the primary flower 1. The internode constituting the peduncle of flower 1 is the section between bract Ill and the bracteole hi. and that of the flower 1-1 is between bracteole b1 and b1-1. These sections of the false axis of the bostryx are much compressed and interposed and each bracteole on this axis is spaced at 90° from the bract or bracteole below in an ascending spiral that is anti-clockwise as indicated in the diagram. This two-flowered unit is a helicoid cyme or bostryx. The two sections of its axis are compressed into thin plates, but the branching is definitely sympodial. The same structural relations exist for the flowers 2 and 2-1 and for their bracteoles, peduncles and pedicels which comprise another two-flowered bostryx.
The compression in vertical growth of the internode between bracts B1 and B2 and of the internodes representing the peduncles of the four flowers and their expansion in diameter result in an aggregate of nodes arid internodes that may be called a synnode. Each enlarged bract (B1 and B2) encompasses a flower and its auxiliary bostryx. Each is keeled in the line of the greatest diameter of the synnode and each has a wide lobe and a narrow one. The nodes at their origin in the stem are a]most at the same level and the two bracts fully encircle the scape and the synnode. Their margins expand laterally and the adjacent margins of the two overlap from the base to the apex of an unopened spathe. The excentric growth in the base of each bostryx is such that the narrow lobe of a bract is always on the side of the lateral in the bostryx and the wide lobe of time other bract is interposed between it and its bracteole. Thus the bract that is outside in the overlap on one side of the spathe is the bract that is inside on time opposite side (see diagrams 1 to 4 in Plate 269). It would be of some interest and perhaps of significance to determine the extent of this modification in the synflorescences of the Amarylladaceae and their relatives.
In the Amaryllis hybrids observed by the writer, three-flowered scapes bear one solitary primary flower. In two-flowered inflorescences both flowers are primary but a second bracteole often represents the position of the aborted false axis of the bostryx. The pedicels of the flowers arise at nearly the same level in the synnode and they are elongated. Though they differ in length and size, as do the flowers, according to sequence in development, the entire compound heterogeneous inflorescence simulates an umbel, and it is so designated in most taxonomic literature of Amaryllis.
It may be reported that an examination of the 8- to 17-flowered scapes of "Clivia nobilis" grown at The New York Botanical Garden in 1944 revealed that there were as many as four different helicoid cymes in a synflorescence and that the arrangement of these units was racemose. The stem of a scape at time level of the bracts was enlarged excentrically to a greater degree than in the Amaryllis hybrids, in response to the increase in time number of collateral primary and secondary branches. Time number of flowers in a bostryx was as many as four and all of their peduncles were much compressed. A diagram of one of the synflorescences observed in Clivia nobilis is shown in 5 of Plate 269. There were four bracts and four bostryxes. In each of the lower two (nos. 1 and 2), the position of the bracts, bracteoles, arid pedicels was quite regular--But in each of the third and fourth bostryxes the bract did not encompass its primary flower which stood well toward the center in the synnode of the entire synflorescence while its bract was more toward the outer border. For each bostryx there was a bract and for the base of each pedicel there was a bracteole. The four bracts of the primary raceme and each series of bracteoles all had a spiral spacing of approximately 90°.
According to Goebel (1) the true umbel, either simple or compound, is exclusively racemose, but there are cymose umbels with dichasial branching which superficially resemble true umbels. More recently Goebel (2) has emphasized that especially in monocots inflorescences which appear to he umbels may be composed of several different cymose inflorescences. For an example of this lie illustrates an inflorescence of Allium Suwarovii and refers to the studies made by Weber (6). This investigator observed that there is little difference in the time of the maturity of the primary flowers, that within each unit inflorescence there is repeated cymose branching, and that the number and relative positions of the many branches are riot always evident. Goebel recognizes that in the many-flowered and much-condensed synflorescences which surmount greatly thickened scapes (as in Allium, Clivia, etc.) the developments involve interpolations and displacements of stem elements and the elimination of bracteoles to the degree that the stem units below the pedicels are not distinct from one another. It may be added that these developments also obscure the disposition of the several units of a compound inflorescence which is heterogeneous in respect to branching.
The character of the inflorescence, and especially the judgment as to whether it is a raceme or an umbel, has been the basis of rather important distinctions in evaluating the status and relationships of genera and of still larger groups of plants. In his evaluation of the Liliaceae and the Amaryllidaceae, Hutchinson (3) considers that the character of the inflorescence is more important than is the position of the ovary. In his revision of the Amaryllidaceae several new families were made by extractions largely on the view that of the legitimate members of this family the "most distinct and constant feature is the umbellate scapose inflorescence."
Frequently in discussions of taxonomy arid phylogeny the reference to a racemose inflorescence actually refers to secondary branches that are helicoid cymes. Also many references to an umbel refer to synflorescences in which the basic or primary branching is racemose and the Secondary branching is cymose. Usually an inflorescence of Hemerocallis is termed "racemose." But the extended secondary branches are helicoid cymes; the arrangement of the primary flowers of these bostryxes is, however, racemose. The sequence of the heterogeneous branching, primary arid secondary, is the same as that in the Amaryllis hybrids. In the genus Hemerocallis (4) there is much diversity and specificity in such features as (a) degree of branching, (b) fusion or stem elements, (c) dichotomy, and (d). displacement of bracts and bracteoles. In one species (H. nana) there are only primary flowers in racemose development, and often scapes bear a single terminal flower whose axis has only one bracteole and one bract. For the species that have much-branched terminal inflorescences there are several secondary units each of which is an extended bostryx. In one species the two branching systems (H. Middendorffii) are condensed into a synflorescence.
|A typical four-flowered
heterogeneous synflorescence of an Amaryllis hybrid. The primary flowers 1 and
2 form a raceme. The flowers 1 and 1-1 form a helicoid cyme or bostryx;
and the flowers 2 and 2-1 form another bostryx. For the arrangement and
position of the bracts and bracteoles see diagram 1 in Plate 269.
|Diagrams 1 to 4
inclusive are of inflorescences of Amaryllis hybrids. No. 1 is for the
inflorescence shown in figure 1, and the numbering of the flowers, bracts and
bracteoles is the same in both. In diagrams I and 4 the spiral disposition is
counter clockwise; in 2 and 3 it is clockwise. In diagram 3 the two flowers are
primary ones; in 4 there is a lateral (1-2) to only one (1) of the
Diagram 5 is of a compound heterogeneous synflorescence of Clivia nobilis in which there were four primary flowers (1 to 4) and bracts (131 to 134). Each of these flowers was the first flower of a bostryx and each flower had a bracteole.
Diagram 6 is for an inflorescence of Hemerocallis Middendorffii which had only two primary flowers (1 and 2); the two bracts overlapped and one was entirely below the other. On the false axis above flower no. I there were three flowers; above flower no. 2 there were two flowers as indicated. Usually in Hemerocallis there is at least one small bracteole of the aborted end of the false axis and occasionally this is present in Amaryllis hybrids.
A recent revision of the genus Amaryllis (5) includes species formerly called Hippeastrum. In the tentative key to the subgenera and species it is indicated that the number of flowers "per umbel" is different for certain species. For three species the number is one; for five species the number is 1-2; for two species the number is 2-6; and for seven species the number is 3-10. These differences suggest that there may be specificity either (a) in reduction to a single primary system of branching that is confined to a main axis or (b) in modifications which retain both the racemose and the helicoid cymose systems seen in the Amaryllis hybrids in horticultural culture. The ten-flowered inflorescenses may involve either (a) increase in the primary units of the raceme or (b) increase in the number of flowers per bostryx or (c) both of these conditions.
At any rate the term umbel is scarcely adequate for complete descriptions and critical comparisons. It needs further qualification in respect to what the branching is, especially when there is heterogeneous branching that includes both the monopodial raceme and the sympodial bostryx. The umbel is the expression of a habit of vegetative growth characterized by a differential repression of elongation in a series of internodes. In the Amaryllis hybrids this condition is localized in the apical internode of the scape anti in the peduncles of the several secondary as well as primary, and racemose as well as cymose in development. To this condition the elongation of the pedicels which are next above is in sharp contrast as is also the greatly elongated and expanded internode which forms the scape below. These highly specialized features of vegetative growth are so fundamental that they may affect, and transform, and combine both a monopodial racemose branching and a sympodial cymose branching.