ORCHIDS

Orchids are well known and very popular plants in the world. The main reason for their popularity is most probably in a very large number of artificial hybrids, which are more numerous than the described species in nature.

Orchids are divided in terrestrial and epiphytic, although there are also species that are rooted although they climb on trees or rocks. The Orchidaceae family is cosmopolitan. The exact number of species is not known and it is only estimated that there are between 20,000 and 25,000 of them. The largest number is concentrated in the tropics; more than 8,000 species from 306 genera can be found in tropical regions of America, 6,800 species from 250 genera in the South-East Asia and more than 3,000 species from 134 genera in tropical parts of Africa. The most interesting fact related to distribution of orchids is that different continents have specific orchid floras. The reason for that is that the Orchidaceae family is relatively young and that a large part of its evolution has been happening after the separation of continents. This refers in particular to epiphytes, which are younger than terrestrial orchids in evolution sense. Considering the number of species, distribution and diverse and unmatched adaptations to entomophilia, orchids represent the final branch in the plant evolution.

Among the genera known for the largest number of species we can single out Dendrobium (1,400) and Bulbophyllum (more than 1,000 species). New Guinea is the region with the largest number of species (more than 1,450 species) and it is followed by Columbia with 1,300 species and Malaga with about 800 species. About 4,000 species belong to terrestrial genera. However, there are also genera that are primarily epiphytic although they also have some terrestrial plant. It is assumed that about 1/4 of the total number of orchid species is terrestrial and that about 5% of them can grow attached both to the ground and to trees.

The size of orchids varies greatly. The smallest orchid in the world is probably the Central American specie Platystele jungermannioides the stems of which are about 1 cm high, while there are several candidates for the largest one as there are many epiphytes that can be highly massive while some orchids with monopodial growth, such as Vanilla for example, can reach several tens of meters in length. The longest flowering stem of 5 m was recorded by one orchid belonging to Oncidium genera.

The orchid flora in a temperate zone, where exclusively terrestrial species can be found, is much poorer in relation to the tropics. In a temperate zone of the north hemisphere there are about 900 species from 75 genera, while there are even less of them in the south temperate zone - 40 genera and about 500 species. In America, to the north of Mexico, there are 170 species and somewhat more than 200 species from 36 genera can be found in the whole Europe.

Having in mind such a large number of species and wide distribution, the orchids occur from flatlands to the highest steep mountains in almost all types of habitats, from marshes to dry steppes and desert oases. In Columbia several orchids live in regions covered with eternal snow and in Australia some of them live even under the ground. The highest diversity of species can be found in humid, mountainous forests, in particular in forests belonging to the cloud and fog zone. The largest number of orchid species grows up to the altitude of 2,000 m while certain number of species can also be found above the upper forest border, even up to 5,000 m above the sea level. Only a few genera have their representatives both in temperate zones and in the tropics (Herminium, Liparis, Malaxis etc.).   

Just like other monocots, the orchids never have any primary roots but the whole root consists of secondary roots. Many terrestrial orchids that have adapted to the climate with strong seasonal contrasts, the root forms storage organs (corms or tubers), namely the organs used in dormant stage with the assistance of which the plants survive during unfavourable periods. The Orchis genera (and the whole family after it) got the name after the round shaped pair tubers resembling to male testicles (orchis means "testicle" in the Old Greek). Terrestrial orchids without corms use rhizome as the organ that stores food and water reserves. The orchid stem is basically the same as the stem of other monocots. Each stem node generates one leaf. Orchid leaves are undivided, of different size and they can be distributed either spirally or in two rows. Orchid flower perianth is composed of six leaves distributed in two circles (whorls) with three leaves each. Outer (external) leaves are usually called "sepals" and internal are called "petals". The upper medial petal leaf can mainly be clearly distinguished from the other two and it is called a "labellum". It is usually bigger and more complex than other petal leaves and it is one of the key elements making the orchid flowers so recognisable. Labellum base is usually elongated into a shorter or longer spur. Labellum can be in one piece or divided in lobes, with a unified colour or with different patterns that often point to the false way to the nectar. Flower colour, together with smell and shape, plays a role in visual attracting of pollinators. Daily butterflies, contrary to bees, can recognise red colour and that is why the flowers that they pollinate are often red, pink or yellow. On the other hand, insects that are active by night can only make the distinction between light and darkness, which is the reason because of which the flower that they pollinate are usually white or pale green and they have the tendency to radiate strong smell by night. One of the main characteristics that occurs in most representatives of   Orchidaceae family is flower resupination. When the flower is in a bud stage, labellum is located on the upper side. However, during further development the whole flower is rotated 180° by means of ovary or pedicel folding so that labellum is placed in the position suitable to form a landing platform for pollinators. The filaments of the stamens are always adnate (fused) to the style to form cylindrical structure called gynostemium, which is a typical determining structure of orchids. Most orchids have one stamen the anther of which is located on the top of gynostemium and pollen grain grouped primarily into tetrads and then in larger masses - pollinias.  The stigma is composed of three lobes with the central lobe being much bigger than lateral ones. A part of the central lobe, in most orchids, except for the primitive ones, is changed into rostelum. Pollinias are always taken from the flower as whole, which reduces the pollen loss. Orchid seeds are small and of relatively simple structure. Due to microscopically small seeds in comparison to most other flowering plant the botanists of the 16^th century were of the opinion that orchids in general do not have any seeds but that they hold dust in pupas. They also believed that orchids are generated from animal sperm.

Mycorrhiza (a symbiosis between fungi and plant roots) of orchids belongs to a special type (endo-mycorrhiza) and contrary to others, it is necessary for germination of seeds whereas most adult orchids depends on that connection up to a much smaller extent. Mycorrhiza has developed very early in the evolution, considering that it has not been lost in any of the recent species, either terrestrial or epiphytic.

Wise and sometimes highly complex pollination mechanisms, in particular with epiphytes, go far beyond even the wildest imagination. Expect for a small number of autogammic species all other orchids are pollinated exclusively by animals. Insects are pollen carriers in most cases, except for a small number of tropical species where birds play the role of pollinators. Just as other Angiosperms, many orchids provide nectar as the reward fro their pollinators. However, certain number of species functions by means of deception, i.e. as defrauders that do not give any rewards. Orchids are most probably the best known defrauders among the Angiosperms. The flower of about 1/3 of orchid species (about 10,000 species) cheat their pollinators. The ways in which orchids deceive their pollinators can be classified into several types: territorial defence (some epiphytic genera), selection of egg laying sites (sapromiophylia), sexual reaction (pseudo-copulation) and most often the quest of pollinators for food. The most interesting is pseudo-copulation that occurs in species of Ophrys genera and some Australian and South American genera. This is a unique phenomenon in the floral world. It is based on the principle of sexual deception where flowers imitate sexual signals of Hymenoptera females (shape of labellum and flower odour) that stimulate males to attempt copulation with them.

ORHIDEJE

 

Orhideje su veoma poznate i popularne biljke u svetu. Njihovoj popularnosti verovatno je najviše doprineo veliki broj veštačkih hibrida, kojih ima više nego opisanih vrsta u prirodi.        

Orhideje se dele na terestrične i epifitske, mada postoje i vrste koje su ukorenjene ali se ipak penju na drveće ili stene. Familija orhideja je kosmopolitska. Tačan broj vrsta nije poznat, procenjuje se da ih ima između 20000 i 25000. Najveći broj je skoncentrisan u tropskim oblastima; u tropskom delu Amerike sreće se preko 8000 vrsta iz 306 rodova,  u jugoistočnoj Aziji  6800 vrsta iz 250 rodova, a u tropskom delu Afrike preko 3000 vrsta iz 134 roda. Najinteresantnija činjenica u vezi sa rasprostranjenošću orhideja u tome je što različiti kontinenti imaju specifične flore orhideja. Uzrok tome je familija Orchidaceae relativno mlada, i što se veliki deo njene evolucije dešavao nakon odvajanja kontinenata. Ovo se posebno odnosi na epifite, koje su evolutivno mlađe od terestričnih orhideja. S obzirom na broj vrsta, rasprostranjenost i raznovrsne i neprevaziđene adaptacije na entomofiliju, orhideje predstavljaju terminalnu granu u evoluciji biljaka.

Od rodova sa najvećim brojem vrsta mogu se izdvojiti Dendrobium (1400) i Bulbophyllum (više od 1000 vrsta). Region sa najvećim brojem vrsta je Nova Gvineja, sa više od 1450 vrsta, zatim sledi Kolumbija sa 1300 vrsta i Malaja sa oko 800 vrsta. Isključivo terestričnim rodovima pripada oko 4000 vrsta, međutim postoje i rodovi koji su prvenstveno epifitski ali imaju i terestrične predstavnike. Pretpostavlja se da je otprilike 1/4 od ukupnog broja vrsta orhideja terestrična, uz oko 5% onih koje mogu rasti i na zemlji i na drveću.

Veličina orhideja veoma je različita. Najmanja orhideja na svetu verovatno je centralnoamerička vrsta Platystele jungermannioides čije su stabljike visine oko 1cm, dok za najveću postoji više kandidata, s obzirom da mnoge epifite mogu biti veoma masivne, a orhideje povijuše, kao što je na primer Vanilla,  mogu izrasti na desetine metara u dužinu. Najduža cvast od 5m zabeležena je u rodu Oncidium.

Flora orhideja u umerenom pojasu gde rastu isključivo terestrične vrste daleko je siromašnija u odnosu na trope. U umerenom pojasu severne hemisfere raste oko 900 vrsta iz 75 rodova, dok ih u južnom umerenom pojasu ima još manje - 40 rodova i oko 500 vrsta. U Americi, severno od Meksika, raste 170 vrsta, a u celoj Evropi nešto više od 200 vrsta iz 36 rodova.

S obzirom na tako veliki broj vrsta i široko rasprostranjenje, orhideje se javljaju od ravnica do strmih najviših planina na skoro svim tipovima staništa, od močvara do suvih stepa i pustinjskih oaza. U Kolumbiji nekoliko orhideja živi u područjima sa večitim snegom, a u Australiji čak i pod zemljom. Najveća raznovrsnost vrsta sreće se u vlažnim, planinskim šumama, naročito u šumama pojasa oblaka i magle. Najveći broj vrsta orhideja raste do nadmorske visine od   2000 m, dok se određeni broj vrsta može naći i iznad gornje šumske granice, čak i do 5000m iznad nivoa mora. Svega nekoliko rodova ima svoje predstavnike i u umerenom i u tropskom pojasu (Herminium, Liparis, Malaxis i dr.).

Kao i druge monokotile, orhideje nikada nemaju primarni koren, već se ceo korenov sistem sastoji iz sekundarnih korenova. Kod mnogih terestričnih orhideja adaptiranih na klimu sa jakim sezonskim kontrastima koren formira organe za magacioniranje (gomolji ili tuberoidi), odnosno organe za mirovanje, pomoću kojih biljke opstaju tokom nepovoljnog perioda. Rod Orchis (a po njemu i cela familija) dobio je ime po okruglastim parnim tuberoidima koji podsećaju na testise sisara (orchis na starogrčkom znači testis). Terestričnim orhidejama bez gomolja rizom služi kao organ za magacioniranje rezervi hrane i vode. Stabljika orhideja u osnovi je ista kao stabljika drugih monokotila. Iz svakog nodusa stabljike polazi po jedan list. Listovi orhideja nedeljeni su, različite veličine, i mogu biti raspoređeni spiralno ili u dva reda.. Perijant cvetova orhideja sastoji se iz šest listića raspoređenih u dva kruga po tri listića. Spoljašnji listići obično se nazivaju „sepalni” a unutrašnji „petalni”. Srednji petalni listić uglavnom se jasno razlikuje od ostala dva, i naziva se usna ili labelum. On je obično veći i kompleksniji od ostalih petalnih listića, i jedan je od glavnih elemenata koji čine cvetove orhideja tako prepoznatljivim. Osnova labeluma često se produžava u kraću ili dužu ostrugu. Labelum može biti ceo ili režnjevit, jednobojan ili sa različitim šarama koje često predstavljaju put ka lažnom nektaru. Boja cvetova, zajedno sa mirisom i oblikom, ima ulogu u vizuelnom privlačenju oprašivača. Dnevni leptiri za razliku od pčela, razlikuju crvenu boju, pa su zbog toga cvetovi koji se preko njih oprašuju često crveni, pink ili žuti. S druge strane, insekti aktivni noću mogu razlikovati samo svetlo od tame, i zbog toga su cvetovi koji se preko njih oprašuju uglavnom beli ili bledo zeleni, i imaju tendenciju da jako mirišu noću. Jedna od osnovnih karakteristika koja se javlja kod većine predstavnika familije Orchidaceae  je resupinacija cvetova. Kada je cvet u pupoljku, labelum se nalazi na gornjoj strani, međutim, u toku daljeg razvoja ceo cvet se rotira savijanjem plodnika za 180°, tako da se labelum postavlja u položaj pogodan za sletanje insekata. Srastanjem vrata tučka i filamenata prašnika nastaje ginostemijum, karakteristična determinaciona struktura orhideja. Većina orhideja ima jedan prašnik čija se antera nalazi na vrhu ginostemijuma i polenova zrna grupisana prvo u tetrade a zatim u veće mase - polinije. Žig je trorežnjevit, a srednji režanj je mnogo veći od bočnih. Deo srednjeg režnja kod većine orhideja je, izuzev primitivnih, izmenjen u rostelum. Polinije se iz cveta uvek odnose cele čime je smanjen gubitak polena. Semena orhideja sitna su i relativno jednostavne strukture. Zbog mikroskopski sitnih semena u odnosu na većinu drugih cvetnica, botaničari XVI veka smatrali su da orhideje uopšte nemaju semena već da se u čaurama nalazi prašina, kao i to da orhideje nastaju iz sperme životinja.

Mikoriza (simbioza između gljiva i korenova biljaka) orhideja pripada posebnom tipu (endomikoriza), i za razliku od ostalih, neophodna je za germinaciju semena, dok, naprotiv, većina odraslih orhideja u manjoj meri zavisi od ove veze. Mikoriza se u evoluciji razvila veoma rano, s obzirom da se nije izgubila ni kod jedne recentne vrste, bilo terestrične ili epifitske.

Domišljati i ponekad jako složeni mehanizmi oprašivanja, posebno kod epifita, prevazilaze maštu. Izuzev malog broja autogamnih vrsta, sve ostale orhideje se oprašuju isključivo preko životinja. Prenosioci polena u većini slučajeva su insekti, osim kod malog broja tropskih vrsta, gde ptice igraju ulogu oprašivača. Kao i druge skrivenosemenice, mnoge orhideje za svoje oprašivače obezbeđuju nagradu u vidu nektara. Međutim, određeni broj vrsta funkcioniše putem obmane tj. kao prevaranti koji ne obezbeđuju nagradu. Orhideje su verovatno najpoznatiji prevaranti među cvetnicama. Cvetovi oko 1/3 vrsta orhideja (oko 10000 vrsta), varaju svoje oprašivače. Načini na koji orhideje obmanjuju svoje oprašivače mogu se svrstati u nekoliko tipova: teritorijalna odbrana (neki epifitski rodovi), izbor mesta za polaganje jaja (sapromiofilija), seksualna reakcija (pseudokopulacija), i najčešći potraga oprašivača za hranom. Najzanimljivija je pseudokopulacija koja se javlja kod vrsta iz roda Ophrys i nekih australijskih i južnoameričkih rodova. To je fenomen jedinstven u biljnom svetu. Bazira se na principu seksualne prevare, gde cvetovi imitiraju seksualne signale ženki Hymenoptera (oblik labeluma i miris cveta), koji stimulišu mužjake da pokušaju kopulaciju sa njima.