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|Vol. 12(6), pp. 4-11||The McAllen International Orchid Society Journal||June 2011|
The arrangement of flowers on a floral axis; a floral cluster.
Fig. 1. Diagrammatic representations of a few inflorescence types. Digital scan from MIOS Journal 3(7): 5, issue of July, 2002.
As a practical point, the term inflorescence does not just refer to the specific arrangement, but the term "inflorescence" is used when one speaks of that portion of the plant directly and only having to do with displaying flowers. Five different types of inflorescences are illustrated below (Fig. 1). It is correct to refer to each simply as an inflorescence, but just referring to it as such, tells nothing about what type of an inflorescence it is. Similarly, just referring to a cabinet-full of containers of ice cream doesn't supply any information about the flavors of the containers of ice cream within the cabinet.
Let us become more specific about an inflorescence, and some of its parts. An inflorescence may be simple (single, spikate) or quite complex (a panicle), but its major axis (sometimes incorrectly called its main "stem") is the rachis.
1.The axis of a compound leaf or fern frond upon which the leafelets are attached. 2. The major axis of an inflorescence.
Referring specifically to each of the simplified illustrations in Fig. 1, we will now define, each type of rachis. Each rachis type is diagramatically illustrated by the five vertical "main lines" from which the flowers extend.
Fig. 2. Lycaste deppei, plant of Betty Dunton. (In her greenhouse at South Pasadena, California) Digital photo DSCN 1330; Thur-16Jun05.
Fig. 3. Maxilariella tenuifolia. Digital photos 4016; Thur-07Apr 11.
The single type of inflorescence is seen in Paphiopedilum species as well as in members of Lockhartia, Lycaste (Fig. 2), Maxillaria, Maxillariella (Fig. 3), Miltonia, and other orchid genera. In passing, note that Lycaste deppei provides an example of single flowering as well as demonstrating lateral flowering; arising from the pseudobulb base. As an aside, it may also be noted that L. deppei flowers may begin opening prior to the new vegetative growth sprouting. Note also the rather dehydrated condition of the pseudobulbs. In their normal annual growth cycle, the vegetation grows; the pseudobulb then forms below; and then the leaves mature, and -- in some species -- wilt, and are discarded and the plant enters the "winter" or dry season as only a cluster of pseudobulbs. This period is often noted by some orchidists as the plant going through a "resting" period, but such is not actually the case. During the winter or dry season, the pseudobulbs gradually dehydrate and as their internal sugars coalesce, and flowering is initiated to coincide with the onset of the wet-season or "spring rains. The flowers open and are hopefully pollinated, followed soon by the new vegetative growth. As the "wet-spring" gives way to the slightly drier summer season, the seed capsules mature. As the "winter-dry season" sets in, the capsules dehisce and the powdery seeds are blown to new locations to (hopefully) sprout with the onset of the following "spring" or "wet" season.
It will be noted in the case of Lycaste deppei above that the foliage is well matured as the flowers are blooming, but this is not necessarily the case with even members of the same genus. When Lycaste aromatica flowers, it normally does so with the new vegetative growths either absent or just barely beginning to sprout. In the case of Maxillariella tenuifolia, its delicate, slim-leaved foliage (L. tenuifolia: delicate foliage) persists all year, making for a plant that is not unattractive even when not in flower.
There is still another variation to "single flowering." Some paphiopedilums and phragmipediums flower "solitary" or "single" but some species put up a single flower and then another and still another bud follows. As the first flower wilts (or is about to do so), a second flower opens and the sequence may go on for three or four flowers. These species are more properly said to be single-flowered, but to flower successively.
Fig. 4. Red Clover (Trifolium repens L.)
The term "head" should actually be labeled composite. It may appear to be seen in a few orchid species where the flowers are densely packed on the inflorescence, but orchids don't utilize the composite mode of flowering. Dicots do. Members of Asteraceae (the Aster or Sunflower family) and the Fabaceae (Bean or Legume Family) are good examples of "head-type" inflorescences (Fig. 4). In the monocot Orchid family, we don't see the composite flowering form, but it's included here merely as an example of one of the inflorescence types. Some orchid species do cluster their flowers terminally, but they do so as an umbel modification or a panicle, not a head or composite.
Fig. 5. Coreopsis species (Fam. Asteraceae). (note small spider on flower at 7 o'clock position) Digital photo 4152a; Sun-22May11.
The flower in the figure (Fig. 5) not only shows a well-camouflaged spider at its 7 o'clock position, and the leg of still another spider beneath the flower at its 12 o'clock position. The beautiful summer flower can be a dangerous place for the unwary and over-eager pollinator! In the case of the flower in Fig. 5, the yellow (petals" are actually ray flowers and the minute yellow dots show the pollen from "head" (seed-forming) individual flowers. In both Figs. 4 and 5, the composite head of florets arises from a single rachis.
In older books the reader may run across the name "Compositae" for the Family Asteraceae, and "Leguminosae" for the Family Fabaceae. These are obsolete terms for these families. Several years ago, the International Congress of botanists agreed to change these family names as listed in the International Code of Botanical Nomenclature in order to reflect a more correct viewpoint of each family. Thus, correctly speaking, the Asteraceae is the Aster (not Sunflower) family and Fabaceae is the Bean (not Legume) family.
This type of inflorescence is well illustrated by members of the Asclepidaceae (Milkweed family), Gentianaceae (Gentian family), and Apocyanaceae (Dogbane family). Observing closely, it will be noted that the inflorescence is actually a group of flower clusters (umbels) held terminally on the rachis. Casually, the whole inflorescence is sometimes referred to as an umbel, but it is better visualized as an "umbel of umbels" or a "compound umbel."
Fig. 6. "Geranium" inflorescence. (Pelargonium hybrid). Digital photo 4151a; Sun-22May11.
Two genera, Pelargonium (Fig. 6) and Geranium are members of the Geranium family (Geraniaceae). Both genera, bear umbel type inflorescences. However, the horticulturally popular garden plants United States citizens commonly know as "geraniums" are actually members of the genus Pelargonium. These have seven stamens and are from Africa, while the (botanically correct) geraniums are temperate zone plants with ten stamens.
Fig. 7. Asclepias lanceolata (Family Asclepiadaceae). (Red milkweed, Butterfly weed;Chigger weed)
The umbel type of inflorescence is also typically seen in the "Butterfly weed" (Fig. 7). This species and other members of the milkweed family are frequented by Monarch butterflies which also lay their eggs on the plants. As the caterpillars dine on the leaves, they ingest the poisonous milky sap. This poisonous sap doesn't harm the caterpillars, and it is passed on to the mature butterflies. As a result, a young bird eating a Monarch butterfly very shortly discovers that eating this particularly colored insect causes an upset digestive tract! The result is the young bird learns to leave monarchs alone! The Viceroy butterfly's caterpillar does not dine on milkweeds, and hence the Viceroy remains a tasty meal for a bird. However, its coloration imitates the Monarch well enough to deceive the bird-predators, and it gets left alone as well. This phenomena is known as Batesian mimicry.
By the way, as a note to humans inclined to closely examine the flowers of "Butterfly weed" and handle them in the wild: they're also known as "Chigger weed," and an encounter with chiggers that have gained access to the human body, particularly in the underarm, joint, and groin areas is not soon forgotten!
Spikes are similar to single or head , but bear sessile flowers (sessile: without a stalk; sitting directly on its base) at various intervals along its support line. It's common to hear orchidists speak of a "flower spike," or a plant "spiking," "being in spike." At the same time, purists have been known to visibly cringe or make a smug side comment about how "orchids don't spike," or "it's not a spike, it's a panicle," or some other such put-down. In actuality, the purists are partly correct in that orchids, in general, don't spike. Spikes are uncommon in the Orchidaceae, but there are species of Orchis in which inflorescences in the bud stage appear to be racemes, but following pollination assume the appearance of a spike because their swollen ovaries, which looked like peduncles prior to fertilization, have now become recognizable as such. In some species of orchids, the axis of a spike can be leaf-like, and in such cases the flowers are borne on a structure called a phylloclade. This is seen in Bulbophyllum falcatum and Bulbophyllum maximum. The sum total is simply this: don't look for spikes as an orchid inflorescence pattern, but realize that, particularly in the orchid family, things are not always what they "obviously" seem to be.
Fig. 8. Spiranthes lacera var. gracilis (terminal portion of inflorescence) Digital photo DSC_1966; Wed-26Sept07.
Fig. 9. Spiranthes cernua (the non-opening form) (terminal portion of inflorescence) Digital photo DSC_2065; Fri-26Oct07.
The figures of Spiranthes (Fig. 8) (Fig. 9) look suspiciously like a spike, but the flowers, although closely held on the rachis, are held by a short peduncle (stem-like; below the flower's ovary) which is subtended by a bract that closely clasps both the peduncle and ovary.
Fig. 10. Part-inflorescence, Tipularia discolor. (terminal portion of inflorescence) Digital photo DSC_0020; Sat-13Aug05.
Fig. 11. Close-in of part-inflorescence, Tipularia discolor. Digital photo DSC_0020ab; Sat-13Aug05.
A clearer look at how the orchid flowers are held on this type of rachis may be provided by the illustration of an inflorescence of Tipularia discolor (Fig. 10). At a glance it will appear the flowers are held sessile on the rachis, but upon a closer look (Fig. 11) it will be seen that the flower, with its inferior ovary, is attached to the rachis by what looks like a "stem." This is actually not a stem but is botanically a different vegetative organ called a peduncle.
In the close-in view of the individual flowers, one also sees the long nectary or spur extending from below the flower's column. In probing for the necter's reward, the pollen is attached to the moth's head and at the insect's next floral visit is deposited to fertilize that flower and a new load of pollen is bestowed on the insect.
Tipularia discolor, known as the cranefly orchid, is widespread across much of the southeastern United States, and it flowers at pretty much the same time in August every year. There may be some minor variation between its extreme southern and northern ranges, but it's generally to be seen flowering sometime during August in most locations.
These photos were taken during a trek in the Frances Marion National Forest of Berkeley County, South Carolina in the company of Jim Fowler, a recognized authority of the native orchids of South Carolina and author of the excellent book about these plants. As an aside, if this book is not in your orchid library, it certainly ought to be! Equally, you would do well to strongly consider taking a trip to see and photograph many of the native orchids of both South and North Carolina!
Fig. 12. Brassia caudata with inflorescence. Digital photo DSC_4018a; Thur-07Apr 11.
A raceme is an unbranched, indeterminate inflorescence, in which the individual flowers are borne on pedicels along the main axis. In this case, the main axis = rachis = raceme. Most orchids flower racemose (meaning "on a raceme" or "raceme-like"). An example may be seen in the showy unbranched Brassia caudata raceme displaying flowers one after another in a long nearly-straight line or a graceful arch. (Fig. 12). By the way, in a raceme, the flowers open from closest to the plant to the outermost flower (it's called acropetal flowering). In a panicle, when such flowering is reversed, with the outermost flowers opening first, this phenomenon is known as basipetal flowering.
Fig. 13. Pl#290496-1. Encyclia alata subspecies parviflora. Digital photo DSC_4157a; Mon-23May11.
The panicle is a rachis with branches and perhaps even with branchlets. As one definition states it: "pedicillate flowers are borne upon the secondary branches." True, but this is not necessarily the whole story. Oncidium sphacelatum panicles have primary branches, and then may have secondary branches and even tertiary branchlets! With a rachis of about 2 meters long and branchlets fronding out to 60 cm. (2 feet!) on either side, and all are bearing flowers, it makes for quite an array of flowers. Considering that a single panicle on a plant that may carry a few dozen flowers, and a plant may bear dozens of panicles, the entire output can easily become a spectacular display! One such example of this is a plant your editor has cultured on the same tree fern plaque since April of 1996 (Fig. 13). It has been in flower for over a month and is currently bearing forty panicles, and a few hundred flowers!
The panicle inflorescence confers an advantage to the plant over the raceme form in that a raceme can only carry so many flowers. Because of its branching, a panicle can carry many more flowers, and more flowers mean greater chances for more seed capsules and hence, greater "plant hope" for more orchid seedlings! In 2002, an Encyclia alata subsp. parviflora alba carried only three panicles, but these held a total of 194 flowers! The paniculiform inflorescence conveys not only a reproductive advantage to the plant, but, for the orchid lover, many more flowers per plant.
For the orchidist, the main compromises are usually the size of the pocketbook gauged against space available, and this--coupled with geographical location--usually affects whether one has a greenhouse and, if so, its size. Other considerations enter into the equation as well. Other considerations include the type of orchids in which one is interested and whether one is interested in only native-growing species or whether the orchidist looks to grow a mix of tropical species and hybrids. Such variables taken care of, the choices may come down to whether one wants very large flowers or a many smaller ones, or perhaps which types and sizes of plants and types of inflorescences one prefers. From the simple single and head types of inflorescences to the elaborate racemose and panniculiform, the space needed will generally go from small to large.