Changes In Leaf Form Research Articles

Convergent morphology and anatomy in the microphyllous leaves of selected heathland Myrtaceae and Asteraceae

Key messageWe examined leaves of a suite of microphyllous woody plants and describe a little-known form of leaf peltation for the first time and also investigate strongly reflexed leaves in two distantly related lineages.Plants cope with a range of environmental conditions, especially related to water relations, and have developed an array of physiological and structural solutions to maintain a functional water balance. There has been considerable recent work on physiological solutions to water deficit but little attention paid to leaf characteristics. In many species there is a change in leaf form from seedlings to adults. We examine such changes in several small-leaved species from the distantly related Asteraceae and Myrtaceae, some of which develop micropeltate or reflexed leaves as adults. All are native to dry or seasonally dry sites. Three major morphological groups were recognised as follows: (1) leaves erect, nonpeltate and scale-like (Ozothamnus hookeri), (2) leaves erect and peltate (Phaenocoma prolifera, Regelia inops), (3) Leaves reflexed (Olearia lepidophylla, Ozothamnus scutellifolius, Ozothamnus reflexifolius, Melaleuca diosmifolia). The microphyllous peltation in P. prolifera and R. inops in the absence of a meristematic fusion/bridge differs from typically peltate leaves. These small-leaved taxa occur in open, high light environments which are very different from the mesic, shaded understorey habitats of typical peltate-leaved plants. Many small-leaved species have leaves closely appressed to the stem and often with recurved margins. The erect leaves are functionally similar to reflexed leaves. Environmental filtering leads to superficially similar plant forms that may have somewhat different ontological origins. Such morphological forms are examples of convergent evolution in distantly related species but within each family are likely phylogenetically related.

Heterochrony underpins natural variation in Cardamine hirsuta leaf form

A key problem in biology is whether the same processes underlie morphological variation between and within species. Here, by using plant leaves as an example, we show that the causes of diversity at these two evolutionary scales can be divergent. Some species like the model plant Arabidopsis thaliana have simple leaves, whereas others like the A. thaliana relative Cardamine hirsuta bear complex leaves comprising leaflets. Previous work has shown that these interspecific differences result mostly from variation in local tissue growth and patterning. Now, by cloning and characterizing a quantitative trait locus (QTL) for C. hirsuta leaf shape, we find that a different process, age-dependent progression of leaf form, underlies variation in this trait within species. This QTL effect is caused by cis-regulatory variation in the floral repressor ChFLC, such that genotypes with low-expressing ChFLC alleles show both early flowering and accelerated age-dependent changes in leaf form, including faster leaflet production. We provide evidence that this mechanism coordinates leaf development with reproductive timing and may help to optimize resource allocation to the next generation.

Molecular Phylogeny Determined Using Chloroplast DNA Inferred a New Phylogenetic Relationship of <i>Rorippa aquatica</i> (Eaton) EJ Palmer & Steyermark (Brassicaceae)—Lake Cress

North American lake cress, Rorippa aquatica (Eaton) EJ Palmer & Steyermark (Brassicaceae), is listed as an endangered or threatened species. Lake cress shows heterophyllic changes in leaf form in response to the surrounding environment. Therefore, this species has received considerable attention from ecological and morphological perspectives. However, its phylogenetic position and taxonomic status have long been a subject of debate. To analyze the phylogenetic relationship of lake cress, we investigated chloroplast DNA sequences from 17 plant species. The results of phylogenetic reconstruction performed using trnL intron, trnG (GCC)-trnM (CAU), and psbC-trnS (UGA) indicated that lake cress is a member of Rorippa. Moreover, we found that the chromosome number of lake cress is 2n = 30. This result indicated that lake cress might have originated from aneuploidy of triploid species or via intergeneric crossing. Taken together, our results suggest an affinity between lake cress and Rorippa at the molecular level, indicating that lake cress should be treated as Rorippa aquatica (Eaton) EJ Palmer & Steyermark.

Heteroblasty in the moss, Aphanoregma patens (Physcomitrella patens), results from progressive modulation of a single fundamental leaf developmental programme

Leaves at the apex of a mature Aphanoregma patens (Hedw.) Lindb. (Physcomitrella patens (Hedw.) Bruch Schimp. in B.S.G.) gametophore differ markedly in size and form from those at its base. To determine how these differences are produced during development, we first examined qualitative and quantitative differences between successive leaves along the stem and among leaves at different developmental stages. Differences between successive leaves were slight and cumulative. Local changes in cell number and size combined to produce a regularly shaped and approximately bilaterally symmetrical leaf suggesting that cell division and cell expansion are regionally regulated and coordinated at the organ level. The midrib and marginal teeth are discrete characters, which were prefigured by changes in cell shape in leaves that lacked these characters. In leaf primordia, cell proliferation was responsible for most of the changes in leaf form and size early in development and may have continued as cell expansion took over as the primary contributor to leaf growth and morphogenesis. Thus, leaf heteroblasty in Physcomitrella probably results from modulation of a single developmental programme by external and/or internal forces, which alter progressively in intensity as a gametophore grows. We applied exogenous cytokinin and auxin separately to growing cultures to explore their effects on leaf growth. Cytokinin and auxin stimulated leaf cell division and leaf cell elongation, respectively. Also, young upper leaves of gametophores exposed to exogenous auxin closely resembled basal leaves of untreated plants. Therefore, endogenous cytokinins and auxins may be among the modulating internal forces involved in leaf morphogenesis and the establishment of leaf heteroblasty.

Toward elucidating the mechanisms that regulate heterophylly

Introduction The leaves of plants show tremendous variation in size and shape, despite all leaf primordia initially developing as simple protrusions from the flanks of the shoot apical meristem (Tsukaya 2006). Aside from variation among species, some species show abrupt changes in leaf form in response to surrounding environmental conditions. This phenomenon is termed heterophylly, and it is generally defined as the variation in leaf form in a single plant in response to environmental conditions (Zotz et al. 2011). Heterophylly allows us to study not only the environmental plasticity of plant development but also leaf development and regulation of leaf form. However, there is limited information on the mechanisms that regulate heterophylly, probably due to the lack of an adequate model, despite numerous morphological studies that have been conducted on this subject (Arber 1920, Bell 2008, Fassett 1957). A thorough understanding of leaf development is required to understand heterophylly. Many recent studies have contributed toward improving our knowledge of leaf development. In addition to understanding the structure and development of leaves, many genes involved in leaf morphogenesis have been isolated, and their networks have been studied in several model species (Blein et al. 2009, Ichihashi et al. 2011, Moon and Hake 2011, Szakonyi et al. 2010, Tsukaya 2006). Moreover, research on the morphological diversification of leaves during the course of evolution is also in progress. In particular, molecular mechanisms, such as those responsible for the difference between simple and compound leaves and the difference in the complexity of leaf form among related species, have been examined in several model plants and their close relatives (Hay and Tsiantis 2006, Kimura et al. 2008, Piazza et al. 2010). Furthermore, the links between phytohormones and leaf development have been examined in detail (Bilsborough et al. 2011, Braybrook and Kuhlemeier 2010, Koenig et al. 2009, Shani et al. 2010, Umehara et al. 2008). Indeed, phytohormones, particularly abscisic acid (ABA) and ethylene, are critical for heterophyllous transformation in various species (Anderson 1978, Hsu et al. 2001, Kane and Albert 1987, Kuwabara et al. 2003). Thus, it appears that the foundation has been laid toward elucidating the mechanisms of heterophylly To facilitate our investigation on heterophylly, we selected Neobeckia aquatica (Eaton) Greene as the model system. N. aquatica is an aquatic member of Brassicaceae and is closely related to the genera Rorippa and Cardamine (Les 1994). N. aquatica exhibits distinct leaf morphology, depending on the environmental conditions; it exhibits simple leaves in terrestrial environments and Plant Morphology vol. 24 pp. 57-63

Living With Limited Water, Part II: Dynamics of Leaf Rolling, Leaf Water Homeostasis and Water Economy by Hybrid Rice*

Change in leaf form or shape has often been enumerated as a potential means of reducing transpiration by plants experiencing water deficit. Because leaf rolling is the first and foremost visible physiological response to water deficit in rice plant, its dynamic nature and impact on leaf water homeostasis and water conservation were studied in hybrid rice (Oryza sativa L.) in depth. Marked diurnal fluctuations were observed in leaf rolling behavior over short as well as extended periods of time during the drying cycle. Leaves were found unrolled (open) during early and late hours of the day in watered plants, whereas stressed plants exhibited partial rolling (score >3) even in the morning, followed by complete rolling at noon (score 5). The rolling of leaves of watered plants fluctuated within a very narrow range (between score 1 to 2) throughout 20 days of drying cycle, whereas stressed plants, although they did not differ from control plants during the first 5 days of stress, displayed wide fluctuations thereafter, culminating into complete rolling (score 5) with no further diurnal variations in space and time. The pattern of changes in leaf water potential closely followed leaf rolling behavior. Examination of leaves by naked eye and hand lens revealed two distinct textures of surfaces in a given leaf blade, i.e. smooth or ridged, located opposite to each other on either side of the midrib. Quite often, the ridged portion rolled first, followed by the smooth one rolling over the former during the drying cycle as the water potential of the ridged portion was less by 0.48 MPa than the smooth portion at the completion of leaf rolling (score 5). Similarly, the water potential of the apical half portion of the leaf blade was less by 0.41 MPa in comparison with the basal half. In the main, two types of leaf rolling were observed: one was classified as acropetal, which consisted of helical and non-helical rolling, and the other was non-acropetal, characterized by either the ridged portion bending first or ridged and smooth portions bending almost simultaneously. Leaf rolling in etiolated leaves was quicker by 14 min than the green ones; however, darkness delayed the rolling (score 5) of green leaves by 32 min. The rate of transpiration decreased by 52% in manually rolled, excised turgid leaves, which reversed back to the initial rate of water loss once the leaves were unrolled (open). In the absence of water supply to the excised leaves, the spontaneously rolled leaves exhibited 87% water saving (only 13% loss in fresh weight) against 68% saving (32% loss) by leaves mechanically prevented from rolling, suggesting its important role in leaf water homeostasis and water economy during limited-water supply conditions. The findings carry biotechnological applications that may further provide important information for the design of strategies for future engineering of an improved external plant defense system against drought with increased water-use-efficiency in hybrid and super hybrid rice.

Leaf patterns, leaf size and ecologically related traits in high Mediterranean mountain on the Moroccan High Atlas

Leaf traits functional relationship is particularly important in plant ecological strategies, but few data are available from Mediterranean high-altitude environments. We analysed leaf general patterns and leaf trait relationships in 84 perennial species on the High Atlas, Morocco. We examined the correlation amongst leaf size, leaf width and length, plant height and seed size, analysed multi-trait relationships using Structural Equation Models and tested leaf size variation amongst growth forms (functional groups). Species spanned 103 range of leaf size (sub-lepto- to microphylls). Nanophylls (48.8%) were dominant and over-represented in half-shrubs. Tree and rosette herbs were more likely to have large leaf size (nano-micro- and microphylls), whereas shrubs have medium leaf size (nano-micro- and nanophylls) and cushion and half-shrubs have small (sub-lepto- to nanophylls) and narrow leaves. Small-leaved species synchronized their leaf phenological activity with the dry summer months (May–August), and large-leaved species extended throughout the spring until the end of summer following the similar patterns found in lowland Mediterranean environments. Regarding woody species, our results showed a positive and significant relationship between leaf size and plant height and a non-significant relationship between leaf size and seed size. Structural Equation Models showed that variation in leaf size was triggered chiefly by changes in leaf form (leaf width) and plant height, seed size being of no relevance. In our study area, large-seeded species have a relatively wide range of leaf size. The hypothesis that the combination of large seeds and small leaves is allometrically unlikely (except for leptophyll Conifers) was supported in this study.

Spatial distribution of leaf form and the self-thinning exponent are affected by the sensitivity of the response to abscisic acid in anArabidopsis thaliana population

The spatial distribution of leaves is related to the exponent of the self-thinning relationship in plant populations. In this study, we evaluated the fractal dimension of rosette leaves of wild-type (WT)Arabidopsis thaliana and of an abscisic acid (ABA) -insensitive mutant (abi2-1) to test a model of the spatial distribution of leaf form in anArabidopsis population based on subdivision of a cube surrounding the leaf into uniform boxes and to investigate ABA’s affect on this model of the leaf. The values of the self-thinning exponent were -1.31 and -1.45 for WT andabi2-1. The mean dimensions of the box used to model the spatial distribution of leaf form, estimated using our model, were 2.08 and 2.03, respectively. By assuming that the box dimension equals the fractal dimension within the populations, the predicted self-thinning exponent equaled -1.40 for WT and -1.49 forabi2-1. When exogenous ABA was applied to both genotypes, the self-thinning exponent became -1.26 and -1.43 for WT andabi2-1, and the exponents predicted using the dimensions of the box were -1.37 and -1.46, respectively. The empirically predicted exponent equaled that predicted using the dimensions of the box (95% confidence interval). Empirical prediction of the spatial pattern using the two genotypes with and without ABA showed that ABA influenced the spatial form of the rosette leaves. Therefore, sensitivity to ABA can affect self-thinning through genetically determined changes in leaf form and its spatial distribution.

Changes in leaf morphology and anatomy with tree age and height in the broadleaved evergreen species, Eucalyptus regnans F. Muell

Relatively little is known about changes in leaf attributes over the lifespan of woody plants. Knowledge of such changes may be useful in interpreting physiological changes with age. This study investigated changes in leaf morphology and anatomy with tree age and height in the broadleaved evergreen species, Eucalyptus regnans. Fully expanded leaves were sampled from the upper canopy of tree ages ranging from 6 to 240 years, and tree heights ranging from about 10–80 m. There were significant changes in leaf form with increasing tree age and height. Leaf size and specific leaf area (SLA; leaf area/leaf mass) decreased, leaf thickness increased, and leaves became narrower relative to their length, with increasing tree age and height. Cuticle thickness and leaf waxiness, including wax occlusion of the stomatal antechamber, increased with increasing age and height. By comparison, there were no clear trends in stomatal frequency or stomatal length with tree age, although there were curvilinear relationships between an index of total stomatal pore area per leaf lamina and both tree age and tree height. The results support the hypothesis that leaves of E. regnans become more xeromorphic with tree age and height. The results are discussed in relation to their significance for changes in water relations in the canopy with age.

Strong, independent, quantitative genetic control of the timing of vegetative phase change and first flowering in eucalyptus globulus ssp. globulus (Tasmanian blue gum)

Analyses of large open-pollinated and controlled-cross field trials of Eucalyptus globulus ssp. globulus show quantitative genetic independence of the times of first flowering and the abrupt change in leaf form. The onset of each of these critical developmental events is under moderate to strong additive genetic control in this taxon, with individual narrow-sense heritabilities of 0.4-0.6 and 0. 2-0.9, respectively, and little nonadditive genetic control. This strong additive genetic control and the independence of these processes provide the genetic environment for rapid heterochronic microevolution.

Photosynthesis and photorespiration in freshwater organisms: Amphibious plants

Amphibious macrophytes are remarkable for their ability to photosynthesise in air and water, since these two media are very different. Specifically, water has a higher water potential, greater thermal stability, but reduced availability of light, CO 2 and O 2 than air. In heterophyllous amphibious plants, production of an appropriate leaf form is triggered by water potential, temperature, photoperiod, ratio of red to far-red light, or CO 2 concentration in some species. These effects may be mediated by endogenous growth regulators. High concentrations of CO 2 are necessary to saturate photosynthesis of submerged shoots of amphibious species underwater, largely because of boundary-layer resistance. The low rate of CO 2 supply underwater is reflected in low activities of carboxylating and other enzymes involved in photosynthesis in submerged shoots. Any limitation of photosynthesis by CO 2 is rarely overcome by HCO 3 use; rather, the aerial portion may allow access to an environment with greater availability of CO 2. Alternatively, submerged-CAM found in amphibious species such as Isoetes howellii Engelm. may reduce CO 2 limitation of photosynthesis by exploiting the higher CO 2 concentration often found at night. Photorespiration is present in submerged and aerial shoots, but incubation underwater at high temperature (30°C) and long days (14 h) reduces photorespiration in both shoot types in Myriophyllum brasiliense Cambess. and Proserpinaca palustris L. Higher relative activities of phosphoenolpyruvate carboxylase in submerged compared with emerged shoots of some amphibious species may be a response which reduces photorespiration. Submerged shoots require less light to saturate photosynthesis and have generally lower light compensation points at air levels of CO 2. It is unclear whether or not this is partly caused by the interacting effect of CO 2. Amphibious macrophytes are poorly understood, and future work should distinguish between the effects of air and water, environmental pretreatment, and linkage to changes in leaf form when photosynthetic and photorespiratory responses of submerged and aerial shoots are compared.

Response of Leaf Water Potential, Stomatal Resistance, and Leaf Rolling to Water Stress

Numerous studies have associated increased stomatal resistance with response to water deficit in cereals. However, consideration of change in leaf form seems to have been neglected. The response of adaxial and abaxial stomatal resistance and leaf rolling in rice to decreasing leaf water potential was investigated. Two rice cultivars were subjected to control and water stress treatments in a deep (1-meter) aerobic soil. Concurrent measurements of leaf water potential, stomatal resistance, and degree of leaf rolling were made through a 29-day period after cessation of irrigation. Kinandang Patong, an upland adapted cultivar, maintained higher dawn and midday leaf water potential than IR28, a hybrid selected in irrigated conditions. This was not explained by differences in leaf diffusive resistance or leaf rolling, and is assumed to result from a difference in root system extent.Stomatal resistance increased more on the abaxial than the adaxial leaf surface in both cultivars. This was associated with a change in leaf form or rolling inward of the upper leaf surface. Both responses, increased stomatal resistance and leaf rolling, were initiated in a similar leaf water potential range (-8 to -12 bars). Leaves of IR28 became fully rolled at leaf water potential of about -22 bars; however, total leaf diffusive resistance was only about 4 to 5 seconds per centimeter (conductance 0.25 to 0.2 centimeter per second) at that stage. Leaf diffusive resistance and degree of leaf rolling were linearly related to leaf water potential. Thus, leaf rolling in rice may be used as an estimate of the other two less obvious effects of water deficit.

Observations on two coastal ecotypes ofSelliera radicansCav. (Goodeniaceae) growing in the Manawatu District of New Zealand

Abstract Morphologically distinct forms of Selliera radicans Cav. growing on silty flats in the estuary of the Rangitikei River and on sand plains in the dunes c. 1.3 km to the south are described. Small pieces of S. radicans rhizome were collected from these two areas and planted in pots containing either sand from the sand plain site, sandy-silt from the estuary site, or a standard potting mix. All were grown for 10 months under identical conditions in a glasshouse. Measurements of leaf dimensions at the beginning and end of the experiment support the view that the two forms are genetically distinct, and can be regarded as ecotypes. A clone of the sand plain form was grown for 34 months in a garden, and the resulting changes in leaf form are illustrated. Several other species have dwarf forms on the sand plains, and it is tentatively suggested that ecotypic differentiation has occurred in these species also.

A CORRELATION BETWEEN THE APICAL CELL AND THE HETEROBLASTIC LEAF SERIES IN MARSILEA

Plants of three species of Marsilea (M. vestita, M. villosa, M. drummondii) were grown in sterile culture under controlled conditions, and stem apices were sampled at one of the three heteroblastic leaf forms typical of this plant: spatulate, bifid, or quadrifid leaves. Statistical analyses were made of the relationship between the area of the apical cell and the leaf form which the plant produces under varied growth conditions. For all three species there is a statistically significant correlation (1% level) between apical cell area and leaf form. The analysis indicates that 83% of the variation in apical cell area in Marsilea vestita, 52% in M. villosa, and 54% in M. drummondii can be related to the change in leaf form. An increase in the glucose concentration of the culture medium increases the average apical cell area, and the addition of the protein synthesis inhibitor 2‐thiouracil at concentrations of 10 mg/liter and 25 mg/liter decreases the average apical cell area. A certain average apical cell area is necessary for the production of a particular leaf form in the series. The area is a relative rather than an absolute size under any one particular growth condition. Any growth condition which inhibits or reverses the increase in apical cell size (which is typical of the normal growth pattern) will inhibit or reverse the normal heteroblastic series. Under constant conditions, the average size at which particular leaf forms develop appears to be species specific. These results confirm a generally held idea that the apex size is related to the heteroblastic leaf series, and they indicate that the area of the apical cell is the key factor rather than the volume or the number of cells of the apex. Undoubtedly the area of the apical cell is only a reflection of the physiological or morphological characteristics of the apical meristem that underlie the heteroblastic leaf series and which currently do not appear to lend themselves to more direct quantitative analysis.

A Correlation between the Apical Cell and the Heteroblastic Leaf Series in Marsilea

Plants of three species of Marsilea (M. vestita, M. villosa, M. drummondii) were grown in sterile culture under controlled conditions, and stem apices were sampled at one of the three heteroblastic leaf forms typical of this plant: spatulate, bifid, or quadrifid leaves. Statistical analyses were made of the relationship between the area of the apical cell and the leaf form which the plant produces under varied growth conditions. For all three species there is a statistically significant correlation (1% level) between apical cell area and leaf form. The analysis indicates that 83% of the variation in apical cell area in Marsilea vestita, 52% in M. villosa, and 54% in M. drummondii can be related to the change in leaf form. An increase in the glucose concentration of the culture medium increases the average apical cell area, and the addition of the protein synthesis inhibitor 2-thiouracil at concentrations of 10 mg/liter and 25 mg/liter decreases the average apical cell area. A certain average apical cell area is necessary for the production of a particular leaf form in the series. The area is a relative rather than an absolute size under any one particular growth condition. Any growth condition which inhibits or reverses the increase in apical cell size (which is typical of the normal growth pattern) will inhibit or reverse the normal heteroblastic series. Under constant conditions, the average size at which particular leaf forms develop appears to be species specific. These results confirm a generally held idea that the apex size is related to the heteroblastic leaf series, and they indicate that the area of the apical cell is the key factor rather than the volume or the number of cells of the apex. Undoubtedly the area of the apical cell is only a reflection of the physiological or morphological characteristics of the apical meristem that underlie the heteroblastic leaf series and which currently do not appear to lend themselves to more direct quantitative analysis.

PHYSIOLOGICAL CONTROL OF THE DIMORPHIC LEAVES OF CYAMOPSIS TETRAGONOLOBA

Cyamopsis tetragonoloba, commonly called guar, produces two leaf forms: a simple leaf and a trifoliate leaf. During ontogeny, the simple leaf is always produced prior to the appearance of any trifoliate forms. The number of simple leaves initiated during the early phase of ontogeny is partially controlled by environmental conditions. Both short daylengths and high temperatures delay the production of the trifoliate form. Treatments using gibberellic acid and reduced light intensity favor production of simple leaves. Removing the cotyledons of young seedlings also favors the simple leaf form. Some consider a change in leaf form as an indication of a change from the juvenile phase to the adult phase of growth. However, in guar when the capability to reproduce is used as the basis for the determination of phase change, a change in leaf form is a separate phenomenon.

PHENYLBORIC ACID, A CHEMICAL AGENT SIMULATING THE EFFECT OF THE LANCEOLATE GENE IN THE TOMATO

Lanceolate, a spontaneously occurring mutant in the tomato, which has 1 dosage of the mutant gene, causes a change in the shape of the leaf from the odd‐pinnately compound leaf characteristic of the normal tomato plant to a simple, much‐reduced in size lanceolate leaf. Homozygous mutant plants, which have 2 dosages of the mutant gene, exhibit extreme lanceolate features, producing a simple narrow leaf, similar in shape to the lanceolate leaf but much smaller in size. The mutant allele of the lanceolate gene causes, in addition to the change in leaf form, the increase in activities of the following oxidative enzymes: tyrosinase, laccase, peroxidase and catalase. The lanceolate gene exhibits a dosage effect with respect to these enzymes, homozygous lanceolate showing the highest activity; heterozygous lanceolate, an intermediate; and normal, the lowest activity. Treatment of germinating normal tomato seeds with phenylboric acid simulated the effect of the mutant allele of the lanceolate gene with regard to both the induction of a lanceolate leaf and the increase in the activity of the above 4 oxidative enzymes; phenylboric acid treatment resulted in relatively higher increase in the activities of the oxidative enzymes in normal and heterozygous lanceolate than in homozygous lanceolate. A direct and close relationship existed between the shape of the leaf and the activity of the enzymes during leaf development. It is suggested that the mutant allele of the lanceolate gene, as well as phenylboric acid, induces an increase in activity of the oxidative enzymes and that this increase in enzyme activities brings about the orderly change in form.

Leaf Development: Index of Plant Response to Environmental Factors1

SynopsisDaily morphologic changes in plants were visually detected and recorded quantitatively. Changes in leaf form were used to determine the relative influences of environmental factors on plant growth. The lag in effect of each environmental factor was used in subsequent multiple regression analyses, responses to individual factors were determined, and a growth‐prediction equation was made.

Variations in Cytology and Gross Morphology of Taraxacum. II. Senescence, Rejuvenescence, and Leaf Variation in Taraxacum

1. Senescent and rejuvenescent change in leaf form is a prime factor in producing variations in Taraxacum vulgare and T. laevigatum. 2. Senescence produces a steadily increasing degree of dissection, and frequently of hairiness; rejuvenescence restores the unsegmented juvenile seedling leaf form. 3. These changes are independent of changes in (a) absolute leaf area, (b) total xylem duct capacity in successive leaves, (c) mean xylem duct capacity in successive leaves, (d) ratio of total capacity to leaf area, and (e) number of vein islets per unit area of leaf. 4. Senescence, however, is accompanied by a marked increase in carbohydrate-nitrogen ratio and rejuvenescence by its abrupt decrease. 5. Environmental factors are of secondary importance in so far as studied. Moist atmosphere causes elongation of leaves.Time of seeding may govern time of flowering and rejuvenation; this in turn may affect degree of dissection in plants at flowering time. 6. Pure stands of distinctive variants in certain cases may be explained by environmental control of seed germination time. 7. The majority of so-called specific characters in Taraxacum are subject to extreme fluctuation. 8. These fluctuations in other than leaf characters are in some cases due to senescent change, in others to environmental factors, but mostly to causes not known. 9. Such fluctuations, together with senescent and ecological changes in leaf form, are potent enough when working upon the two common species to produce phenotypes duplicating many so called species, 10. Many such species may be valid, but so far the physiological evidence has not been obtained.