The genus Micromeles revisited

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Transcript of The genus Micromeles revisited

  • i l IA)SLAV KOVANDA l and JAMES CItALLIC:E 2

    t Botanical Institute, Czechoslovak Academy of Sciences, 252 43 Prflhonice near Praha, Czechoslovakia 2 Long Ashton Research Station, University of Bristol, BS18 9AF, England

    The genus Micromeles rev is i ted

    Keywords

    Sorbus L. emend. CRA~TZ, Micromeles DECAISNE emend. KOEHNE, Raphiolepis LINDL., Leaf venation, Flower and fruit morphology, Generic limits in the Mcdoideae, Chemotaxonomy, Flavone C- and O-glycosides, Phylogenetic implications

    Abst rac t

    Micromeles DW.CAISNE emend. Kop.~2cE (Rosaceae, Maloideae) is shown to be generically distinct from Sorbus L. emend. CRANTZ. Problems in the taxonomy of the genus are reviewed. The genus appears to he most closely related to Raphiolepis L1NDL, but is readily distinguished by a constant set of characters. Both genera may be derived from Sorbus L. emend. CI~ANTZ subg. Aria PER$. Analysis of morphological characters is provided. A survey for flavonoids revealed the presence of vitexin and]or luteolin 7-O-glucoside in some members of the genus in a pattern similar to that found in certain Asian species of Sorbus. There are also brief taxonomle notes on some of the species which were examined.

    INTRODUCTION

    Micromeles DEC~SNE emend. KOEH~E is an Asian genus in the Maloideae (Rosa- ceae). I t contains around 25 species of shrubs and trees distributed from Nepal to the South Kuriles and extending as far south as the Malay Peninsula and Sumatra. The main concentration and possibly the evolutionary centre of the genus as well lies in SW. China and adjacent Burma. The majority of the species are confined to the mountains; some extend up to 4,000 mal t . in the HimMayas. The most widely distri- buted species are M. alni/olia (SIEB. et ZUCO.) KOI~HNE and M. corymbi/era (MIQ.) ](Tm,KM.; the others are more or less localized. The pomes of Micromeles are edible and severM species, such as M. alni/olia (SrE~. et Zvcc.) KOEHt~E, M. japonica (DEcAIS~E) KOI~HNE and M. FoOneri C. K. SCHNEIDER are cultivated as ornamentals.

    Folia Geobot. Phytotax., Praha, 16:181-193. 1981

  • 182 FOL IA GEOBOTA~ICA. ET PHYTOTAXONO~[ ICA, 16, 1981

    Morphologically, the genus is distinguished by its deciduous, simple, serrate or shallowly lobate leaves, flowers in corymbs, dimerous to pentamerous gynoecium with styles coalescent at least at base, inferior ovary, deciduous upper part of the hypanth ium and fruit with membraneous cndocarp and distinct annular cicatricc at the apex. Thus circumscribed, Micromeles DECAISNE emend. KO]~HNE differs clearly from the genus Sorbus L. emend. CRANTZ with which it is often merged. In the past, members of the genus have been variously combined also with other genera in the Maloideae, such as Crataegu8 L., Mespflus L., Pyrus L. or even Photinia LI~DL., and in many herbaria it is still necessary to sort out virtually all material of the Maloideae, to get the sheets of Micromeles together.

    This review is intended to draw attent ion to the problems in the taxonomy of the genus.

    H ISTORY OF THE GENUS

    Micromeles is one of the numerous genera in the ~laloideae whose recognition we owe to DECAISNE (1874). He diagnosed it in detail, recognizing five species, all described as new from the Himalayas: M. verrucosa DEeAISNE, M. ca~'tanei/olia DEeAISN~, ~I. rhamnoides DECAISNE, M. kha~iana DECAISN~ and M. Crri/fithii DECAIS~E. Surprisingly, three more species, M. japonica (DEcAISNE) KOE~E, 2}1. alni]olia (SIE~. et Zucc.) KOE~NE (originally described as belonging to Crataegus L.) and M. tiliae/olia (DEcAIs~-E) KOEn~E, clearly belonging to the genus Micromeles as defined by DECAISNE, were referred by him to the genus Aria HosT, now usually included in Sorbus. This obvious error was corrected by KOEHNE (1890) who transferred these species to Micromeles and demonstrated that, on the morphology of reproductive organs, Micromeles is distinct from all other genera in the Maloideae. Alas, KOEHNE'S work has never become generally known, so that doubts as to DECAIS~E'S taxonomic intention continued, even though the genus Micromeles, as circumscribed by KoE~-~E, was adopted by HEDLUND (1901) and SC~NEIDE~ (1906) who both supplied additional distinguishing characters. Revising SE. Asian Maloideae, REHDE~ (1915) concluded that no distinction could be found between DECAIS~E'S genera Aria and Micromeles and submerged the latter as a section into Sorbus, while Aria had been included in that genus by FRITSCH (1898-- 1899). In the absence of a modern monograph of the subfamily, this treatment has been followed by most subsequent authors (see e.g. HA~DEL-MAzZETTI 1933, Yi) et Kux~ 1963, OawI 1965, GAB~IELJA~," 1978). Exceptions include POJ~KOVA (1939) who adopted the genus in the Flora of USSR, and K~LK~AX (1973) who advocated its generic status in .a review of the Malesian Maloideae. DECAIS~E'S treatment is admittedly inconsistent but this objection does not apply to the genera Micromeles and Aria as conceived by KOEHNE. Here, as will be demonstrated further below, the deciduous upper part of the hypanthium combined with an inferior ovary constitutes a distinct dividing line which cannot be ignored.

    MOt~PHOLOGICAL CONSIDERATIONS

    Interestingly, two distinct types of leaf venation occur within Micromeles. Straight lateral veins ending in the teeth or leaf lobes (eraspedodromous venation, Fig. 1, A) are found throughout the subfamily Maloideae and in most species of Micromeles. The other type, with curving veins dissolving at a distance from the leaf margin (camptodromous venation, Fig. 1, B), is characteristic of the SE. Asian genera Raphio- lepis LINDL. and Photi~ia LINDL. and of the N. American Aronia PEas. which are all quite distinct from Micromeles (the first two having evergreen leaves). I t is most surprising therefore to find that both types of venat ion occur in the genus Micro- meles. I t was apparently the craspedodromous venat ion that made DEC~JSN~ exc]ude

  • KOVANDA A~D ( ' t tALL I ( 'E : THE GENUS MI ( 'ROMELES 183

    M. japonica, M. alni/olia, and M. tiliac/olia froin his Micromeles. Straight veins are usually fewer in number and tend to be correlated with leaves doubly serrate to lobu- late and globose fruit with granulate pulp, whereas species with curving veins usually have leaves simply serrate aatd ovoid or ellipsoid fruit with soft pulp. These correlations are not very well marked, however, and many intermediates occur. I t is questionable therefore whether a subdivision of the genus based solely on the leaf venation would be useful.

    9 "J cm" '

    Fig. 1. Leaves: A, M. ~d~i]olia (~I[EB. et Zucc.) KOEHNE; B, M. aronioides (REHDER) ]~.OVANDt. et CHALL ICE .

    Flower and fruit morphology of Micromeles combirms both primitive and advan- ced characters -- a situation not uncommon in the Malofdeae. The original penta- merous gynoecium has survived in M. caloneura STAPF (Fig. 2, A) but in most species the number of carpels varies from 2 to 3, similarly as in Sen'bus subg. Aria PERS. Regardless of the number of carpels, the styles are always fused at least in their basal parts. The degree of fusion is fairly constant for each species. All members of the genus have an inferior ovary. Contrary to the common belief, this is not character- istic of the subfanfily as a whole, the majority of genera having the ovary semi- inferior. Typical inferior ovary is found, besides Micromeles, only in the genera Cra- taegus L., Pyrus L., Malus MILL., Aronia PERS., Cydonia MILL., Peraphyllum NUTT., Raphiolepis LIN])L. and in one of the five subgencra of Sorbus L. era. CRANTZ, Tormin- aria (DC.) C. KOCH (see KO~lZNE 1890, KOVANDA 1961, 1965).

    The deciduons calyx requires some comment. I t emerges in various genera of the Maloideae, at different evolutionary levels and associated with a variety of other characters. While it is diagnostic in one genus, it may be subject even to intra- specific variation in the other. Therefore its value us a t&xonomic character is different in different situations, a fact which even monographers have failed to appreciate. Care should be taken to distinguish between deciduous (partly or totally) calyx teeth and deciduous calyx (or, rather, deciduous upper part of the floral cup).

  • 184 FOLIA GEOBOTANICA ET PHYTOTAXONO~ICA, 16, 1981

    For instance, in Sorbus subg. Torminaria (DC.) C. KOCH it is only calyx teeth that die and are shed after florescence, while the remainder of the hypanthium becomes fleshy and continues as an inseparable part of the fruit. All other subgenera of Sorbus have calyx teeth persistent or even pulpous in fruit. A degree of variation is known to occur in some Caucasian members of Sorbus subg. Aria P]ms. The mar- cescence and falling off of the upper part of the hypanthium is typical of the genera Micromeles DECAISNE emend. KOEH=~E and Raphiolepis LINDL. Here, the whole portion of the floral cup overtopping the ovary withers and is severed immediately after florescence, leaving a distinct cicatrice on the fruit (Fig. 3, A, B). No deviations have been observed, contrasting with Pyrus L., where the behaviour of the calyx is rather variable (see CHALLIC~. et WESTW0OD 1973) and differences may occasionally be found even within one species (DosT~LEK 1979). It follows that for Micromeles and Raphiolepis the deciduous "calyx" is an important distinguishing feature.

    Fig. 2. Longi tudina l sections of flowers: A, M. ca!oneura STAPF; B, M. aronioides (I~EHDEa) KOVANDA et CHALLICE.

    SUBDIV IS ION OF THE GENUS

    The taxonomic subdivision of the genus presents serious problems. With floral morphology uniform throughout the genus (except for the number of carpels and the degree to which the styles are coalesced), the infrageneric taxonomy is largely dependent upon vegetative features, combi