Seeds

The orchid seed consists of an undifferentiated embryo enclosed in a transparent air- filled seed- coat , which has cells devoid of protoplasm in the mature state . A highly sculptured sclerotic and opaque seed -coat is seen in the primitive forms like Apostasieae , Selenipedium and Vanilla .

According to Dressler and Dodson , relationships between tribes in Orchidaceae are as hazy as the affinities within are clear. Extinction might have occurred at this level this obscuring the interconnecting webs . On the basis of the primitive and advanced characters as exhibited by the living species of orchids which are discussed in the previous pages , all orchidologists agree that Apostasieae are the most primitive of all existing orchids and genera like Neuwiedia provide a connecting link between Orchidaceae and the other monocotyledons with trimerous liliaceous pattern of flowers like Hypoxis and Curculigo. It is suggested that from this Neuwiedia -like ancestor , with fertile anthers present in both the whorls, Cypripedieae might have evolved by the suppression of the outer whorl and the rest of the orchids by the suppression of the inner whorl of anthers .

In evolutionary status , Cypripedieae come nearest to Apostasieae . Diandrous condition , substigmatic insertion of the anthers , free pollen , tricarpellary ovary with axile placentation etc. are characters which bind the two groups together as the most priomitive tribes of orchids . But in the gross morphological appearance ,the Cypripediae appear more related to the monandrous orchids than to Apostasieae . Therefore , it is generally assumed that Cypripediae are not direct descendants of Apostasieae .

These two groups , Apostasieae and Cypripedieae have been variously treated by various authors . Some did not consider them as orchids at all , for example, Pfitzer , Schletcher and Mansfeld . Others went so far as to include them in the family but kept them as a distinct tribe on a par with Orchideae , Epidendreae etc. Holttum and Dressler and Dodson separated Apostasieae and Cypripedieae into a separate sub-family on the basis of their pleuriandrous condition . Bentham and Hooker went to the other extreme and included the genera Apostasia and Neuwiedia along with Cypripedium under the same tribe suggest that the Apostasieae be treated as a distinct tribe nearest in status to Cypripedieae on the one hand and NEottieae on the other .

The assemblage of primitive characters as well as the worldwide and sparse distribution of the four genera of Cypripedieae , justify the belief that they might have existed at a time prior to the origin of the monodrous orchids. Many of the living representatives of this group today exist in secluded and almost inaccessible regions , which are themselves isolated from each other by hundreds of miles of land or sea or both . Breiger (1960) has pointed out that the great continental disjunction separated genera in Cypripedieae , whereas in the monandrous orchids, groups were separated. This clearly shows that geographically Cypripedieae are more ancient than the monandrous orchids.

Although morphological appearances warrant a closer reltationship between Cypripedieae and monandrous orchids than between the former and Apostasieae , a direst line of descent from Cypripedieae to Monandrae is a possibility ruled out by all orchidologists . The main reason is the radical difference in the structure of the flower between the two groups . As mentioned earlier ,two different staminal cirlces are functional in the two groups. The fertile stamen of the monandrous orchids is the odd one of the outer whorl which is generally accepted patterns of evolution that the staminode of Cypripedieae should turn fertile to give rise to the monandrous orchids . The difference in the size of the chromosomes also support this argument. When compared to the other tribes of Orchidaceae , a particularly interesting point regarding the chromosomes of the Cypripedieae , is their remarkable size . The chromosomes of Paphiopedilum spiceranum range in length from 2.5 to 10 and are disproportionately large when compared to such genera as Eulophia and Geodorum , where they are only 1- 1.5 . For these reasons Cypripedieae cannot be considered as the direct progenitors of the monandrous orchids. It is true that Selenipedium has resemblances with Apostasia on the one hand and Cephalanthera on the other . But the best we can deduce is that the Cypripedieae and the monandrous orchids evolved from some remote ancestor along two distinct and separate lines. As Darwin (1877) remarked " an enormous amount of extinction might have swept away a multitude of intermediate forms and left this single genus (Cypripedium ) as a record of the former and more simple state of the great Orchidean order " . The fact that many of the living species of this group are now on the verge of extinction prove the validity of this assumption . Both in vegetative and floral characters Cypripedieae are unadapted for survival . They do not have either underground tubers or pseudobulbs to tide over unfavourable conditions. Their floral structure is such that self fertilisation is difficult to take place , in case cross fertilisation fails. It is seldom that a plant of this group is seen in the wild bearing pods.

As to which group of monandrous orchids comes nearest to this so-called common ancestral stock, which is supposed to have also given rise to Cypripedieae at a much earlier geological age , there is a great deal of speculation . Both in Neottieae and Orchideae aer comprised of comparatively primitive orchids. Orchideae , like Cypripedieae , are recognised as a specialised offshoot from the ancestral stock . They are characterised by the possession of two sectile pollinia each of which has a caudicle and a viscidium and is enclosed within an antherbag . The viscidium here , however , is morphologically rostellum. Outside Orchideae , sectile pollinia are seen only in the subtribe Spiranthinae of Neottieae . This could point to a relationship , but there are differences in the basic structure of the anther between Orchideae and Spiranthinae . In Orchideae ,the anther is erect and persistent , whereas in Spiranthinae it is incumbent and opercular. Dressler and Dodson are of opinion that Orchideae would have been quite isolated if it is not for the tow genera Epipogium and Stereosandra . Here the pollinia are sectile and anther persistent as in the Orchideae . In Epipogium aphyllum , the anther is incumbent as in Epidendreae , but E. roseum and Sterosandra have erect anthers. The earlier authors like Hooker and Schletcher treated these two genera as part of the tribe Neottieae . But Dressler and Dodson have put them under Orchideae threating them as connecting links between Orchideae and the rest of the orchids. They further venture to assume that Orchideae might have originated from ancestors closely akin to Cephalanthera .

Taxonomically the tribe Neotieae had a rather chequered history . The earlier taxonomists considered the group as nearly well defined . Dorsifixed anthers separated them from Orchideae where the anther was basifixed and mealy or granular pollinia separated them from Epidendreae ,where the pollinia were waxy . But later trends of thought put considerably less value on single characters , preferring the sum total of all characters i deciding reationships . According to Mansfeld (1937) , the subtribe Arethusinae of Neottieae has mealy pollinia but their relationships are more with Bletiinae of Epidendreae than with other subtribes of Neottieae . Accodingly he transfeffed this subtribe to Epidendreae . Besides , further knowledge on the morphology of the various genera revealed that what were considered as key characters till then , were not reliable enough to be made the basis of tribal delineations. Mansfeld (1937) has recorded the occurrence of mealy pollinia in certain genera of Bletiinae of Epidendreae , while waxy pollinia are seen in Sobraliinae of Neottieae. Naturally he transferred Sobraliinae also to Epidendreae . Dressler and Dodson went further and transferred three more subtribes , Pogoniinae , Vanillinae and Gastrodiinae , to Epidendrae , arguing that the affinities of these three subtribes are more with Epiidendreae than with the rest of the Neottieae a more natural group . But ,while these suggestions helped to break down the boundaries between tribes which were once well defined and firm , it still left the rest of Neottieae a very heterogeneous collection of subtribes ,which are not mutually as closely knit as those of Orchideae or Epidendreae.

Of the 9 subtribes recognised under Neottieae, Limodorinae with Cephalanthera , Epipactis and Limodorum ,are the more generalised and consequently the most primitive of all monandrous orchids. The genus Cephalanthera comes nearest to the probable ancestral group and provides connecting links with Apostasieae , Cypripedieae and Orchideae . The chromosomes of Cephalanthera and Epipactis and those of the related Neottiinae are the largest of all monandrous orchids and remind one of the situation in Cypripedieae .

We do not know anything about the cytology of Chloraeinae , Rhizanthellinae , Prasophyllinae etc. Isolated in the continent of Australia ,the genera included in these subtribes have undergone considerable development by way of specialisation .

Spiranthinae is a very distinct group , but with doubtful affinities . Dressler and Dodson are of opinion that they could be raised to the status of a tribe because of their isolation . The lowest chromosome number reported so far from all Orchidaceae is , from this group (Zeuxine - n = 10 ) .

If Neottieae represents a particualr state in the several evolutionary lines which fed into the vast and diverse tribe Epidendreae, as is often postulated, then there is no validity for its existence as a tribe , because in a taxonomic system based on phylogenetic principles , grouping together of genera on the basis of a character which represents only particular phase through which the family has passed in its evolutionary history , is not warranted . A great deal of further work is necessary for understanding the relationships of Neottieae . Here there are further complications . This is a primitive tribe and as such , extinction might have created large voids , thus obscuring relationships further .

The tribe Epidendreae is the largest in Orchidaceae and includes the advanced forms with waxy pollinia . Hooker treated this group under two tribes Epidendreae and Vandeae , the division being based on absence and presence of appendages to the pollinial aparatus .Pfitzer , Schletcher , Mansfeld and Dressler and Dodson considered them as one section , but the first three authors proceeded to divide it into further subsections . Pfitzer and Schletcher divided it into three sub-sections . Acranthae , Sympodiales and Monopodiales ; Mansfed followed Hooker and divided the section into Epidendreae and Vandeae , but Dressler and Dodson postponed such subdivisions to a future date, expecting that as more information accumulated they will provide clues to several of the missing links . They treat the genera included under this tribe directly under 27 sub-tribes .

Epidendreae is a vast and diverse group and is considered to be polyphyletic in origin . But the chromosome numbers in the tribe show a remarkable degree of uniformity . Out of the nearly 800 species investigated so far, about 53 % have 19 and nearly 40 % have 20 as their haploid number . But this uniformity evident in the number of chromosomes , does not extend to the morphology or size of the chromosomes . For example ,the genus Calanthe and Epidendrum have 20 aa the haploid number . But in Calanthe , the chromosomes are as long as 5 in length , while in Epidendrum , they range in length between 0.8 - 1.5 . It is to be assumed that these two genera ,though possessing the same number of chromosomes , neverthless , are representatives of two distinct lines of development within Epidendreae . On the other hand , other 20- chromosomed group like Coelogyninae and Cymbidiinae do possess chromosomes which resemble those of Calanthe in all morphological aspects . Here we can indeed speak of an affinity ,which is further emphasized by the same geographical distribution .

The differentiation into species of the Eastern and Western world is far more pronounced in the epiphytic rather than in the terrestrial species . The tropical Asiatic representatives of Epidendreae constitute a section very distinct in development from their occidental relatives . The two great Eastern tropical groups are the Dendrobiinae and the Sarcanthinae . As mentioned earlier, there are remarkable similarities in number , size and morphology of chromosomes between these two groups . In both , the chromosomes measure between 1 and 3 in length and 19 is the prevalent haploid number through 20 is also reported in a few species of Dendrobium and in a lesser of species in Sarcanthinae . Sarcanthinae are considered to be the most advanced group orchids of the Eastern world ,because of their monopodial growth pattern and possess naked pollinia without appendages . This type of pollinia are supposed to be primitive and accordingly Dendrobiinae have been allotted a low position among sympodial forms. But Holttum (1964) has suggested that this naked condition of the pollinia could be achieved through loss of appendages which at one time the group might have possessed in its evolutionary history . On the other hand Dressler and Dodson have suggested that limitations in the range of floral variations seen in Dendrobium are the result of their naked pollinia , which has seriously hampered the progress of evolution in this subtribe . Sarcanthinae have escaped this sort of stagnation , because of the complexity of their pollinial apparatus ,which enabled them to have wide range of floral variation . This has occured also in subtribes like Genyorchidnae which are closely related to Dendrobiinae . It is alos probable that both Dendrobiinae and Sarcanthinae might have achieved the 19-chromosomed condition independently . It would be interesting to assess the cross-compatibility between these two major groups.

According to Holttum , sympodial forms that come nearest to the monopodial types are species like Dipodium and Grammatophyllum . This does not mean that the monopodial species have evolved from these two species . On the othere hand, this change must have occurred in more than one line of descent . Dipodium and Grammatophyllum might represent relics of such forms which monopodial forms might have originated . These two genera have elongate stems , lateral inflorescences and pollinia with stipe as in monopodial species. Dipodium pictum is even monopodial in growth .

Discussing the various evolutionary trends within Sarcanthinae group of orchids, Holttum suggests that types like strap-leaved Vanda are the most generalised in this subtribe and thus represent the central primitive figure , from which other types seen in the group might have evolved . Vandas are large plants with coriaceous leaves at moderately spaced nodes , large flowers which are long -lasting with no complicated appendages on the lip and with two progress in evolution which is aparent in such genera as Saccolabium and culminates in such specialised forms as Taeniophyllum , where the leaves are reduced to scales and epiphytic roots with chloroplasts take up the function of leaves. With the reduction in size flowers are short-lived , and the lip gets more complicated with warts , and other appendages . Commensurate with this there is advancement from the two notched pollinia of Vanda to four pollinia in two pairs as is seen in Arachnis and Renanthera , from there to four equal pollinia as in Taeniophyllum and Microsaccus and finally to two undivided pollinia as in Taeniophyllum and Microsaccus and finally to two undivided pollinia as is seen in Saccolabium , Chroniodium , Microtachorchis , Hymenorchis etc. The state of two undivided pollinia may also be achieved by the elimination of the notch from the Vanda type .

The tropical American representatives of Epidendreae form two main lines of development , one represented by the comparatively primitive Epidendrinae and the other constituted by the relatively advanced Maxillariinae ad Oncidiinae . In Epidendrinae we see the rudiments of the stipe which reach its full development in the Oncidiinae , which is comparable to the condition seen in the Asiatic Sarcanthinae . But in the American Epidendreae , there is no stabilisation of chromosome numbers which characterises the Asiatic forms. The Epidendrinae have a stabilised number 20 or its multiples but in the advanced Maxillariinae and Oncidiinae , the number ranges from 26 to 112 . The American orchid flora as we see it today is to be considered as the result of independent lines of evolution distinct from their oriental relatives . Breiger (1960 ) has pointed out that the northern limits of Asiatic tropics were at a much more northern latitude in the early Tertiary . This faciliated free migration of the flora between North East Asia and North America . In mid-Tertiary , however, the northern limit moved down considerably thus separating the American and Asiatic tropics. It appears that the 20-chromosomed condition has been attained in the Asiatic Coelogyniae ,Cymbidiinae etc. and the American Epidendriinae independnetly.