Similarities and gradients in growth unit branching patterns during ontogeny in "Fuji" apple trees: a stochastic approach

This study aims to explore and model the changes in growth unit (GU) branching patterns during tree ontogeny. The question was addressed in apple trees cv. "Fuji", by analysing the relative impact of GU length and within-tree position. The development of two 6-year-old trees was recorded over 6 years. The fate of axillary buds along each GU was represented as a sequence of symbols corresponding to five types of lateral growth: latent buds, short, medium, long, and floral lateral GUs. Based on an exploratory analysis of data and a priori hypotheses, a hidden semi-Markov chain was estimated from all of these GU sequences. This model was composed of six transient states representing successive branching zones along the GUs. The accuracy of this global model was a posteriori assessed by fitting the characteristic distributions computed from model parameters to the corresponding empirical characteristic distributions extracted from the observed sequences. The observed sequences were then grouped hierarchically according to the GU length, year of growth, and branching order. Comparing model parameters between these sub-groups revealed similarities between GUs. These similarities were based on particular branching zones whose composition and relative position within the GUs remained invariant across the subgroups: the latent zones, floral zone, and short-lateral zone. The probability of occurrence of the floral zone varied with the year, showing the alternate fruiting of "Fuji". It is shown that, during tree ontogeny, as GU length decreases, branching patterns tend to progressively simplify due to the disappearance of the most central zones and a progressive reduction in the length of the floral zone.     

Modeling Nymphoides architecture: A morphological analysis of Nymphoides aquatica (Menyanthaceae)

? Premise of the study: Species in the aquatic genus Nymphoides have inflorescences that appear to arise from the petioles of floating leaves. The inflorescence-floating leaf complex can produce vegetative propagules and/or additional inflorescences and leaves. We analyzed the morphology of N. aquatica to determine how this complex relates to whole plant architecture and whether whole plant growth is sympodial or monopodial. ? Methods: We used dissections, measurements, and microscopic observations of field-collected plants and plants cultivated for 2 years in outdoor tanks in south Florida, USA. ? Key results: Nymphoides aquatica had a submerged plagiotropic rhizome that produced floating leaves in an alternate/spiral phyllotaxy. Rhizomes were composed of successive sympodial units that varied in the number of leaves produced before the apex terminated. The basic sympodial unit had a prophyll that subtended a renewal-shoot bud, a short-petioled leaf (SPL) with floating lamina, and an inflorescence; the SPL axillary bud expanded as a vegetative propagule. Plants produced either successive basic sympodial units or expanded sympodia that intercalated long-petioled leaves between the prophyll and the SPL. ? Conclusions: Nymphoides aquatica grows sympodially, forming a rhizome composed of successive basic sympodia and expanded sympodial units. Variations on these types of sympodial growth help explain the branching patterns and leaf morphologies described for other Nymphoides species. Monitoring how these two sympodial phases are affected by water depth provides an ecologically meaningful way to assess N. aquatica's responses to altered hydrology.     

Tool-using and -making behavior in wild chimpanzees at Bossou,Guinea

The behavior of wild chimpanzees at Bossou, Guinea, was studied from November 1976 to May 1977 recognizing each chimpanzee without artificial feeding. During the study period some tool-using and tool-making behavior was observed, as follows: (1) Although water drinking using a “leaf-sponge” was not seen, that using a “leaf-spoon” was observed for taking water from the hollow of a tree. (2) “Termite fishing” was not seen in this group although there were many termite hills in the moving range of the chimpanzees. They dug termites from the hollow of a tree by pounding with a small stick. Similar use of a stick was made for digging up the resin from a tree. (3) “Aimed throwing” was frequently observed in adult males for attacking an observer, and in adolescents and juveniles as mischief against an observer or for their own play. (4) “Nut cracking” with a pair of stones was seen for removing the ovule from palm-seeds. Particular stones were repeatedly used by many chimpanzees for a long period. (5) “Branch hauling” represented difficult work. Patient and inventive manufacture of proper sticks was necessary for capturing branches which they were unable to reach normally. Local variations in the tool-using patterns and manufacturing ability of chimpanzees are discussed.     

Branch development in custard apple (cherimoya Annona cherimola Miller × sugar apple A. squamosa L.) in relation to tip-pruning and flowering, including effects on production

Custard apple has cryptic axillary buds, hidden from view by the base of the petiole. This has led to confusion about custard apple’s flowering habit. Flowering only occurs during early branch development, and can be forced at any time of the growing season simply by removing leaves. Here, we show that flowering is terminal, not extra-axillary, and that the apparent continuation of the main stem beyond the flower is, instead, a sympodial branch. Secondary (including sympodial) branching only occurs during early branch development. Thereafter, axillary bud release is inhibited by the subtending leaf. Here, we show that summer tip-pruning of all branches arrests canopy development until the following spring owing to this inhibition. Although summer tip-pruning prevented new vegetative growth in the canopy, fruit size decreased relative to the control trees by ca. 23%. The reason for this decrease was probably related to increased carbon limitation to growth given that dawn water soluble and total non-structural carbohydrate concentrations were lower in the tip-pruned trees. Thus, it appears that the reduced competition between fruit development and new vegetative growth in the tip-pruned trees was more than matched by lower photosynthetic capacity in the arrested canopy. Trees grown inside a shade-house were more vigorous than those grown outside. The difference in vigour had little effect on fruit size.     

Architecture of the pruned tree: impact of contrasted pruning procedures over 2 years on shoot demography and spatial distribution of leaf area in apple (Malus domestica)

BACKGROUND AND AIMS: Demography and spatial distribution of shoots are rarely studied on pruned trees. The present 2-year study deals with the effect of pruning strategies on shoot demography and development, and consequences on the spatial distribution of leaf area in three architecturally contrasted - from type II to IV - apple cultivars: 'Scarletspur Delicious', 'Golden Delicious' and 'Granny Smith'. METHODS: All trees were initially subjected during 5 years to Central Leader training with winter heading on all long shoots. For 2 years, half of the trees were further trained with Centrifugal training, where removal of flowering shoots - called extinction pruning - was carried out along the trunk and at the bottom of branches at flowering time. During these 2 years, shoot type (vegetative, inflorescence) and length, and the three-dimensional spatial distribution of all shoots were assessed with an electromagnetic digitizer. KEY RESULTS: Shoot demography, frequency of transitions toward an inflorescence from either an inflorescence (bourse-over-bourse) or a vegetative shoot (trend toward flowering), and the number of bourse-shoots per bourse were strongly affected by cultivar, with little influence of tree manipulation. In contrast, the proportion of vegetative long shoots developing from previous year latent buds was significantly lower in Centrifugal-trained trees for the three cultivars. Canopy volume showed large variations between cultivars, but only that of 'Granny Smith' was affected by tree manipulation in the 2 years. Spatial distribution of shoots varied significantly according to cultivar and manipulation. In 'Scarletspur Delicious' and, to a lesser extent 'Golden Delicious', the distribution of vegetative and flowering shoots in the outer and the inner parts, respectively, was not affected by tree manipulation. In contrast, in 'Granny Smith', vegetative shoots were stimulated in the periphery of Central Leader trees, whereas flowering shoots were stimulated in the periphery of Centrifugal-trained trees. CONCLUSIONS: In apple, the variability of responses to contrasted pruning strategies partly depends on the genetically determined growth and flowering habit of the cultivar.     

Study of tree architecture of apple (<Emphasis Type="Italic">Malus</Emphasis> × <Emphasis Type="Italic">domestica</Emphasis> Borkh.) by QTL analysis of growth traits

Apple tree architecture is naturally very diverse, but for fruit production, certain tree habits are more desirable than others. Here we describe the results of a QTL analysis performed to study the genetic control of growth traits in apple. This was carried out on the progeny of a cross between two apple cultivars of contrasting tree architectures. “Telamon” has a columnar tree form and “Braeburn” has a more standard, “normal” growth habit. The growth traits were measured on the F ₁ seedlings of the Telamon × Braeburn population for two consecutive years of growth on own roots and for the first year of growth on M9 rootstock. QTL analysis was carried out using either the Kruskal–Wallis method or the Multiple QTL Method. For all but one growth characteristic, significant QTLs were detected. A major cluster of QTLs was located in the Co gene region of “Telamon”, confirming the major influence of the Co gene on tree architecture, although this influence changed as the plant material aged and was generally more pronounced for rootstock-grown plants. Additional QTL results suggest the occurrence of genes with pleiotropic effects on tree architecture. The observed QTL instability over different years and for different root systems indicates that the genetic control of tree architecture is largely influenced by environmental factors and probably changes as the tree matures. Finally, a major influence of the root system on all the traits determining tree architecture was clearly demonstrated.     

Phenological models for blooming of apple in a mountainous region

Six phenological series were available for ‘Golden Delicious’ apple blooming at six sites in Trentino, an alpine fruit-growing region. Several models were tested to predict flowering dates, all involving a “chilling and forcing” approach. In many cases, application of the models to different climatic conditions results in low accuracy of prediction of flowering date. The aim of this work is to develop a model with more general validity, starting from the six available series, and to test it against five other phenological series outside the original area of model development. A modified version of the “Utah” model was the approach that performed best. In fact, an algorithm using “chill units” for rest completion and a thermal sum for growing-degree-hours (GDH), whose efficiency changes over time depending on the fraction of forcing attained, yielded a very good prediction of flowering. Results were good even if hourly temperatures were reconstructed from daily minimum and maximum values. Errors resulting from prediction of flowering data were relatively small, and root mean square errors were in the range of 1–6 days, being <2 days for the longest phenological series. In the most general form of the model, the summation of GDH required for flowering is not a fixed value, but a function of topoclimatic variables for a particular site: slope, aspect and spring mean temperature. This approach allows extension of application of the model to sites with different climatic features outside the test area.     

Levels of indole-3-acetic acid in vigorous and genetic dwarf apple trees

Seasonal levels of indole-3-acetic acid (IAA) present in buds, meristematic tissues, and leaves of 1-year-old shoots of two selections (vigorous and dwarf) of a F₂ generation apple population with original “Goldspur” × “Redspur” parentage were determined using gas chromatography—selected ion monitoring—mass spectrometry (GC-SIM-MS) with a¹³C-IAA internal standard. A comparison of IAA levels through one growing season was made in the shoots of two trees differing in growth. The levels of IAA in the leaves of the vigorous tree were higher in the early and late growing seasons, as compared to those in the dwarf tree. Levels of IAA in buds of the dwarf tree shoots were found to be higher than in those of the vigorous tree shoots. On leave from Indian Institute of Horticultural Research, Bangalore-560080, India.     

Apple dwarfing rootstocks and interstocks affect the type of growth units produced during the annual growth cycle: precocious transition to flowering affects the composition and vigour of annual shoots

Background and Aims: Precocious flowering in apple trees is often associated witha smaller tree size. The hypothesis was tested that floral evocationin axillary buds, induced by dwarfing rootstocks, reduces thevigour of annual shoots developing from these buds comparedwith shoots developing from vegetative buds. Methods: The experimental system provided a wide range of possible treevigour using ‘Royal Gala’ scions and M.9 (dwarfing)and MM.106 (non-dwarfing) as rootstocks and interstocks. Second-yearannual shoots were divided into growth units corresponding toperiods (flushes) of growth namely, vegetative spur, extensiongrowth unit, uninterrupted growth unit, floral growth unit (bourse)and extended bourse. The differences between the floral andvegetative shoots were quantified by the constituent growthunits produced. Key Results: The dwarfing influence was expressed, firstly, in reduced proportionsof shoots that contained at least one extension growth unitand secondly, in reduced proportions of bicyclic shoots (containingtwo extension growth units) and shoots with an uninterruptedgrowth unit. In treatments where floral shoots were present,they were markedly less vigorous than vegetative shoots withrespect to both measures. In treatments with M.9 rootstock,vegetative and floral shoots produced on average 0·52and 0·17 extension growth units, compared with 0·77extension growth units per shoot in the MM.106 rootstock treatment.Remarkably, the number of nodes per extension growth unit wasnot affected by the rootstock/interstock treatments. Conclusions: These results showed that rootstocks/interstocks affect thetype of growth units produced during the annual growth cycle,reducing the number of extension growth units, thus affectingthe composition and vigour of annual shoots. This effect isparticularly amplified by the transition to flowering inducedby dwarfing rootstocks. The division of annual shoot into growthunits will also be useful for measuring and modelling effectsof age on apple tree architecture.     

Clarifying the effects of dwarfing rootstock on vegetative and reproductive growth during tree development: a study on apple trees

BACKGROUND AND AIMS: Despite the widespread use of dwarfing rootstocks in the fruit-tree industry, their impact on tree architectural development and possible role in the within-tree balance between growth and flowering are still poorly understood, in particular during the early years of growth. The present study addressed this question in apple trees, through a detailed analysis of shoot populations, i.e. both vegetative and flowering shoots, during tree development. METHODS: Architectural databases were constructed for trees of two cultivars that were either own-rooted or grafted on dwarfing rootstock. Within-tree shoot demographics and annual shoot characteristics, i.e. their dimensions, number of laterals and flowering, were observed from the first to the fifth year of growth and compared among scion/root system combinations. KEY RESULTS: Differences in axis demographics appeared among scion/root system combinations after the second year of growth. Differences were found (a) in the number of long axes and (b) the number of medium axes. Dwarfing rootstock reduced the total number of axes developed in a tree, and this reduction resulted from proportionally more medium axes and spurs than long axes. The life span of spurs was also shortened. These phenomena appeared after an increase in flowering that started in the second year of growth and involved both axillary and terminal positions. Flowering regularity was also increased in grafted trees. CONCLUSIONS: These results confirm that the number of long shoots and flowering potential depend on the cultivar. They indicate that tree architectural plasticity in response to its root system mainly derives from the number of medium shoots developed and follows priorities within the whole tree axis population. There was also evidence for dwarfing rootstock involvement in adjusting the flowering abundance and that differences in flowering occurrence take precedence over those regarding vegetative growth during tree development.     

Risk of spring frost to apple production under future climate scenarios: the role of phenological acclimation

In the context of global warming, the general trend towards earlier flowering dates of many temperate tree species is likely to result in an increased risk of damage from exposure to frost. To test this hypothesis, a phenological model of apple flowering was applied to a temperature series from two locations in an important area for apple production in Europe (Trentino, Italy). Two simulated 50-year climatic projections (A2 and B2 of the Intergovernmental Panel on Climate Change - Special Report on Emission Scenarios) from the HadCM3 general circulation model were statistically downscaled to the two sites. Hourly temperature records over a 40-year period were used as the reference for past climate. In the phenological model, the heat requirement (degree hours) for flowering was parameterized using two approaches; static (constant over time) and dynamic (climate dependent). Parameterisation took into account the trees’ adaptation to changing temperatures based on either past instrumental records or the downscaled outputs from the climatic simulations. Flowering dates for the past 40 years and simulated flowering dates for the next 50 years were used in the model. A significant trend towards earlier flowering was clearly detected in the past. This negative trend was also apparent in the simulated data. However, the significance was less apparent when the “dynamic” setting for the degree hours requirement was used in the model. The number of frost episodes and flowering dates, on an annual basis, were graphed to assess the risk of spring frost. Risk analysis confirmed a lower risk of exposure to frost at present than in the past, and probably either constant or a slightly lower risk in future, especially given that physiological processes are expected to acclimate to higher temperatures. An erratum to this article can be found at     

Gametophytic development of <Emphasis Type="Italic">Brassica napus</Emphasis> pollen in vitroenables examination of cytoskeleton and nuclear movements

Isolated microspores and pollen suspension of Brassica napus “Topas” cultured in NLN-13 medium at 18°C follow gametophytic pathway and develop into pollen grains closely resembling pollen formed in planta. This culture system complemented with whole-mount immunocytochemical technology and novel confocal laser scanning optical technique enables detailed studies of male gametophyte including asymmetric division, cytoskeleton, and nuclear movements. Microtubular cytoskeleton configurationally changed in successive stages of pollen development. The most prominent role of microtubules (MTs) was observed just before and during nuclear migration at the early and mid-bi-cellular stage. At the early bi-cellular stage, parallel arrangement of cortical and endoplasmic MTs to the long axis of the generative cell (GC) as well as MTs within GC under the plasmalemma bordering vegetative cell (VC) were responsible for GC lens shape. At the beginning of the GC migration, endoplasmic microtubules (EMTs) of the VC radiated from the nuclear envelope. Most cortical and EMTs of the VC were found near the sporoderm. At the same time, pattern of MTs observed in GC was considerably different. Multiple EMTs of the GC, previously parallel aligned, reorganized, and start to surround GC, forming a basket-like structure. These results suggest that EMTs of GC provoke changes in GC shape, its detachment from the sporoderm, and play an important role in GC migration to the vegetative nucleus (VN). During the process of migration of the GC to the VC, multiple and thick bundles of MTs, radiating from the cytoplasm near GC plasma membrane, arranged perpendicular to the narrow end of the GC and organized into a “comet-tail” form. These GC “tail” MTs became shortened and the generative nucleus (GN) took a ball shape. The dynamic changes of MTs accompanied polarized distribution pattern of mitochondria and endoplasmic reticulum. In order to confirm the role of MTs in pollen development, a “whole-mount” immunodetection technique and confocal laser-scanning microscopy was essential.     

Root suckering patterns in Populus euphratica (Euphrates poplar, Salicaceae)

To understand the spatial structure of monospecific Tugai forests (Xinjiang Province, China) growing as gallery woods nourished by ground water, root suckering in Populus euphratica was studied by a combination of morphological and molecular analyses. Seedlings grow a deep tap root and keep this as adult trees, whereas root suckers never develop a tap root but utilize the horizontally stretching root of their parent trees. The resulting reverse “T” root architecture distinguishes reliably even adult root suckers from generatively grown trees. Due to assimilate input from the root sucker, the distal root (pointing away from the parent tree) becomes thicker soon than its proximal root, which allows determination of the direction of vegetative growth. One stand including 279 young trees germinated from seeds and 267 root suckers was mapped completely, and selected suckers were assigned to parent trees by genotyping with microsatellite DNA. Root suckers develop up to 40 m away from parent trees on horizontal “spacer” roots, usually originating not deeper than 20 cm below surface. Trees begin with root suckering between 10 and 15 years, shortly before reaching flowering age. Cutting experiments indicated reduced survival of young root suckers disconnected from the parent tree. Without a tap root and with a rooting point close to the surface, declining ground water levels should lower the fitness of root suckers even more than that of generatively grown trees.     

Flowering response oflemna paucicostata in japan

Twenty-two strains ofLemna paucicostata collected from various districts in Japan are classified into 4 types having different morphological features and photoperiodic behaviors. (I) The “N-1” type, widely distributed in north and middle Japan, consists of many short-day strains with similar morphological characteristics. The strain of northern origin, however, has a shorter critical dark period than that of southern origin, and most of the strains flower in response to a single short day though some from middle Japan require 2 short days. (2) The “N-2” type, found in a limited area of northern Japan, is also a short-day type, but has very long critical dark periods and requires 3–5 short-day cycles for flowering. (3) The “S” type, distributed in southern Japan, never flowered in any photoperiod or culture conditions tested. (4) The “K” type, strain 351, collected at the experimental farm of Kyoto University, is a day-neutral type and flowers independently of photoperiod. Application of some SH inhibitors, tungstate or EDTA, or exposure to blue, far-red or low-intensity light caused daylength-independent flowering in short-day strain 6746 (California origin), but none of these treatments caused flowering in any short-day strain collected in Japan.     

Md-ACS1 and Md-ACO1 genotyping of apple (<Emphasis Type="Italic">Malus</Emphasis> x <Emphasis Type="Italic">domestica</Emphasis> Borkh.) breeding parents and suitability for marker-assisted selection

Fruit ethylene production genotypes for Md-ACS1 and Md-ACO1 were determined for 60 apple cultivars and 35 advanced breeding selections. Two alleles for each gene are commonly found in cultivated apple. Earlier studies showed that genotypes homozygous for the ACS1-2 allele produce less ethylene and have firmer fruit than ACS1-1/2 and ACS1-1/1 genotypes. ACO1 plays a minor role compared to ACS1, with homozygous ACO1-1 having lower ethylene production. In this study, ACS1-2 and ACO1-1 homozygotes had firmer fruit at harvest and after 60 days of 0–1°C cold storage compared to other genotypes. These genotypes, ACS1-2/2 and ACO1-1/1, were observed for the following 8 of 95 cultivars/selections: “Delblush”, “Fuji”, “Pacific Beauty”, “Sabina” and four breeding selections. Cultivars/selections that were homozygous ACS1-2 but not ACO1-1 were: “Ambrosia”, “Aurora Golden Gala”, “CrimsonCrisp”, “Gala”, “GoldRush”, “Huaguan”, “Pacific Rose, “Pacific Queen”, “Pinova”, “Sansa”, “Sonja”, “Sundance”, “Zestar”, and 17 breeding selections. Cultivars with the heterozygous ACS1-1/2 genotype were “Arlet”, “Braeburn”, “Cameo”, “Delicious”, “Delorgue”, “Empire”, “Enterprise”, “Ginger Gold”, “Golden Delicious”, “Granny Smith”, “Honeycrisp”, “Orin”, “Pink Lady”, “Silken”, “Suncrisp”, “Sundowner”, “Sunrise” and 11 breeding selections. No cultivars were detected homozygous for both ACS1-1 and ACO1-1, or for both ACS1-2 and ACO1-2. This study is the first large-scale allelic genotyping of both ethylene synthesis genes for a comprehensive set of apple breeding parents used in an ongoing breeding project. The data reported here are important for informative selection of parent combinations and marker-assisted selection of progeny for breeding low ethylene-producing apple cultivars for better storability and improved consumer acceptance.     

Inheritance of Flowering Time in Apricot (<Emphasis Type="Italic">Prunus armeniaca</Emphasis> L.) and Analysis of Linked Quantitative Trait Loci (QTLs) using Simple Sequence Repeat (SSR) Markers

Time of flowering was studied during 3 years in a BC1 apricot progeny of 73 seedlings derived from a cross between the F1 selection “Z506-07” (“Orange Red” × “Currot”) and the Spanish cultivar “Currot”. Results indicated a quantitative inheritance of flowering time in apricot with an influence of juvenility and environmental conditions (chill accumulation) on the evaluation and expression of this trait. Genetic maps consisting of 11 linkage groups for both parents representing the eight chromosomes of apricot were developed using 46 apricot and peach simple sequence repeat (SSR-microsatellites) markers and were used for the identification of quantitative trait loci (QTL). QTL analysis for flowering time allowed the identification of one significant QTL on the linkage group 5 (G5) of “Z506-07”, and explaining most of the phenotypic variation. Two microsatellite loci (UDAp-423r and AMPA-105) were found to be tightly linked to this important agronomic trait. Finally, we discuss the stability of the QTL described during the 3 years of the study and the development of efficient marker-assisted selection strategies applied to apricot and other Prunus breeding programs.     

Tree specific traits affect flowering time in Indian dry tropical forest

Dry tropical forest tree species show variations in leafless duration (i.e. deciduousness), stem wood density (SWD), leaf mass area (LMA) and leaf strategy index (LSI, reflecting resource use rate) to overcome water limitations. We investigated the role of these tree traits in the seasonal timing of flowering and subsequent fruiting. Flowering and fruiting time of 24 tree species was recorded over two consecutive annual cycles and their relationships with the abovementioned tree specific traits were examined across the species. In leaf-exchanging species having higher SWD and LMA, low LSI and short deciduousness, flowering coincides with leaf transitional state when vegetative growth is at its minimum, and fruit formation and leaf flushing are both supported at the same time. However, >4-months-deciduous species with lowest SWD and LMA, higher LSI and longer deciduousness showed predominantly dry season flowering, subsequent fruiting on leafless shoots and distinct separation of vegetative and flowering phenophases. In contrast, intermediate species (<2 months-deciduous, 2–4-months-deciduous) showed wider flowering range through summer, rainy, autumn or winter seasons. Fruiting duration varies considerably with variation in the flowering time; ca. 5–14 months in summer flowering species; 7–12 months in rainy flowering species; 6–10 months in autumn flowering species, 4–9 months in dry season flowering and 3–7 months in winter flowering species. In most species, fruit maturation occurred just prior to the onset of rains, ensuring seedling survival. The ability of tree species to withstand (leaf-exchange) or avoid (deciduousness) drought stress and varying seasonal flowering timings appear to be the principal mechanisms for successful survival and reproduction under extremely dry and seasonal climate. Since environmental characteristics affect flowering and fruiting either directly (e.g. through conditions in the habitat) or indirectly (e.g. through deciduousness, LMA, SWD and LSI), the impact of probable global climatic change will have long implications on reproduction of dry tropical trees.     

Sympodial and monopodial branching inAcer (Aceraceae): Evolutionary trend and ecological implications

The evolutionary trend and its ecological implications in sympodial and monopodial branching patterns has been investigated in 20 JapaneseAcer spp. through comparison of shoot tip abortion and terminal bud formation. The genus is divided into two species groups according to its branching pattern, one (6 species) predominantly exhibiting sympodial branching with frequent monopodial branching in short shoots (sympodial species), and the other (14 species) exhibiting only monopodial branching (monopodial species). The early ontogeny of leaf and bud scales is described. Despite the difference in branching patterns, the bud scales of terminal buds are essentially the same in having a leaf base developed to function as a protecting organ. In all the sympodial species, during the abortion of a sympodium shoot tip, one or two pairs of primordia were found to occur on the apex, and later wither. These primordia resemble bud scales of terminal buds in their ontogeny and morphology, and appear to be rudimentary. It is suggested that a rudimentary terminal bud develops together with the establishment of sympodial branching, and that sympodial branching has originated from monopodial branching. Based on this proposed evolutionary trend, it is suggested thatAcer has moved from less shady habitats into shady habitats with monopodial branching (advantageous for vertical growth) changing into sympodial branching (advantageous for lateral spread).