INTRODUCTION

Although reproductive axes in palms have attracted much attention, amongst others because of the commercial value of many members of the group, investigations concerned with their

basic structure are scarce. Therefore, even the general features of flowering in palms is a subject rarely considered i n botany textbooKs. In 1824, Van Mohl gave descriprions of the reproductive axes of several palms without much detail on their structure. Micheels (1892) described frrrit bearing axes in 32 species belonging to 19 genera of the palmae. His description, however, was liurited to the usefulness of mechanical stresses in such axes.

A revival of interest in the subject was shown late1y by Tomlinson and Moore (1968) who analysed the reproductive axis in Nannorrhops ritchiana and described the branching pattern up till the individual flowers which rnay be used for at least the majority of pa1ns. Morphology and anatomy of palm flowers was given by Uhl for At isteAera (1966), Nannorrhops ritahiana (1968) and Rhapis excelsq. (Uhl et al. r97l). Il became evident that many more investigations should be done to add to the survey of floral urorphology in palrns which is necessary as a basic knowledge on which function of the organs could be understood.

It was wlth that purpose in rnind that the present investlgation was carri,ed out. The material selected was Arenga potphyroearpa (Bl ,) H.E. doore (Backer and Bakhuizen v.d. Brink, 1968), which appears to have a similar vegetative structure as Arenga pinnata of which familiarity with its vegetative structure was acquired earlier (Ilidajat and Utomo, 1975).

MATERIALS AND METHODS

Several reproductive axes growing in the Bandung area have been cut dorun and examined in detail. Illustrations are made largely fron these shoots. Male and female inflorescences rdere f ixed ln FAA. Followlng Uh1 (1966) , flowers aE several stages of development were softened by treatment for 3 weeks to one month ln one third connerciaL (522) hydrofluoric acld. Serial sectlons of flowers and parts of the inflorescences were prepared by the parafin method and cut at 8-10 mlcrons, and then stained wlth safranln and fast green. Some flowers and portions of axes were cleared by treatnent overnlght in 5Z sodir:m hydroxlde ln an oven at 58 degrees centigrade, followed by 2-4 hours j-n one third strength connercial sodtrd hypochlorite and then approximateLy L2 hours i,n a saturated soLution of chloralhvdrate.

OBSERVATIONS

Arenga po!,phA!,oearpa is an unarmed, erect dwarf palm growlng in clunps up ti1l 3 neters hlgh. The short internodes at a vegetative shoot tip results ln a rosette appearance. Each iuparlpinnate leaf which may reach Lr" - 2 meter ln lenght ccinsists of a blade, petlole, sheath and a prominent llgule.

1. The reproductiue aris cnd inflorescenees

a. Morphology

The onset of the reproductlve phase involves a transformation Of a vegetatlve shoot into one wlth many Lnflorescences. The axls lnitlally produces a long internode but then shows a reduction in lnternode length. A narrowlng of the axls occurs accompanied by modlflcatlon of leaf shape and slze. The splral phyllotaxls of the vegetatlve shoot Ls belng contlnued lnto the inflorescence axis. Nornal follage leaves as descrlbed above undergo progressive reduction along the repro[uctlve axls. Blade and petiole are abruptly dlninlshed at the flrst leaf next to the last norual follage leaf. The flrst three successl-ve leaves are represented by a sheath and a ligule, and thereafter only a vestlgial sheath remalns which does not appear as a closed tube but as a thin open flap lnstead (Fle. 1).

At a reproductive axis possesslng 6 nodes, 3-5 axlllary buds nay be present at each node, partly or wholl-y enclosed by the leaf remrant. After the flrst lnflorescence, whlch is a fermle one has developed frorn the ternlnal node, male lnflorescences as well as additlonal female inflorescences may develop from the axlllary buds at the lower riodes. Male inflorescences My then also develop from the remainlng buds at the termlnal node, showing that the fltst female lnflorescence ls deilved from the dlstal shoot tip. An lnflorescence conststs of several axes each with florers or flower clusters attached to it (Fig. 2). Those flowerlng axes are termed rachl11ae.

In the young inflorescence, rachLllae seem to depart from a connon slte at the dlstal end of a peduncle, the rachillae being of the same length throughout. At the base of the com-Don pedrmcle 1s formd a hairy bract. At a later stage lntercalary grorth seems to occur as each rachllla appear to branch monopodially fron a common axls, whlch ls l-ndlcated by the bract scar at the base of each branch except the ternlnal one. The latter ls forned by transfornation of the nost distal part of the inflorescence axis and has no bract. Monopodlal branching nay be repeated up t111 5 tlmes resultlng l-n a rachillae cluster whlch represents an lnflorescence. No obvious reduction l-n dianeter of srrccesslve branches of the axis

Flgs. 1-2. Fig. 1, Diagram of the reproductlve axls wlth one fenale inflorescence. Fig. 2, Diagram of a 'female l-nflorescence consisting of 5 rachillae while nonopodial branching is shown. Bracts at the four lower rachillae not drawn. DE-TAILS: A, B, C, D, E, F, nodesl Pr Petiole of the last foliage leaf; 1, ligu1e; sh, sheath.

is observed. There is a tendency for lower and terninal rachlllae to be shorter than the others.

b. Anatorny

The following description is taken from a nale inflorescence. outline about circular in transverse section, although at several parts or areas an elliptlcal form is acquired as result of a flattening agains! the main stem. Epidernis consl-sting of snall cells elongated parallel to the main axis with a thin cutlcle and wlthout sinuous marglns. Eairs conslsting of a nu1tice11ular stalk and a head oade of many ou1 ticelled fllanents form a dense covering over the epidermis, coloured white in the beginning but becomes a brown tomentum later on. Stonates with bean shaped guard cells are irregularly dlstributed. Corte.s consisting of 2-3 cell layers composed of thiclq.ralled cells whlch are larger than the epidermal ones. At the inner side of the narror/ cortex a layer of fibrous bwrdles is found. Some of the bundles sholr vascular elements. Many sline cells each containing a bundle of raphlde crystals occur close to the margin. Sline ce1ls also occur at other places but not as dense. Central eylinder marked by a large amount of vascular bundles especially at the outer edge. Each vascular bundle composed of phloen and xylen l-n collateral arrangement and a sclerenchynatous sheath. Phloen apparently consisting of sleve tubes and conpanion cells on1y. No isthmus is present. Xylen composed of two, rarely three large tracheas and several srna11 diametered rylem cells. No protorylem is found at the inner vascular bundles. Sclerenchym sheath generally thicker at the phloen side whlle the thicker sheaths occur at the outer vascular bundles. Some vascular bundles link together. Orientation of vascular bundles not uniform.

The anatomy of a fenale inflorescence is essentially sinilar to that of the male inflorescence axis. Often a larger axis dianeter is found, while with increased vigor sclerenchym sheaths tend to become thicker.

2. Raehillae

a. Morphology

Male rachilla can be distinguished fron feoale rachillae by their more numerous number of slender flowers usually arranged in pairs. In the female inflorescence the number of rachillae tend to be lower with a larger diameter and roundish solitary flowers. In male as well as fe'uele inflorescences 6-7 bracts ensheath the group of rachillae within. These bracts show sinilar areas of disintegration at their distal

margin. At a female inflorescence up till 7 branches may develop while the lowest branch may branch again forming a second order branch. Flowers are arranged in a spiral. Close to the distal end flowers are often not well developed while the very distal ones usually abort. At a male inflorescence branching takes place in a similar pattern. The male rachilla often exceeds that of the female in length. Abortive flowers occur throughout the pistillate rachillae between the normal flowers. In the staminate rachillae superficially there are less abortive flowers, but there are always some abortive ones at the distal tip. Along a rachilla, flowers mature acropetally.

b. Anatomy

Anatomically, all rachillae are similar. Epidermal cells are small and slightly papillose. A thin cuticle is present. Three to four layers of sclerenchyma cells are found beneath the epidermis. Many slime cells containing raphide crystals are found in the cortical part and a few more in the central part. Tannin occurs mainly in the central region.

The vascular complement consists of larger bundles in the centre and smaller ones at the periphery. The total number of vascular bundles is progressively reduced distally (see table I). In pistillate rachillae rounded or pointed projections of the axis extend beyond the flowers at the distal end. Some vascular bundles are found in this reduced tip.

Examination of longitudinal sections through parts young rachillae reveals a triad system for the flowers which is still present in early ontogeny (Fig. 13, 14). Flowers are subtended by small bracts or bracteoles which number 3 flower cluster mentioned above. In staminate rachillae central flower in a triad system does not develop further and In other cases it does not develop at all and therefore does not leave a mark behind. In pistillate rachillae the solitary flowers always show two dark pits, one It can therefore be assumed that the two sites of necrotic tissues may represent aborted flowers. Bracteoles subtending flowers are directed perpendicularly to the main axis in early ontogeny but owing to increase in size of the flower, bracteoles later assume a sidewards direction. is most obvious in a male flower pair.

3. Flowers

1. Pistillate flower (Fig. 7-12)

Each flower is subtended by three bracteoles of which one is of a much smaller size and situated inside one of the lar-

Table I. Flowers and bundles per cm of Length in Rachillae.

Figs. 3-14. Fig. 3, part of a male rachilla; Fig. 4. Several flowers at the male rachilla, enlarged. Fig. 5, longitudinal section through a male flower. Fig. 6, one stamen. Fig. 7, part of a female rachilla, also showing two aborted flowers. Fig. 8, female flower, side view. Fig. 9, female flower, inner view of calyx. Fig. 10, female flower, frontal view. Fig. 11, transverse section through gynoecium showing two ovules. Fig. 12, female flower, longitudinal section. Fig. 13, young flower cluster composed of three individual flowers. Fig. 14, hypothetical manner of sympodial branching in the cluster of figure 13.

ger bracteoles (Fig. 13) have 5-15 traces. According to size, a bracteole may

a. Morphology

In the flower, the 3-3 floral plan is obvious both morphologically and anatomically. The three sepaLs are connate at the base forrning a shallow parenchymatous cup 1 nun ta11. After a short imbricate region the three dentate lobes are free. The total height of a sepal is about 2 unn. Petals are connate about half their length and then separate into three lobes after a very brief imbricate region. Before anthesis, edges of the petal lobes still join each other forming a tightly closed cavity in whj-ch the upper region of the gynoecium is situated. Length of a petal is about 5 mn. During anthesis, increase of gynoecial length night also be a factor in the opening up of the petal lobes. Before separation three dark lines mark the regions where petal edges join. Adnate to each petal base in a position close to its lateral margin, a staminodiurn is found composed of a thin membranous filament and a tanniri fi11ed slightly widened distal region. The g1obose gyttoeci.um tapers distally into a two or three pointed tip which composes the stigmatic area.

b. Anatonry

Although the flower appears sessile, a short stalk containing vascular bundles can be recognized anatomically. In both pistillate and staminate flowers an abscission zone is found in which parenchymatous cells are smaller and where the fibrops bundle sheath ls absent.

In the sepaL, small epidermal cells are found having a thin cuticula and are slightly papillate. At the adaxial side a row of sclereids is present, while large slirne cells containing raphide bunciles are found in a scattered distribution. The vascular complement consists of approxinately 12 vascular bundles which mav branch dichotomously. At the young flower, peLal lobe margins which sti11 join each otner are marked by tu/o cell layers which show an increasing degree of papillation distalwards and where interlocking of the ce1ls of the two lavers occurs at the cortrnon interface. At the distal region the adaxi.rl epidermis is papi-llose also, and cells interlock with the papillose abaxial surface of the gynoecium. r\ row of sclereids is present underneath the abaxial epidermis rvith a irigrrt'r concenLration at t.he distal region. Slime cel1s are disrrilrri.,ed throughout the pefa1. There are approximately 33 vaiiiniirr bundles mostly consisting of fibers. Darker co1- ,rured ':..rraF-e petal margins consist of sclerified cells wtrile

some necrosis also occurs. At the base of the separation region, petals may show a brief imbrication. The stanii.nodiun is composed of a parenchymatous filament and a slightely wider distal tip where tannins occur in abaxial cells and anticlinally elongated cel1s are present at the adaxial region. A stamlnodium may reach 21 rm in lenght. The presence of the staminodium leaves a shal,low region at the gynoecium where it nas appressed during growth.

The globose gAnoeeium ls 4 unn in diameter and 3 3/4 mm in helght approaching anthesis. The epidermis consists of anticlinally elongated ce1ls with thickened walls. In the centre, parenchymatous tissue may be present in an immature stage. The outer regions of the gynoecium show 3-4 layers of sclereids which are separated from the epidermis by 2-3 layers of parenchynatous cells. This sclereid layer starts a litt1e above the gynoecial base and ends somewhat below the stigmatic region. Raphide cells are found in smal1 groups or solitary throughout but less dense at the gynoecial base. Occasionally a short canal opening to the gynoecial surface is found.

Although the gynoeciurn superficially shows a tri radiate organization, anatomically only two carpels can be recognized which is indicated by the ventral suture along a plane through its ventral region. At the very base, the abaxial epidermis shows to be continuous with ce1ls of the ventral suture. Higher up, partial fusion occurs. The structure of the ventral suture which constitutes the stylar canal is therefore variable. At different heights, only the two outer ends form canals while the region in between these may consist of thro tightly interlocking papillose surfaces or may be entirely closed (Fig. 15, L6, I7). The canals are lined with glandular cel1s (p.ig. 18). About two third of the gynoecial height, two stylar clefts are found perpendicular to the ventral suture and opening into this septal canal. Apparently these clefts are contlnuations of locular canals which are narrow cavities occuring to the left and right of the stylar canal.

About 100 micron of the distal region, the gynoecial tip is divlded lnto three regions each one papillose to a high degree resultlng in two or three celled filaments and interlocklng at the common interfaces. After pollination these stignatic cel1s tend to dry out and become brownish black col-oured. Tannin is fotnd about the stigrnatic region and also surroundlng the locular canals. The vascular bundles are concentrated in a central plane para1le1 to the ventral suture. About 12 vascular bundles are found at each half and about 4 more bundles at its respective opposlte side. Each loculus contalns a hemianatropous ovule. The vascular supply to an ovule consists of about 23 vascular bundles which originate from the funiculus. The two ovules often srow into a dissimilar size.

Iigs. 15-18. Carnera lucida drawings of the female flower. Fig. 15, Ior:gitudinal section. Fig. 16, transverse section. Fig. 17, part of the stylar canal enlarged to show interlocking cf the epiderr,'ral ce1ls at the conmon interface. Fig. 18, locular canal of figrlre 16 enlarged to shor^r the epithelium. DETAILS: t, staninodium; sc, sclereid layer; st, stylar canal; sr, sfigr,atic region; vb, vascular bundle; oc, locular cana1.

2. Staminate flower (Fig. 3-6)

a. Morphology

In comparison to the pistillate flower similarities as well as dissimilarities are found. The bracteoles have a similar structure. The staminate sepal which is not as fleshy as its pistillate counterpart, is imbricate and about 3 mm in length. The three petals are free at anthesis and reach a lenght of \(6\frac{1}{2}\) mm (Fig. 5). The stamens number 19-20 and are attached to an elevated receptaculum. In a stamen the filament measures \(1\frac{1}{2}\) mm and is half a mm thick while the latrorse anther may reach a length of 4,4 mm. The central part of an anther is densely filled with tannin.

b. Anatomy

In the sepal, epidermis consists of small, thin, tangentially elongated cells. Beneath the epidermis, one or two layers of thickwalled cells occur. Slime cells containing raphide crystals are present. Some sclereids are found adaxially. Seven vascular bundles may occur, each of them branching dichotomously. At the petal, the outer epidermis is papillose. Chloroplasts may be found in the 5-6 layers of cells beneath the epidermis. Adaxial to the latter 4-5 cells layers of sclereids are found. Slime cells containing raphide bundles also occur. Right underneath the inner or adaxial epidermis another layer of brachysclereids is found. The epidermis consists of anticlinally elongated cells. The stamen consists of a thin parenchymatous filament in which one vascular bundle is present and the long anther.

DISCUSSION

In Arenga porphyrocarpa, transformation of the vegetative axis into a reproductive one is accompanied by a reduction of leaf parts, a shortening of internodes after an initial elongation and some reduction in axis diameter. These features are also found in Rhapis (Uhl et al. 1960) or Nannorrhops (Tomlinson and Moore, 1968) and may also be common in many other monocotyledonous plants (Troll, 1964, in Tomlinson and Moore, 1968). Monopodial branching is shown in the reproductive axis as well as in the inflorescence. This is in accordance with the general pattern in palms as suggested by Tomlinson and Moore (1968). In each case the terminal part of an axis transforms into a flowering branch.

At the development of a flower cluster the branching pattern changes. The young stages of a flower or flower cluster still show three flower primordia in an arrangement which suggests similarity with the organization in a cincinnus such as found in Atzisteyera (Uhl, 1966) or Nannorrhops (Uhl, 1969 a). This indicates that the type of branching in th.e Anenga porphyrocatpa flower cluster is also sympodial. Therefore the monopodial pattern of branching in the rnaln axes, which converts to a sympodial type of branching in the flower clusters is sinilar to that found in Nannorrhops (TonLinson and Moore, 1968). Tn Az.enga porphyrocarpa, however, differentlation ln rachillae takes place becarlse the central flower does not develop further or does not develop at all ln a male rachilla, while the two marginal flowers abort ln a female rachllla. In the genus a triad consists of one feoale and two fenale floners (Moore, I973). Therefore reduction of oqe sex gives specializati.on of rachillae. Within the individual flower specialization also occurs. It Mettorglon sagu (Tomlinson, 1971) for instance, the perfect flower seems to be in a central position due to the fact that the male flower accompanying it may be found to its right to left side. In Arenga por-phArocqrpar the central flower too reminds of a perfect flower because of the presence of staninodia. An ontogenetical study of the flower cluster in Mettorglon sagu would probably show it to have a sympodial branching a1so. It is interesting to note that in Arenga porphyrocar.pa tl:'e fenale sex is always found at the terminal part of an axis whether it turns to be the reproductive axis or the one in a flower cluster.

In a triad, the angle of divergence and the position of the bracteole and its subtended prinordium will determine the shape and consequent definition of the flower cluster (Uhl, 1969) but in Arenga porphyrocarpc further development of each prinordiun also influences the ultimate shape and direction of the flower or flower pair.

Carpel features have often been discussed (Eames, 1961). The presence of two carpels it Arenga potphyroeatpa is a reduction of the three which is common in Caryotoideae (Moore, L973). The relatively thick area of the ventral suture suggests a closely appressed region of the ventral surface of a carpel as would be postulated by the conduplicate concept (Swany and Periasarny, 1964). The shape of the gynoecium, absence of a promi,nent stigna and the presence of locular canals in Arenga porphyrocatpa resembles Ia.tania (Ifhl and Moore, l97I). In our species, however, everything is situated somewhat higher. The hemianatropous, 1atera1ly placed ovule is an intermediate form which is also common in Caryotoideae (Moore, L973). The short canal ending into a pore at the rynoecial surface, which is found occasionally in the material investigated may represent a nectary (Uh1 and Moore, L97L).

Arenga porphAroeqtpa seeme to occupy a relatively specialized position among the Caryotoideae judging from rachillae differentiation. However, a stipitate gynoecitm, is a feature

regarded as primitive. The high degree of ovule protection as suggested by the presence of fibrous sheaths and bundles in sepals and petals, presence of sclereid layers as well as raphide bundles may be appreciated.