Orthotropic Growth Research Articles

Numerical approximation of variational problems with orthotropic growth

Numerical approximation of variational problems with orthotropic growth

PIN-FORMED is required for shoot phototropism/gravitropism and facilitates meristem formation in Marchantia polymorpha.

PIN-FORMED auxin efflux transporters, a subclass of which is plasma membrane-localised, mediate a variety of land-plant developmental processes via their polar localisation and subsequent directional auxin transport. We provide the first characterisation of PIN proteins in liverworts using Marchantia polymorpha as a model system. Marchantia polymorpha possesses a single PIN-FORMED gene, whose protein product is predicted to be plasma membrane-localised, MpPIN1. To characterise MpPIN1, we created loss-of-function alleles and produced complementation lines in both M.polymorpha and Arabidopsis. In M.polymorpha, gene expression and protein localisation were tracked using an MpPIN1 transgene encoding a translationally fused fluorescent protein. Overexpression of MpPIN1 can partially complement loss of an orthologous gene, PIN-FORMED1, in Arabidopsis. In M.polymorpha, MpPIN1 influences development in numerous ways throughout its life cycle. Most notably, MpPIN1 is required to establish gemmaling dorsiventral polarity and for orthotropic growth of gametangiophore stalks, where MpPIN1 is basally polarised. PIN activity is largely conserved within land plants, with PIN-mediated auxin flow providing a flexible mechanism to organise growth. Specifically, PIN is fundamentally linked to orthotropism and to the establishment of de novo meristems, the latter potentially involving the formation of both auxin biosynthesis maxima and auxin-signalling minima.

From dark to darkness, negative phototropism influences the support-tree location of the massive woody climber Hydrangea serratifolia (Hydrangeaceae) in a Chilean temperate rainforest

ABSTRACT Climbing plants rely on suitable support to provide the light conditions they require in the canopy. Negative phototropism is a directional search behavior proposed to detect a support-tree, which indicates growth or movement away from light, based on light attenuation. In a Chilean temperate rainforest, we addressed whether the massive woody climber Hydrangea serratifolia (H. et A.) F. Phil (Hydrangeaceae) presents a support-tree location pattern influenced by light availability. We analyzed direction and light received in two groups of juvenile shoots: searching shoots (SS), with plagiotropic (creeping) growth vs. ascending shoots (AS), with orthotropic growth. We found that, in accordance with light attenuation, SS and AS used directional orientation to search and then ascend host trees. The light available to H. serratifolia searching shoots was less than that of the general forest understory; the directional orientation in both groups showed a significant deviation from a random distribution, with no circular statistical difference between them. Circular-linear regression indicated a relationship between directional orientations and light availability. Negative phototropism encodes the light environment’s heterogeneous spatial and temporal information, guiding the shoot apex to the most shaded part of the support-tree base, the climbing start point.

Yield of cocoa under different agroforestry systems in a dry tropical forest in western Colombia

The expansion and modernization of the cocoa area under new strategies, such as the use of adapted genetic material and the establishment of Agroforestry Systems with cocoa, under criteria of competitiveness and sustainability, require selecting sites with adequate biophysical conditions, which facilitate the optimization of resources for production. In this sense, we conducted a study in the Estación Agraria Cotové, of the Universidad Nacional de Colombia, located in a tropical dry forest life zone (TDF), at 540 meters of elevation, with an average temperature of 27 ºC, average annual precipitation annual of 1,031 mm and relative humidity less than 70 %. The yield components and productive potential of four cocoa clones, ICS 95, TSH565, CCN 51, and ICS 60, were evaluated. The cocoa clones were planted under two controlled sunlight habitats, generated by the timber species Gmelina arborea Roxb. (single-row and double-row arrangement), and two different canopy management of the cocoa plants (plagiotropic and orthotropic growth stimulus). The clones TSH 565 and CCN 51 showed the highest yields in the two harvest years. ICS 95 showed the lowest bean index. Regarding the pod index, no differences were observed between the cocoa clones. Clones TSH 565 and CCN 51 stood out as the earliest and most productive clones.

Архитектура кроны дуба черешчатого (Quercus robur L.) в генеративном периоде онтогенеза в фитоценозах северной лесостепи Тульской области

The diversity of the habitus in reproductive individuals of Quercus robur L. in phytocoenoses of the northern forest-steppe within the Kulikovo Pole museum-reserve in the Tula Region was analysed. The individuals grow-ing in the ravine and upland closed and open forests as well as open-growth individuals and groups of individu-als in the forest outliers were researched. One-trunk, a few-trunk, coppice and clump life forms were detected. It was shown that the types of the habitus or architectural types (AT) in reproductive individuals resulted from those AT that were firstly described in the virginal ontogenetic state and in the beginning of the young reproduc-tive state in the mixed forest and broadleaved forest biomes. The quantitative and qualitative changes of the shoot systems were found out. The ways of the crown transformation are ten in number. They include acropetal mortality of the branches from the trunk, basipetal mortality of the shoot systems, decrease of the trunk branch-ing frequency, secondary crown formation, trunk deviation from the orthotropic growth direction, increase of the polyarchic plan of organization in trunk, trend of branch drooping, disarrangement in crown zoning, increase of the polyarchic plan of organization in branches, immediate reiteration. The most survived initial crown architec-ture is expressed in the open-growth individuals that grow on the slow slopes. The most common way of trans-formation is acropetal mortality of the branches from the trunk. Its intensity depends on the closeness of the stand. Besides light level, crown architecture in the reproductive period of ontogenesis is influenced by the indi-vidual’s position in relief, which affects the aerologic and hydrologic regimes of the phytocoenosis.

Preservation of biodiversity of the genus Hedera L. in the Nikita botanical garden

Representatives of the genus Hedera L. have been cultivated in Nikitsky Botanical Gardens over two centuries. The inventory of the collection has revealed thirty five species, cultivars, and forms, including four relics of cultivation – H. helix ‘Angularis’, ‘Rugosa’, ‘Cavendishii’, and H. hibernica ‘Scutifolia’. The presence of three species, thirty-two cultivars and forms belonging to four species, of which two species and four varieties have reached the generative stage and twenty-six cultivars are at the juvenile stage, has been established. There are seven of nine garden groups in the collection. Within morpho-decorative characteristics, two groups have been identified in terms of the color of the leaf blade: monochromatic green (twenty-four varieties and forms); and variegated (eight varieties and forms, of which there are seven white-spotted and one yellow-spotted varieties). In terms of the color type (multicolored), marginal, median, and suffuse colors have been identified. In terms of the shape of the leaf blades, the following groups have been identified among the species and cultivars: finger-cut, rounded-reniform, ovate-elongated, and irregularly asymmetric. In terms of the growth type at the juvenile stage, most species of the genus proved to be plagiotropic; yet cultivars with orthotropic growth type have also been found (in our collection, this is H. helix ‘Conglomerata’). Promising directions for increasing biodiversity of the genus include their further introduction by way of the generic complex, as well as enlarging the collection, primarily with variegated varieties of different color types and varieties from two garden groups that have not yet been present in the collection.

Role of Dormant Buds in Crown Architecture in Coniferous and Deciduous Trees of the Temperate Zone

The goal of this study is to investigate the localization of dormant buds and the features of the structures arising from the buds in coniferous and deciduous trees in response to age-related changes and various growing conditions. The crown is analyzed using architectural models and the reiteration concept. It is found that the dormant buds in deciduous trees are usually at the bases of annual growth. The dormant buds in conifers are at the bases of annual growth, in the axils of bud scales, or along the entire length of the shoot in the axils of needles and in the shoot distal part. It is revealed that the terminal buds of short shoots can function as dormant buds. The natural aging of skeletal axes is an endogenous factor of the development of shoots from dormant buds in deciduous trees. In evergreen conifers, it is the plant’s response to needle aging. This leads to the induction of the growth of dormant buds and to the acropetal appearance of the buds on the mother axis in conifers. The reiterated complexes of deciduous trees form in response to the completion of the axis development cycle, which initiates dormant buds in the centripetal direction. The reiterated complexes of conifers look like bundles: such structures form due to the easy initiation of dormant buds, which leads to the formation of the shoots of other orders. The reiterated complexes of deciduous trees are characterized by prolonged growth, acrotonicic branching, and orthotropic growth direction. The crown of the old generative and senile deciduous trees consists almost entirely of reiterated complexes. It has been found that conifers have a different degree of participation of dormant buds in the crown formation, which can vary significantly within the same genus. The reiterated complexes of Picea abies (L.) Karst. replace small twigs. In some other species (for example, Picea schrenkiana Fisch. et E. Mey., Abies sibirica Ledeb., Pinussibirica Du Tour.), the reiterated complexes can also replace the skeletal branches extending from the trunk. No secondary crown is formed in Pinus sylvestris L.

ENXERTIA DE PROPÁGULOS TRUNCIFORMES NOS RAMOS DE <i>Araucaria angustifolia</i> E MULTIPLICAÇÃO DE MATRIZES

A enxertia em Araucaria angustifolia é uma alternativa em relação à produção de mudas por sementes. No entanto, a disponibilidade natural de propágulos é muito limitada, uma vez que somente material obtido do tronco (ortotrópico) pode ser utilizado. O objetivo deste trabalho foi avaliar a técnica de enxertia sobre ramos da araucária e o tropismo dos enxertos visando à multiplicação de propágulos. A enxertia foi realizada no início de julho de 2014 por meio da técnica de placagem lenhosa em 20 plantas de Araucaria angustifolia com três anos de idade. Foram utilizadas borbulhas ortotrópicas obtidas de troncos e enxertadas em 43 ramos localizados nos três últimos verticilos superiores, em uma distância de 10 cm do tronco da planta porta-enxerto. Foi avaliada a sobrevivência dos enxertos, a qualidade da cicatrização, o tropismo (ângulo de crescimento), o comprimento e o diâmetro das brotações dos enxertos, bem como o diâmetro dos ramos porta-enxerto e sua relação com o diâmetro da brotação dos enxertos. Os dados foram analisados por meio de ferramentas de estatística descritiva. A enxertia nos ramos da araucária é viável. Apesar das diferenças morfológicas e funcionais, a interação entre enxertos de origem ortotrópica e ramos porta-enxertos plagiotrópicos não interfere no pegamento, na cicatrização e no crescimento inicial dos enxertos, pelo menos até 180 dias após a enxertia. Portanto, a possibilidade de enxertia em vários ramos de uma mesma planta configura uma alternativa para multiplicação de borbulhas ortotrópicas. O tropismo das brotações depende da origem do enxerto. Propágulos trunciformes (obtidos de tronco) não são afetados pela interação quando enxertados em ramos porta-enxertos plagiotrópicos, dando origem a brotações com crescimento ortotrópico e morfologia de tronco juvenil. Os ramos do verticilo superior são os mais indicados para enxertia visando à multiplicação de propágulos ortotrópicos, pois estão menos sujeitos à dominância apical.

Specific features of morphogenesis of sedge (Carex hirta L.) on oil-contaminated soil

Specific features of sedge (Carex hirta) development on oil-contaminated soils are studied for the first time. A decrease in morphometric parameters of leaves and rhizomes and the enlargement of roots under the effect of oil are revealed. The plants of sedge on oil-contaminated soils are characterized by a reduced period of morphogenesis and early ageing of leaves. Rhizomes of these plants are less branched. Their plagiotropic growth is short-term and soon gives way to orthotropic growth. Our investigations yield new data on the morphology, morphogenesis, and ecology of C. hirta.

Irradiance and Developmental Stages of Crown Architecture Affect Shoot Production in <i>Rhododendron reticulatum</i>

Plasticity in crown architecture, contributing to leaf arrangement within crown, is an important feature for whole plant carbon assimilation and survival. In this study, I examined the plasticity in crown architecture to light condition and developmental stage by the changes in shoot production. Rhododendron reticulatum expands crown with orthotropic growth in monopodial branching in young stage, but orthotropic growth is ceased in adult stage. Main stem of young crown is described with monopodial branching regardless of light environment. But multi-layer crown is observed in sun-lit environment rather than mono-layer crown in adult stage. Long shoot production for each branching system (foliage derived from sympodial branching) in young crown is associated with local light environment, but not in adult crown. Long shoot production rate is correlated with long shoot production rate of its mother shoot in young crown, but not in mono-layer crown. These results suggest that young crown expands branches to sun-lit position whereas adult crown reduces congestion of shoots with stochastic shoot production regardless of shoot production of mother shoots. I concluded that both light and developmental stage are important factors for shoot production and constructing crown architecture.

Amplified Fragment Length Polymorphism Marker-Based Genetic Diversity in Tamarind (Tamarindus indica)

Tamarindus indica, commonly called tamarind, is a medium-sized evergreen tree that gives a high yield. The fruit is commonly used as a spice. Despite its commercial importance in the international market, it has been little explored. The genetic diversity and genetic relatedness of 36 tamarind genotypes were studied using amplified fragment length polymorphism (AFLP) markers. Twelve primer pairs were used for the screening of genotypes based on the number and quality of polymorphic fragments produced. These primer combinations produced 702 bands, 627 (89.3%) of which were polymorphic. The mean Nei's gene diversity index was 0.4868, and the Shannon's information index was 0.6802, indicating the abundant genetic diversity within the tamarind population. Among the tamarind genotypes, the dissimilarity coefficient ranged from 4.4 to 14. Cluster analysis grouped all tamarind genotypes into two major clusters (A and B) at 37 linkage distance and two minor clusters each. Cluster A included the genotypes predominantly with brown to dark brown fruit pulp and dark green leaves. However, the straight to semi-curved fruits were sub grouped in sub-cluster ‘A1’ and semi-curved to curved fruits in sub-cluster ‘A2’. Cluster B contained a collection of genotypes predominantly characterized with trees of orthotropic growth and semi-curved fruit shape. A good correlation of these amplified fragment length polymorphism-based groupings with their morphological traits was observed. We found moderate genetic diversity in these tamarind genotypes. The use of AFLP markers and the level of genetic variability detected within Southern Indian tamarind germplasm suggested that this is a reliable, efficient, and effective marker system that can be used for diversity analysis and subsequently in tree breeding programs.

Indução de brotações epicórmicas ortotrópicas para a propagação vegetativa de árvores adultas de <i>Araucaria angustifolia</i>

This study evaluated 4 epicormic (lateral sprouts) sprout-induction methods with adult ( > 20-year-old) araucaria trees: a) branch pruning up to 2/3 total height and leaving 3 different branch sizes in the stem (2-3 cm, 20 cm, and 50 cm); b) 50-cm long cuttings from branches with 30-60 mm diameter and placed in greenhouse or under shade; c) sprouts from stumps; and d) top pruning. It was observed that branches pruned at 20-50 cm long produced more sprouts. Cuttings 50 cm long under shade registered more sprouts than those placed in the greenhouse. Stumps produced 60% sprouts 5-37 cm long, varying from 7-59 single sprouts per stump. One year later, 50% of top-pruned trees produced enough sprouts ( > 5 cm long) to be harvested. It is concluded that branches pruned at 20 to 50 cm long, and basal stumps, were the most efficient sprout producers, especially with orthotropic growth. Meanwhile, long cuttings were not efficient due to high mortality and low sprout induction. Top pruning has a potential, especially for grafting purposes, but still showing low sprout production.

Chara cf. globularis Thuill. (Charophyceae) orthotropic growth under soil culture conditions

Chara cf. globularis Thuill. (Charophyceae) orthotropic growth under soil culture conditions

Transplantation as a method for restoring the seagrass Posidonia australis

Transplant trials of the seagrass Posidonia australis were carried out after loss of seagrasses following eutrophication and increased turbidity in two marine inlets on the south coast of Western Australia. A pilot study in Oyster Harbour measured survival and growth in situ for 4 years. Long-term survival rates were high (96–98%), providing plants were anchored into the sediment. All unanchored plants were lost in the first winter. Following the success of the pilot study, a more comprehensive program began 3 years later with over 500 transplant units collected from either actively growing edges of nearby patches (plagiotropic growth form) or within established meadows (orthotropic growth form). Transplant units from edges expanded at a faster rate compared to units from mid-meadow but increases in shoot numbers were similar. Growth rates in the first 2.5 years averaged 10–20 cm yr −1 horizontal rhizome extension, depending on the source of the transplant units, and 4–12 shoots per initial shoot yr −1, depending on the initial shoot number of the transplant unit. After 5 years, shoot numbers of individual transplants were similar to shoot densities recorded for natural meadows, >500 shoots m −2. Approximately, 10% of transplants from mid-meadow flowered in the first year, whereas transplants from edges flowered only after 5 years. Transplant trials were also established in nearby Princess Royal Harbour at a site selected to test the effect of disturbance by bioturbation from large sand-burrowing worms or by sediment erosion. Survival was lower than in Oyster Harbour, 75–89% in areas with bioturbation but only 14% in areas where sediments were eroded. Growth was poor, <1–2 shoots per shoot yr −1 with high shoot mortality, and low rates of increase in rhizome length, <5 cm yr −1. In areas affected by worm bioturbation, there was almost no horizontal expansion of plants because rhizomes grew vertically to keep pace with sediment deposition. This study showed that Posidonia australis could be transplanted with a high degree of success into a protected embayment previously vegetated with seagrass meadows. After 4 years, flower and seed production had occurred, and transplants spaced 1 m apart began merging. By the fifth year, individual transplant units could no longer be distinguished and planted areas resembled a natural meadow. These findings have important implications for restoration of impaired Posidonia habitats in Australia, using conventional low-cost techniques for transplantation once the sources of injury have abated.

Ecophysiology of the cacao tree

Cacao, one of the world's most important perennial crops, is almost exclusively explored for chocolate manufacturing. Most cacao varieties belong to three groups: Criollo, Forastero and Trinitario that vary according to morphology, genetic and geographical origins. It is cropped under the shade of forest trees or as a monocrop without shade. Seedlings initially show an orthotropic growth with leaf emission relatively independent of climate. The maturity phase begins with the emission of plagiotropic branches that form the tree crown. At this stage environmental factors exert a large influence on plant development. Growth and development of cacao are highly dependent on temperature, which mainly affects vegetative growth, flowering and fruit development. Soil flooding decreases leaf area, stomatal conductance and photosynthetic rates in addition to inducing formation of lenticels and adventitious roots. For most genotypes drought resistance is associated with osmotic adjustment. Cacao produces caulescent flowers, which begin dehiscing in late afternoon and are completely open at the beginning of the following morning releasing pollen to a receptive stigma. Non pollinated flowers abscise 24-36 h after anthesis. The percentage of flowers setting pods is in the range 0.5 - 5%. The most important parameters determinants of yield are related to: (i) light interception, photosynthesis and capacity of photoassimilate distribution, (ii) maintenance respiration and (iii) pod morphology and seed fermentation, events that can be modified by abiotic factors. Cacao is a shade tolerant species, in which appropriate shading leads to relatively high photosynthetic rates, growth and seed yield. However, heavy shade reduces seed yield and increases incidence of diseases; in fact, cacao yields and light interception are tightly related when nutrient availability is not limiting. High production of non-shaded cacao requires high inputs in protection and nutrition of the crop. Annual radiation and rainfall during the dry season explains 70% of the variations in annual seed yields.

Micropropagation of Taxus mairei from mature trees

Micropropagation of mature Taxus mariei was achieved using bud explants derived from approximately 1,000-year-old field-grown trees. Bud explants derived from 1-year-old stecklings raised from rooted cuttings of these trees were included in this study for comparison. Steckling-derived explants collected in different seasons showed high survival rates and good growth performance in vitro, whereas the survival of mature tree-derived explants was highly season dependent, with satisfactory survival in early spring only. During successive subculturing, tissue browning was observed in cultures of both origins, but the problem was circumvented by supplementing the basal medium with 1 g l–1 activated charcoal and 100 mg l–1 silver nitrate. The steckling-derived cultures performed better than mature tree-derived cultures in terms of shoot multiplication and rooting ability. New shoots of both origins exhibited plagiotropic growth, but steckling-derived shoots had a smaller plagiotropic angle (near vertical) than mature tree-derived shoots. In both cases, the plagiotropic angles decreased after these subcultures were maintained in shoot multiplication conditions for half a year. Orthotropic growth was restored in 25% and 10% of the steckling- and mature tree-derived cultures, respectively.

Interactions betweenSphagnum mosses and field layer vascular plants in the development of peat-forming systems

The interactions between field layer vascular plants andSphagnum mosses in peat-forming systems are discussed in terms of differences in growth strategies, access to light, acquisition of mineral nutrients and water and the processes involved in the formation of the micro-topographical structures characteristic for these systems.To keep pace with the vertical growth ofSphagnum, the co-occurring vascular plants require a growth strategy involving continuous movement of the growing point and meristematic tissue upwards and a frequent formation of adventitious roots.The growth form and architecture of the vascular plants determine the occurrence and distribution of the structural units on a mire, the hummocks, lawns and hollows. Dwarf shrubs and other vascular plants with an orthotropic growth pattern characterise hummocks, where they form a firm matrix which reinforces and supports the spongy biomass ofSphagnum. In a similar way, clonal herbs stabilise the lawns because of the predominantly plagiotropic, or only weakly orthotropic, growth pattern of the rhizomes and coarse roots in the upper, oxic layers.Extended periods of drought often may have deleterious effects on the mosses but smaller impacts on the vascular plants because of their more efficient water conducting system. Different sources of mineral nutrients are used bySphagnum (atmospheric deposition) and the vascular plants (mineralisation of the organic matter). The presence ofSphagnum, therefore, reduces the supply of nutrient resources to the vascular plants.Sphagnum thereby gains a competitive advantage. A high rate of mineralisation would be beneficial to the vascular plants by increasing their growth rates causing shading of theSphagnum mosses and covering the moss layer by the above-ground litter fall. However, the slow decomposition ofSphagnum litter keeps the system in balance as it will reduce the nutrient supply to the vascular plants.

Genetic variation in plagiotropic growth in a provenance hybrid cross with Piceaabies

Plagiotropism (angle departure from orthotropic growth) was measured on 360 Norway spruce (Piceaabies (L.) Karst.) clones propagated in 2 subsequent years. The clones were members of 20 full-sib families (18 clones per family) from a provenance hybrid factorial cross involving 10 parents of Norwegian (N) and 10 parents of east-European (EE) origin. The clones from the EE × EE crosses showed an average of 17° and 20° in plagiotropism, those from the N × N group showed 3° and 7° in plagiotropism, while the hybrids (EE × N and N × EE) showed 8° and 13° in plagiotropism in 1988 and 1989, respectively. A large family variation was found in the EE × EE and the hybrid groups (11 and 8.5% of total variation, respectively), but variation was small (3.5%) in the N × N group. The clonal component accounted for 10, 12, and 8.5% of total variation in EE × EE, hybrids, and N × N, respectively. Among the plagiotropic clones, some were heterogeneous with a large within-clone variation and others were homogeneous. The year × family and year × clone components were rather large in EE × EE (5 and 9% of total variation, respectively) but smaller in the hybrids and smallest in N × N. The clones with best growth were not among those with the worst plagiotropic growth. The clonal material is ideal for use in future studies on the genetic and physiological regulation of maturation.

The effect of crown form and plagiotrophy on the growth of Picea abies f. pendula cuttings

Seven‐year old cuttings of pendula spruce were taller and had larger diameters than seedlings or cuttings of Norway spruce of normal crown type. Pendulous cuttings still had plagiotropic growth at age 7 years and the rate of change to orthotropic growth form was weaker than in cuttings with normal crown type. In pendulous cuttings the effect of the origin of ortet and clone on the crown symmetry was significant at age 5 years but not at age 7 years. The effect of clone on the growth form was evident at ages 5 and 7 years, but the effect of the origin of ortet was significant only in 7‐year‐old cuttings. There were no differences in plagiotrophy between clones from 4‐ and 7‐year‐old ortets. For pendulous cuttings the average height and diameter growth decreased in 5‐ and 6‐ year‐old cuttings with less pendulous crown form and with more plagiotropic growth form but this decrease had almost disappeared in 7‐year‐old cuttings.

Mixoploidy and cytotypes: a study of possible vegetative species differentiation in stapeliads (Asclepiadaceae)

Mixoploidy is common in proembryos and embryos as well as in meristems of radicles, primary, secondary and adventitious roots and in innovation shoots of stapeliad species (Asclepiadaceae). The proportion of polyploid cells in the meristem of single adventitious roots is significantly higher than in meristems of primary and secondary roots. This may lead to a complete polyploidisation of adventitious roots. Innovation shoots display a low percentage of polyploid cells, comparable to the condition found in primary and secondary roots. Nevertheless cells of different euploid levels are frequently found. During field studies, however, individuals of a given population were always found to have the same chromosome number: infraspecific polyploidy was found in only three out of 305 investigated stapeliad species. Genera with an orthotropic growth form were found to be diploid throughout, whereas the genera of which the members spread vegetatively by means of innovation shoots, comprise di-, tetra- and polyploid species. This phenomenon can be ascribed to ecological rather than morphological factors. The often proposed hypothesis that new cytotypes can have a vegetative origin is not found to be acceptable.