The Family Orchidaceae and Its Morphology

" If we visualize the sequence of the flowering plants form the evolutionarly point of view as a genealogical tree ,the branches of which represent the taxonomic concepts of botanical classification ,we find one branch given over to those families which are grouped together as the mobocotyledons. At the tip of one branch ,the family of the Orchids is placed as the most highly organized group set apart ,among other characters ,by complexity of floral structure. In brief ,among the grasses ,palms ,lilies and irises ,to mention only for the forty-five families that constitute the monootyledoes ,the Orchids hold the loftiest position ." So wrote that great student of the orchids ,the late Professor Oakes Ames of Harvard University , in his fervor for the family of plants to which he had devoted a life-time of study and research . The number is legion of those , scientist and layman alike ,who agree in awarding to the orchid a very special place in man's concept of the Plant Kingdom .

The ORCHIDACEAE , without question the largest family not only of the MONOCOTYLEDONEAE but also all the PHANEROGAMAE ,comprise from 20,000 to 25,000 known speceis and varieties distributed in from 500 to 600 different genera. Some specialists place the number of species and genera higher , even as high as 35,000 in 800 genera ,but we prefer to maintain the more conservative estimate .It is true ,however, that our concervative estimate may one day be out-of-date ,since new species are being discovered by almost every botanical expedition to the tropical regions. The orchids , represented in the flora of every part of the globe with the exception of the two polar regions ,are today eypically and outstandingly a tropical family , and their greatest speciation and diversification are expressed in the highland tropics of both the New and Old Worlds.

It has sometimes been maintained that the COMPOSITAE , the most highly evolved of the dicotyledonay families ,may perhaps constitute a larger family in number of species than the Orchidaceae, but there can no longer be any uncertainty as to the numerical superiority of the orchids. And with every year of exploration the difference widens ,because those floras known to be rich in orchids and relatively poor in composites are situated in the most inadequately explored tropical parts of the globe.

The Orchidaceae are grouped in the order MICROSPERAMAE, a principal character of which is the presence of extremely numerous and minute , tunicate seeds. The Microspermae are commonly divided into 4 families: the BURMANNIACEAE with provided with endosperm and usually regular flowers with or 6 or 3 fertile stamens ; and the APOSTASIACEAE and Orchidaceae in the suborderGYNANDRAE , with seeds devoids of endosperm and usually zygomorphic flowers with 1, 2 , or rarely 3 fertile stamens.

The Apostasiaceae ,sometimes included amongst the orchids , comprise the nearest allies of the Orchidaceae .A small group of genera native to Malasia , the Apostasiaceae are now usually excluded from the orchids mainly because of their regular perianth. Furthermore ,the Orchidaceae possess more consistently than the Apostasiaceae , a true column , that is , a columnar structure resulting from the complete fusion of stamens and pistils in all their parts.

The Orchidaceae have been conventionally subdivided into two large subfamilies : the DIANDRAE and the MONANDRAE , the principal difference residing in the possession by the former of 2 functional anthers , by the latter of one .

The Orchid Family may justifiably lay claim to our deepest attention because of excessive variation in floral structures , giving rise to the most bizarre of forms . No other family of plants shows such an extraordinary range of floral variability .Studies have shown that evolution has, mainly through a process of supperssion and union of parts ,so changed the orchid flower from what its prototype must have been that the modern orchid often would not be recognized , on superficial examination , as representing the typical monocotledonous flower composed of 5 whorls each of 3 floral segments , 3 sepals , 3 petals , 6 (2 whorls) of stamens , and 3 separate styles. Change has been so drastic that Ames whimsically wrote in this connection :" What may be called the behavior or certain orchids may awaken a suspicion that their flowers are endowed with something analogous to intelligence . Certain floral structures and the study of their probable evolution intensify the belief that orchids possess qualities which set them apart from other plants.... it is delightrul to regard orchids as beings which have shaped their own destiny , and to speak of their floral structures, of the rostellum , for example , as organs purposely developed to serve special and sometimes fantastic functions.

" The gynandrous flowers are hermaphroditic ,sometimes polygamous .... In some species , they are polymorphic .They are always normally zygomorphic and epigynous and are borne singly or in a several -to many flowered spicate, racemose or paniculate inflorescence that arises terminally or laterally from the axil of a small leaf or bract . They vary from being minute and inconspicuous to large , dully hued , showy or even tawdry and brightly coloured.

" When an orchid flower is examined , it becomes clear ..... that is a complex structure made up of simple and compound organs ." The perianth consists of an puter whorl of 3 sepals and , alternatating with them , an inner whorl of 2 normal petals and a modified one known as the lip or labellum , usually larger than the normal petals . The pedicel of the flower bears the three-coluar and many-ovulate ovary which ,in most , but not all , species is resupinate . Resupination , a phenomenon sometimes of appreciable taxonomic value as a diagonostic character in the orchids , is the twisting of the ovary or pedicel about 180 after the opening of the flower to bring the lip , at first uppermost , to a position in which it is directed downwords . " The term resupination is important in our studies of the Orchidaceae because the vast majority of the species possess resupinate flowers . Glanced at casually , the position of the lip rarely awakens a question ; and yet here is a fascinating condition .The lip has not always been the lowermost segment of the corolla . In the early years of evolution , the lip was uppermost , and to-day it is uppermost when the buds are formed .... To was uppermost segment of the corolla. In the early years of evolution , the lip was uppermost , and to-day it is uppermost when the buds are formed ..... TO our eyes , there is something quite proper in the usual position of the labellum ..... yet in the bulk of the species the labellum owes its satisfying position .... (to) some vagary of nature , or shall we say ..... (to) a sympathetic physiological response to the behavior of those food-seeking insects which accomplish pollination ."

The 3 sepals may be equal in size and similar in shape and hue , or the dorsal sepal may be larger or smaller than the lateral and , rarely however , somewhat different in coloration .The laterals are usually or less oblique , whereas the dorsal is normally quite summetrical .The three may be freee or variously jointed or connate . The sepals , ulike those in most other families of monocotyledons , are usually be provided with prominent veins or be conspicuously ciliate along their margins .

The lateral or normal petals are similar and usually quite regular in shape . They often strongly resemble the lateral sepals and are commonly similarly colored and veined . When we consider the labellum , which as we have seen , represents the uppermost petal , although , through resupination , it is most often lowermost , we find an organ that exhibits an unbelievably complex series of variations .

In the primitive orchid , the lip must have been merely another normal petal, but in evolution it became " the crowning glory of the modern orchid." It is so highly modified that one might not, without careful examination , presume that it formed a part of the inner perianth whorl . It is the conspicuous segment of most of the orchids and differs from the other , usually smaller, perianth segments not only in shape and form but also usually in texture and coloration . It may be divided but more often it is lobed in sundry patterns , the usual one being a three-lobate one with the lateral loes often strongly divergent and sometimes in another plane from the midlobe. Its margins may be entire or variously notched , toothed , fringed or lacerate ,or, rarely ,even ciliate . It may be sessile , or drawn out basally into a long claw. Sometimes at its base there is the development of long or short , occasionally somewhat complex spur or calcar that may function to deeply or very complexly saccate or slipper-shaped , and may , in some genera, be differntiated into a most complicated series of parts , designated as epichile , mesochile and hypochile. The upper surface of the lip , called nated as epichile , mesochile and hypochile. The upper surface of the lip ,called the disk , may be , and most ingenious assortment of glands,calluses , papillae, beards , keels of plate-like lamellae . The lip is , in short , the petal that evolved to form an organ for the attraction of insects and to provide them with a surface on which they may conveniently alight from flight , and it is oriented in such a way that insect's entrance into the flower must disturb the pollinia which , brushed aganist and loosened , often stick to the insect's body to be carried off to another flower.

As complex and intricately ingenious as the labellum may be , the extreme evolutionary advance in the Orchidaceae is probably nowhere more clearly shown than in the true column , a structure which , among the monocotyledons, is peculiar to the orchids. As a consequence of the column or gymnostenium , the orchid flower has either free styles nor stamen -filaments .The column shows much variation in size and morphology , providing excellent characters for the separation of genera and sometimes even of species. It may be straight , arcuate or bent in more or less sigmoid fashion ; it may be variously winged laterally ; and it may or may not possess a foot- a more or less conspicuous elongation projecting from the base of the column.

At or near its apex or laterally , the column carries the polle-bearing anther or anthers which may be mobile or rigidly attached . On the ventral surface of the column are the stigma or stigmas . The two orchidaceous subfamilies , Diandrae and Monadrae ,differ primarily in the number of anthers and stigmas and in the presence or absence of rostellum and clinandrium . In the Diandrae , the column bears ;laterally two fertile stamens and centrally a petaloid staminode which serves to protect the column; it has also at the apex a large stigmatic surface consisting or three separate , functional stigmas . The Monandrae differ in having only one fertile stamen at or near the apex of the column , generally two functional stigmas either confluent or more or less separated and the rostellum .

The pollen grains are borne singly in the Diandrae ; in the Monadrae , they are united into tetrads and are compressed into 2,4, 6 ,or 8 definite masses or pollinia of varying form. These pollinia may be powdery , granular-farinaceous , waxy ,or cartilaginous -a character used in separating tribes in the family .

The number of pollinia oftenn utilized as a basic character for generic distinction . The only other family known to have the pollen grains formed into coherent masses or pollinia is the dicotyledonary ASLEPIADACEAE , a family which , like the Orchidaceae , has unusually complex relationships with insects in the mechanics of pollination .The pollinia of orchids are removed unbroken by insect visitors which , in coming into contact with the column, exert pressure on them.

" In the early period or orchid development ,many millions of years ago, there were six anther-bearing ,stamens in the flower , and the pollen was composed of loose ,separable grains. Then a reduction in the number of pollen-bearing stamens took place -where or why we do not know -and the pollen grains began to cohere together -first in fours (tetrads) , and later became compacted into definite masses of varying texture and ifferent form."

The rostellum period of orchid development ,many millions of years ago , there were six anther-bearing stamens in the flower , and the pollen was composed of loose ,separable grains. Then a reduction in the number of pollen bearing stamens took place -where or why we do not know -and the pollen grains began to cohere together -first in fours (tetrads), and later became compacted into definite masses of varying texture and different form."

The rostellum is essentially a modified third stigma , and , in one tribe (POLYCHONDRAE), remains fertile and still fulfills the functions of a stigma . The rostellum projects out over the stigmatic surface on the ventral surface of the column and serves to affix the pollinia to insect intruders . It is " a sterile female structure which has resigned itself to the altruistic function of bringing to its fertile sisters the fertilizing pollen of an anther." In other words : " It ceased to function as a fertile female to which pollen might be applied to ensure fertilization , and became the vehicle whereby the pollen, a male substance , could be transported to a fertile female" .

Present only in the monandrous orchids, it has probably done more to shape orchid evolution than any other floral adaptation .As we contemplate the basic significance of this remarkable mechanism , the crowning achievement of floral evolution in the Orchidaceae,we cannot but agree with Ames when he stated : " It is said that Hamlet would be a very dull play were Hamlet to be omitted . Surely the orchids would have put on a very commonplace show had the rostellum failed to appear on the biological stage."

There can be no doubt that the rostellum represents one of the most ingenious modifications for a special function of any organ in the Plant Kingdom . As Darwin has written: " If the homologies of Orchids had not been pretty well made out , those who believe in the separate creation of each organism might have advanced this as an excellent instance of a perfectly new organ having heen specially created and which could not have been developed by a successive modification of any pre-existing part" (as quated in Ames (3)).