STUDIES ON THE FLORAL HISTOGENESIS AND PHYSIOLOGY OF PERILLA. III. EFFECTS OF INDOLEACETIC ACID ON THE FLOWERING OF APICAL BUDS AND EXPLANTS IN CULTURE

Raghavan , V. (Princeton U., Princeton, N. J.) Studies on the floral histogenesis and physiology of Perilla. III. Effects of indoleacetic acid on the flowering of apical buds and explants in culture. Amer. Jour. Bot. 48(10): 870–876. Illus. 1961.—The responses of apical buds and explants of a short-day plant, Perilla frutescens (L.) Britt. var. 'Tall Late,' when grown in vitro in White's medium supplemented with indoleacetic acid (IAA) and subjected to short-days (SD) or long-days (LD), are described. Additions of varying concentrations of IAA to the medium inhibited the flowering of the photoinduced apical buds in 2 ways. There was a progressive delay in the appearance of the first signs at the apex and a gradual transition from the more flower-like structures in lower concentrations of IAA (0.1 mg/liter) to sterile cones in higher doses. The sterile cones had florets with well-developed calyx and corolla lobes, but lacked the sporogenous tissues. When subjected to LD, visible signs were observed only in buds grown in 0.1 and 1.0 mg/liter IAA, the pronounced effect of the auxin being in the step-wise inhibition in the formation of the non-sporogenous tissues of the differentiating florets. Flowering of the explants with the 1st pair of unfolded leaves was also inhibited by IAA in either SD or LD, but the 1st signs appeared relatively faster than in apical buds. When photoinduced, explants with the 1st and 2nd pairs of unfolded leaves flowered in all concentrations of IAA tried (up to 100 mg/liter) while those kept in LD remained entirely vegetative.     

DEVELOPMENT OF FLORAL ORGANS IN THE SITES OF LEAF PRIMORDIA IN PINGUICULA VULGARIS

Plants of Pinguicula vulgaris L. have either clockwise or counterclockwise spiral phyllotaxy. The inception of floral primordia occurs in leaf sites as a normal sequence of development. Only two leaf primordia initiated late in the season develop into floral primordia in the following year. They do not represent a direct modification of the apical meristem nor of the detached meristem. The apical meristem continues to produce leaves in the vegetative phase and flowers in the reproductive phase, and thus the plants show a monopodial growth. Axillary buds are not developed in this perennial species and instead additional buds of adventitious ontogeny appear. Such buds are produced on the older leaves of larger plants, and they are extremely useful in the vegetative propagation of the species.     

GIBBERELLIN-INDUCED INHIBITION OF FLORAL INITIATION IN FUCHSIA

The ‘Lord Byron’ cultivar of Fuchsia hybrida is a long day plant for which GA acts as an inhibitor of flower initiation. At the dosages required to inhibit initiation (0.025 μg per plant) GA also promotes increased stem elongation but causes no other departures from normal development. Similar tests with auxins, antiauxins, kinins, and other substances showed no effect on flower initiation at dosages equivalent to that for GA. At 10- to 100-fold greater dosages, auxins, kinins, and anti-auxins inhibit not only flower initiation but also vegetative development. Thus the effect of GA on flower initiation appears to be unique, although other hormonal substances, such as abscisin, have not been tested. GA-induced inhibition is directly proportional to the dosage applied and inversely to the strength of long day induction (as measured by the number of long days). GA is most effective when applied to the terminal bud rather than to the mature leaves, suggesting that it is active at the site of flower initiation rather than in the leaves. If it is applied after translocation of the floral stimuli from the leaves, GA does not prevent flower initiation. Regardless of the dose applied, GA is less effective if applied later rather than earlier during LD induction. The inhibitory effect persists for several days. For example, an 0.85 μg dosage causes an 8–10-day delay in initiation; lower dosages have reduced effects. GA inhibits flower initiation but has no effect upon flower development. The rate of bud development is the same in GA-treated and control plants. Apparently no more than one to two axillary buds immediately below the apical meristem are receptive to long day-induced floral stimuli from the leaves. Regardless of the daylength conditions axillary buds more basal do not initiate flowers but develop into branch axes. The effect of a long day treatment persists for a very short time, perhaps no longer than the inhibition caused by minimal GA dosage. Thus flower initiation continues for a very short time following the end of long day induction. The significance of these findings is discussed in relation to the many reports of GA-induced inhibition as well as promotion of flower initiation. In particular, the discussion concerns the nation that flower initiation in fuchsia may be controlled by a gibberellin-like transmissible inhibitor.     

FLORAL BIOLOGY OF MAGNOLIA

The floral biology of eight species of Magnolia native to the United States is described. The flowers are protogynous. They are pollinated by several species of beetles that enter buds as well as closed and open flowers to feed on nectar, stigmas, pollen, and secretions of the petals. Individual flowers persist from two to four days and undergo a series of petal, stigma, and stamen movements that assure pollination by beetles. It is suggested that the flowers of Magnolia are highly specialized for exclusive pollination by beetles. These specialization mechanisms produce large quantities of food for the beetles and deny other types of insects (bees, moths, etc.) access to the flowers at critical stages in the pollination process, i.e., when stigmas are mature and pollen is shed.     

Transition from absolute to quantitative short-day control in Nicotiana tabacum cv. Maryland Mammoth

The response in vitro of thin cell layers, excised from different stem regions of Nicotiana tabacum cv. Maryland Mammoth plants at various developmental stages, was studied under different photoperiodic treatments. The aim was to determine at which stage of plant development, and in which region of the stem, the absolute short-day requirement, indispensable for the induction of the flowering process in this genotype, becomes quantitative and whether it remains short-day. The explants were cultured on a medium suitable for flower neoformation, and were exposed for 30 days to the following treatments: continuous darkness, 8 h light/16 h dark per day, 16 h light/8 h dark per day, and continuous light. The first flowers on explants were observed from plants that were still in the vegetative state, but whose apex showed an accelerated production of axillary vegetative buds, as observed histologically. These explants were excised from the first 10 internodes below the first node with a leaf ≥ 5 cm in length (apical site), and produced flowers only under short-day treatment. When the apical dome initiated the organization of the terminal flower, the apical site explants developed flowers under both short-day and long-day treatments. At the same stage, explants from the 15th to the 20th internode below the first leaf ≥ 5 cm in length also formed flowers, but only under short-day. When the plant showed a complete inflorescence, flowers were also present on explants from the most basal stem internodes and from the inflorescence branches. At this stage, flower neoformation occurred under all treatments; however, under short-day the number of explants showing flowers not associated with vegetative buds on the same sample greatly exceeded that observed under other treatments, as did the mean number of flowers per explant (except the basal regions). In conclusion, in the post-inductive phases of the flowering process, the photoperiodic requirement of this genotype is always short-day. The superficial tissues of the stem require either absolute or quantitative short-day treatment, depending on their position on the stem and the stage of evolution of the flowering process in the terminal apex.     

The apical bud as a novel explant for high-frequency in vitro plantlet regeneration of <Emphasis Type="Italic">Perilla frutescens</Emphasis> L. Britton

In this study, we established an in vitro regeneration system to maximize the recovery of leafy perilla (Perilla frutescens L. Britton) plantlets as part of developing a molecular biotechnology-based metabolic engineering program for this crop plant. Hypocotyl segments including the apical buds were used as explants for the direct production of shoots without an interim callus phase. The number of shoots produced from the apical buds peaked within 3–4 weeks, and the shoots were subsequently cultured on Murashige and Skoog (MS) media supplemented with 2 mg l⁻¹ benzylaminopurine (BA). Spontaneous rhizogenesis was observed after 7–10 days of culture on MS media without hormonal additives. The rooted shoots developed into normal plants in soil after hardening on distilled water for 3–4 days. The average plantlet regeneration frequency was higher for the apical buds (64.33%) than for the top (15.66%), middle (4%), and basal (1.33%) segments of the hypocotyls. This regeneration system demonstrates a capacity for high-frequency plantlet recovery and thus should be considered for use in the genetic manipulation of leafy perilla.     

FLORAL DEVELOPMENT OF A FLOWER-STRUCTURE MUTANT IN SOYBEANS,GLYCINE MAX (L.) MERR. (LEGUMINOSAE)

A flower-structure mutant with cleistogamous flowers (but often with an exposed style and stigma) and very low seed set was found in soybeans (Glycine max (L.) Merr.). The mutant, assigned Genetic Type Collection Number T269, is controlled genetically by duplicate recessive genes, fs₁ and fs₂. A study of flower development in T269 plants was undertaken to determine the cause of the low seed set. Both normal and mutant flower buds were observed with a light microscope by using paraffin serial sections and with a scanning electron microscope. Measurements of various floral structures were taken to verify differences observed between mutant and normal flowers. Young mutant flower buds had longer carpels and larger receptacles than did normal flower buds. These two factors caused the sepals to be positioned abnormally, which, in turn, prevented normal development of the petals. The abnormal petal development prevented staminal tube elongation, and a spatial separation between the anthers and stigma existed at anthesis, preventing self-pollination. Observations of the gynoecium of mutant flowers revealed that megasporogenesis and megagametogenesis were normal but that other features of ovule ontogeny were abnormal. In all ovules examined, the outer integuments failed to form micropyles. In addition, many ovules were positioned abnormally. The degree of aberration varied even within a carpel, but we estimated that at least 75% of the ovules were too aberrant to be functional. Therefore, the low seed set on T269 plants was due both to a lack of self-pollination and to partial female sterility. It is the only naturally occurring structural sterile reported in soybeans to date.     

Bud regeneration from inflorescence explants for rapid propagation of geophytes in vitro

Bulbs, corms and other subterranean storage organs are commonly used as explant source material for the establishment of geophytes in vitro. The inflorescence stalk was found to be a good alternative source of explants to overcome explant contamination originating from underground storage organs. Inflorescence explants of Allium, Dichelostemma, Eucrosia, Gladiolus, Haemanthus, Hyacinthus, Narcissus, Nerine and Ornithogalum were used to establish cultures in vitro. The regeneration potential of the inflorescence was compared with regeneration from bulb twin scales or from apical buds isolated from corms. Gladiolus (Iridaceae) explants isolated from the floral stem just below the expanding florets, still enclosed in the bracts, were highly regenerative in the presence of naphthalene acetic acid (NAA) and kinetin. In the presence of 2,4-dichlorophenoxyacetic acid and benzyl aminopurine (BA) in the medium, explants isolated from the tissue at the junction between the peduncle and the pedicels of a young Nerine (Amaryllidaceae) inflorescence regenerated several buds. The scapes of young unemerged inflorescences taken from sprouting bulbs of Narcissus (Amaryllidaceae), following a 15 °C storage treatment, regenerated buds in the presence of NAA, BA, elevated phosphate and adenine sulfate in the medium. The number of buds regenerated depended on the location on the scape from which the explant was isolated, and on the duration of the 15°C treatment. In Allium (Alliaceae), capitulum tissue between the flower pedicels regenerated buds. Explants excised from the peduncle, as well as the pedicel-peduncle junction of Dichelostemma (Alliaceae), Ornithogalum, Hyacinthus (Hyacinthaceae) and Eucrosia (Amaryllidaceae) regenerated several buds in each type of explant. In the case of Haemanthus (Amaryllidaceae), pedicel-peduncle junction explants regenerated buds only when excised from inner whorl florets. Propagation protocols and the potential use of expediently isolated inflorescence explants for efficient micropropagation of geophytes are discussed. Received: 1 September 1999 / Revision received: 13 December 1999 / Accepted: 13 December 1999     

SYMMETRY AND DEVELOPMENT OF BUTOMUS UMBELLATUS (BUTOMACEAE) AND LIMNOCHARIS FLAVA (LIMNOCHARITACEAE)

The prostrate rhizome of Butomus umbellatus produces branch primordia of two sorts, inflorescence primordia and nonprecocious vegetative lateral buds. The inflorescence primordia form precociously by the bifurcation of the apical meristem of the rhizome, whereas the non-precocious vegetative buds are formed away from the apical meristem. The rhizome normally produces a branch in the axial of each foliage leaf. However, it is unclear whether the rhizome is a monopodial or a sympodial structure. Lateral buds are produced on the inflorescence of B. umbellatus either by the bifurcation or trifurcation of apical meristems. The inflorescence consists of monochasial units as well as units of greater complexity, and certain of the flower buds lack subtending bracts. The upright vegetative axis of Limnocharis flava has sympodial growth and produces evicted branch primordia solely by meristematic bifurcation. Only certain leaves of the axis are associated with evicted branch primordia and each such primordium gives rise to an inflorescence. The flowers of L. flava are borne in a cincinnus and, although the inflorescence is simpler than that of Butomus umbellatus, the two inflorescences appear to conform to a fundamental body plan. The ultimate bud on the inflorescence of Limnocharis flava always forms a vegetative shoot, and the inflorescence may also produce supernumerary vegetative buds. Butomus umbellatus and Limnocharis flava exhibit a high degree of mirror image symmetry.     

Endogenous levels of abscisic acid,indole-3-acetic acid and benzyladenine during in vitro bud growth induction of Wild cherry (Prunus avium L.)

Levels of endogenous growth substances (abscisic acid: ABA; indole-3-acetic acid: IAA) and applied benzyladenine (BA) were quantified during the eight first days of in vitro propagation of Wild Cherry (Prunus avium L.). Axillary buds from the middle part of the explants started to grow at day 2, thus were released from apical dominance. Hormone levels were measured in the apical, median and basal parts of the explants using an avidin-biotin based enzyme-linked immunosorbent assay (ELISA) after a purification of the extracts by high performance liquid chromatography (HPLC). All hormones showed rapid and considerable changes during the first eight days of growth. Exogenous IBA was probably transformed into IAA mainly in the basal part of the explant, and BA penetrated quickly. ABA levels were transiently enhanced in the apical part of the explants bearing young leaves. These phenomena are discussed in connection with the axillary bud reactivation.     

THE EVOLUTION OF FLORAL COLOR CHANGE: POLLINATOR ATTRACTION VERSUS PHYSIOLOGICAL CONSTRAINTS IN FUCHSIA EXCORTICATA

Field experiments showed that the green-to-red color change in the flowers of Fuchsia excorticata is age-dependent, rather than pollination-induced. Nectar is produced only in green and, to a lesser extent, intermediate-phase flowers; red flowers are postreproductive and are avoided by pollinators (bellbirds). Additional experiments suggested that the red flowers are retained because pollen tubes require at least three days to reach the ovaries, and abscission of the floral tube and accompanying style requires at least another 1.5 days. The change in color directs pollinators away from the postreproductive flowers while these physiological processes are occurring, thereby increasing foraging efficiency and visitation to flowers that are still capable of receiving and donating pollen. No evidence was found to suggest that red-phase flowers function to attract pollinators. Finally, we suggest that the color change evolved through neotenous retention of the green coloration of buds and is a derived trait reflecting an interaction between natural selection and physiological constraints.     

DEVELOPMENT OF THE INFLORESCENCE AND “CROWN” OF ANANAS COMOSUS AFTER TREATMENT WITH ACETYLENE,NAA, AND ETHEPHON

The vegetative apex of the Masmerah cultivar of pineapple, Ananas comosus (L.) Merr., is dome-shaped and has the typical tunica-corpus organization. Treating the plant with acetylene, NAA or ethephon transforms the vegetative apex to a reproductive one. As early as two days after treatment the apical height and width increase, attaining maximum dimensions in 8–14 days, after which they progressively decrease. During 4–6 weeks, when the full complement of florets has been initiated, the dimensions of the apex are again increased and leaves that form the fruit crown are initiated. A cambium-like zone at the base of the corpus is noticed 2–4 days after the application of acetylene and ethephon. Acetylene and NAA result in a more pointed apex initially, while ethephon results in a more broadened apex. The largest increase in apical height and width is seen when NAA is used. The development of the inflorescence from the time the full complement of florets is formed to fruit maturity is described. Cell multiplication is the main cause of peduncle elongation and inflorescence head enlargement, although towards the later stages of growth, cell enlargement contributes more to the increase in size of the organs.     

HORMONAL REQUIREMENTS OF EXCISED DIANTHUS CARYOPHYLLUS L. SHOOT APICAL MERISTEM IN VITRO

Whereas a medium containing kinetin alone enabled a few Dianthus caryophyllus L. apical meristem dome explants to develop into rooted plants, the highest frequency of plants was obtained in one containing supplements of both IAA and kinetin. In an unsupplemented medium, continued development required that explants have 2 pairs of primordial and a pair of expanding leaves. Kinetin alone caused production of many new leaves, but the development was significantly less than when it was furnished in combination with IAA. IAA given alone caused meristem explants to develop primarily callus, roots, and a few leaves. Gibberellin and abscisic acid were without promotive effects on leaf and shoot formation. A balance of hormonal substances, synthesized in young leaf structures and relocated to the meristem, is proposed as the fundamental mechanism that regulates new leaf initiation in the shoot apex.     

LABILE SEX EXPRESSION IN ANDROMONOECIOUS SOLANUM HIRTUM: FLORAL DEVELOPMENT AND SEX DETERMINATION

Sex expression (the proportions of hermaphrodite and staminate flowers produced) of the andromonoecious species Solatium hirtum is labile, and this lability of whole plant sex expression is due to labile sex expression of individual floral buds. In this paper I examine the developmental processes that underlie the differences in floral sex expression of hermaphrodite and staminate flowers of Solarium hirtum, focusing particularly on the processes responsible for the observed lability of floral sex expression. Differences in bud growth rate and relative growth of floral organs in these buds are evident at about the time of megasporocyte meiosis (11–12 days before anthesis). However, gynoecial sterility in staminate buds does not occur until just 6–7 days before anthesis. At this time, abnormalities in ovule development occur in staminate buds: the ovules begin to appear necrotic, the integumentary tapetum collapses, and the megagametophytes of many ovules cease normal development. These observations are consistent with the predictions of labile floral development.     

INFLORESCENCE AND FLORAL DEVELOPMENT IN HOUTTUYNIA CORDATA (SAURURACEAE)

The inflorescence of Houttuynia cordata produces 45–70 sessile bracteate flowers in acropetal succession. The inflorescence apical meristem has a mantle-core configuration and produces “common” or uncommitted primordia, each of which bifurcates to form a floral apex above, a bract primordium below. This pattern of organogenesis is similar to that in another saururaceous plant, Saururus cernuus. Exceptions to this unusual development, however, occur in H. cordata at the beginning of inflorescence activity when four to eight petaloid bract primordia are initiated before the initiation of floral apices in their axils. “Common” primordia also are lacking toward the cessation of inflorescence apical activity in H. cordata when primordia become bracts which may precede the initiation of an axillary floral apex. Many of these last-formed bracts are sterile. The inflorescence terminates with maturation of the meristem as an apical residuum. No terminal flowers or terminal gynoecia were found, although subterminal gynoecia or flowers in subterminal position may overtop the actual apex and obscure it. Individual flowers have a tricarpellate syncarpous gynoecium and three stamens adnate to the carpels; petals and sepals are lacking. The order of succession of organs is: two lateral stamens, median stamen, two lateral carpels, median carpel. The three carpel primordia almost immediately are elevated as part of a gynoecial ring by zonal growth of the receptacle below the attachment of the carpels. The same growth elevates the stamen bases so that they appear adnate to the carpels. The trimerous condition in Houttuynia is the result of paired or solitary initiations rather than trimerous whorls. Symmetry is bilateral and zygomorphic rather than radial. No evidence of spiral arrangement in the flower was found.     

Flower formation in vitro in a quantitative short-day tobacco: interrelation between photoperiod and infructescence development

The flowering response of thin layers excised from branch internodes of Nicotiana tabacum cv. Maryland Catterton (quantitative short-day plant for induction) was studied under three photoperiodic treatments. The explants were excised from inflorescences bearing flowers only, flowers and green fruits, or from infructescences with green fruits only. The aim of the study was to investigate the post-inductive photoperiodic effects on in vitro flower bud formation in a quantitative short-day tobacco and the relation with infructescence development. Short days quantitatively enhanced the flower bud regeneration capacities of explants in all stages of development, both as number of explants induced to produce flowers and as mean number of flowers per explant. There was no significant difference in flower bud formation on explants of the first two stages, which produced much more flowers than those of the third stage. Observations in planta showed that, during the 20 days separating the second stage from the first stage, there was no significant difference in the number of floral buds and flowers present on the inflorescence; however, the branch internodes lengthened, as did the floral buds and flowers. During the 10 days leading to the third stage, the number of capsules did not change significantly, but a high rate of floral abscission occurred. The present results show that in Nicotiana tabacum cv. Maryland Catterton short day quantitatively controls not only the inductive step of the flowering process, but also affects the capacity to regenerate flower buds during the late post-inductive phases. The responsiveness to the photoperiodic signal decreases only when the plant exhibits only fruits.     

THE DEVELOPMENTAL ANATOMY OF LONG-BRANCH TERMINAL BUDS OF PINUS BANKSIANA

A study of the composition of long-branch terminal buds (LBTB) of Pinus banksiana Lamb. and the yearly periodicity associated with their formation, development, and elongation was undertaken. Each LBTB has lateral bud zones and zones of cataphylls lacking axillary buds. When present, staminate cone primordia differentiate from the lowest lateral buds in the lowest lateral bud zone of the LBTB. Ovulate cone primordia and lateral long-branch buds can differentiate from the upper lateral buds in any lateral bud zone. When both types of buds are present, lateral long-branch buds are uppermost. Dwarf-branch buds occur in all lateral bud zones. During spring LBTB internodes elongate, new cataphylls are initiated, dwarf branches elongate, needles form and elongate, pollen forms and is released, and ovulate cones are pollinated. During summer buds form in the axils of the newly formed cataphylls. By early fall the new LBTB are in overwintering condition and the four types of lateral buds are discernable. The cytohistological zonation of the LBTB shoot apex is similar to that of more than 20 other conifer species. Cells in shoot apices of pine are usually arranged in distinct zones: apical initials, subapical initials, central meristem, and peripheral meristem. Periclinal divisions occur in the surface cells of the apex; therefore no tunica is present. At any given time, shoot apex volume and shape vary among LBTB in various positions on a tree. In any one LBTB on a tree, shoot apex shape changes from a low dome during spring to a high dome during summer to an intermediate shape through fall and winter.     

Changes in the Protein Composition of the Shoot Apical Bud of Sinapis alba in Transition to Flowering

Vegetative plants of Sinapis alba L. grown in short days were induced to flower by expsoure to one or continuous long days. In both inductive conditions, the first flowers were initiated about 60 h after the start of the treatment. Soluble protein extracts were prepared from apical buds and just-expanded leaves of both vegetative and induced plants. Rabbit antisera were prepared using extracts from vegetative and reproductive buds. Immunodiffusion tests were performed. Analysis of the precipitin bands indicated that: (1) one antigenic protein was present in the vegetative buds and disappeared from the buds of induced plants between 96 and 240 h after the start of the inductive treatment; (2) the concentration of a another antigenic protein increased in buds of induced plants 30 h after the start of the inductive treatment; (3) the concentration of a third antigenic proteín increased in buds of induced plants at 96 h.     

Development of shoot inHydrangea macrophylla II. sequence and timing

The development of axillary buds, terminal buds, and the shoots extended from them was studied inHydrangea macrophylla. The upper and lower parts in a nonflower-bearing shoot are discernible; the preformed part of a shoot develops into the lower part and the neoformed part into the upper part (Zhou and Hare, 1988). These two part are formed by the different degrees of internode elongation at early and late phases during a growth season, respectively. Leaf pairs in the neoformed part of the shoot are initiated successively with a plastochron of 5–20 days after the bud burst in spring. The upper axillary buds are initiated at approximately the same intervals as those of leaf pairs, but 10–30 days later than their subtending leaves. Changes in numbers of leaf pairs and in lengths of successive axillary buds show a pattern similar to the changes in internode lengths of the shoot at the mature stage. The uppermost axillary buds of the flower-bearing shoot often begin extending into new lateral shoots when the flowering phase has ended. The secondary buds in terminal and lower axillary buds are initiated and developed in succession during the late phase of the growth season. Internode elongation seems to be important in determining the degrees of development of the axillary buds. Pattern of shoot elongation is suggested to be relatively primitive. Significances of apical dominance and environmental conditions to shoot development are discussed.     

EFFECTS OF LEAF EXCISION ON FLOWERING OF XANTHIUM APICAL BUDS IN CULTURE UNDER INDUCTIVE AND NONINDUCTIVE PHOTOPERIODS

Apical buds of Xanthium were grown in aseptic culture under short-day cycles known to induce flowering in the intact plants or under “light-break” conditions known to prevent flowering. The total light provided in each 24-hr cycle was the same under the two photoperiods. Various numbers of leaves were excised from the apical buds. Excision of leaves did not change the response to photoperiod: even with all leaves excised the apical buds cultured under short-day conditions reached the same average floral stage as the control buds, and those under light-break conditions all remained vegetative. Fresh weight was not significantly changed by the excisions, either. However, excision of the young leaves resulted in an increase in the number of new leaves developed by the apical bud during the two-week culture period.