The Geoelectric Effect in Plant Shoots: II. SENSITIVITY OF CONCENTRATION CHAIN ELECTRODES TO REORIENTATION

A concentration chain, static-drop electrode system has beenused by several investigators to measure the geoelectric effectin plant shoots. This paper describes investigations of theinherent sensitivity of this electrode system to reorientationwith respect to gravity. When the gelatine plug of the electrodeis made up with distilled water, and the contact solution is0.1 mM KCl, a potential difference develops immediately afterelectrodes in direct contact are rotated through 90° intothe vertical plane. A similar response is found when the contactsolution is 5 mM CaCl₂. Increasing the concentration of thecontact solution, or incorporating KCl, K₂SO₄, or ZnSO₄ intothe gelatine plug, drastically reduces the potential differencedeveloped after reorientation. The potential difference acrosselectrodes in direct contact decreases as the electrodes age.The potential difference measured with these electrodes acrossa decapitated, horizontally placed, hypocotyl of Helianthusarmuus also decreases as the electrodes age. The polarity ofthe charge is reversed as compared with that found when theelectrodes are in direct contact. The kinetic characteristicsof the geoelectric potential difference developed across a non-decapitated,horizontal coleoptile of Zea mays change as the electrodes age.With fresh electrodes the potential develops immediately afterreorientation and continues to increase with time. With 4-day-oldelectrodes, however, no potential difference develops until9 min after the moment of reorientation, but then it increaseswith time. The characteristics of the geoelectric potentialdifference developed with the aged concentration-chain, static-dropelectrodes are similar to those found with several other typesof electrodes which do not themselves have an inherent sensitivityto reorientation with respect to gravity. The results supportour earlier suggestion that the potential difference which apparentlydevelops with the static-drop electrode system, immediatelyafter a shoot is turned through 90° in reality developsin the electrode system itself and not in the plant tissue.The geoelectric effect which arises in the living plant shootbegins to develop approximately 10 min after reorientation.     

The Geoelectric Effect in Plant Shoots: III. DEPENDENCE UPON AUXIN CONCENTRATION GRADIENTS AND AEROBIC METABOLISM

The development of the geoelectric effect has been followedin Zea coleoptiles with a flowing-solution electrode system,and its dependence upon auxin concentration gradients and aerobicmetabolism assessed. A symmetrical source of IAA can effectively replace the coleoptiletip in allowing the geo-electric potential to occur. The diffusatefrom coleoptile tips, when applied asymmetrically to the apexof a vertical decapitated coleoptile, generates a potentialdifference across the coleoptile indistinguishable from thatinduced by the asymmetrical application of IAA. Asymmetricalapplication of IAA to vertical Avena and Zea coleoptiles andHelianthus hypocotyls induces closely similar responses. Neither the geoelectric effect nor a geotropic response developswhen intact Zea coleoptiles are placed horizontally after beingdeprived of oxygen, but they both occur when an aerobic atmosphereis restored. The lateral potential difference induced by theasymmetrical application of IAA to the apex of a vertical coleoptiledoes not occur under anoxic conditions. With a static-drop electrode system and a decapitated Zea coleoptile,a potential difference develops immediately after reorientationof the coleoptile into the horizontal position, and attainsa maximum value after about 10 min. This potential differencecan be further increased by the asymmetrical application ofIAA to the lower half of the apical cut surface of the coleoptile. Our data support the view that both the geoelectric potentialand the geotropic response are due to the IAA concentrationgradient which arises from the lateral transport of this substancefrom the upper to the lower half of the horizontal shoot. Theyalso bear out our previous conclusions that the ‘geoelectricpotential’ observed with static-drop electrodes and anintact shoot, is the resultant of two processes. The first isa physical phenomenon arising in the electrodes, or betweenthe electrodes and the plant tissue, and the second arises inthe living tissues of the shoot as the result of gravity-inducedchanges in auxin distribution.     

CONTROL OF SHOOT-RHIZOME DIMORPHISM IN THE WOODY MONOCOTYLEDON,CORDYLINE (AGAVACEAE)

In Cordyline terminalis negatively geotropic leafy shoots and positively geotropic rhizomes develop from single axillary buds on either shoots or rhizomes. All axillary buds have similar morphogenetic potential when released from apical dominance. Experiments in which the orientation of the apex is changed, organs removed, or growth regulators applied indicate that after a rhizome is initiated, it is maintained as a rhizome by auxin originating in the leafy shoot. When auxin levels are lowered by changes in the orientation of the axis or shoot removal, the rhizome apex becomes a shoot apex, which appears to be the stable state of the actively growing apex. Benzyl adenine when applied exogenously to the apex or lateral buds has the same effect as lowering the auxin level. Gibberellic acid has no effect on the apex or lateral buds. High levels of exogenous naphthaleneacetic acid cause bud release and development of rhizomes from previously inhibited axillary buds of the shoot. However, it was not possible to convert a shoot apex into a rhizome apex by auxin treatment. It is suggested that the release of buds on the lower side of horizontal branches and of buds directly above a stem girdle is caused by high auxin levels on the lower side or distal to the girdle. The experimental results are discussed in relation to naturally occurring shoot-rhizome dimorphism.     

Correlative Inhibition in the Shoot of Agropyron repens ( L.) Beauv

Correlative inhibition was investigated in plants of Agropyronrepens at two temperatures. Reciprocal inhibition ocrurred betweenthe main shoot apex and the outgrowing axillary shoots, withthe balance of inhibition varying with temperature. Apical dominancewas stronger at 10 °C than at 20 °C , but even at 10°C release of apical dominance by decapitation had onlyminor effects on the timing of outgrowth, growth pattern andrate of dry weight aocumulation of the axillary shoots. Dominanceof the main shoot apex by the axillary shoots was stronger at20 °C than at 10 °C. Removal of axillary buds preventeddecline in size and activity of the main shoot apex ard resultedin increased rates of primordium initiation, leaf emergenceand dry weight accumulation in the main shoot. It is suggestedthat a system of reciprocal dominance provides a mechanism formaintaining the characteristic habit of the grass plant andlimits growth in height of vegetative shoots. Agropyron repens (L.) Beauv, couch grass, correlative inhibition, apical dominance, shoot, apex     

Bud Structure in Relation to Shoot Morphology and Position on the Vegetative Annual Shoots of Juglans regia L. (Juglandaceae)

The length and basal diameter of all lateral and terminal budsof vegetative annual shoots of 7-year-oldJuglans regia treeswere measured. All buds were dissected and numbers of cataphylls,embryonic leaves and leaf primordia were recorded. Each axillarybud was ranked according to the position of its associated leaffrom the apex to the base of its parent shoot. Bud size andcontent were analysed in relation to bud position and were comparedwith the size and number of leaves of shoots in equivalent positionswhich extended during the following growing season. Length andbasal diameter of axillary buds varied according to their positionon the parent shoot. Terminal buds contained more embryonicleaves than any axillary bud. The number of leaves was smallerfor apical and basal axillary buds than for buds in intermediatepositions on the parent shoot only. All new extended shootswere entirely preformed in the buds that gave rise to them.Lateral shoots were formed in the median part of the parentshoot. These lateral shoots derived from buds which were largerthan both apical and basal ones. Copyright 2001 Annals of BotanyCompany Juglans regia L., Persian walnut tree, branching pattern, preformation, bud content, shoot morphology     

Intercellular Transport and Photosynthetic Differentiation in Chara corallina

Intercellular transport of ¹⁴C-labelled photoassimilates, bothin isolated upper shoots and in isolated internode-branchletcomplexes of Chara corallina, was measured. The isolated uppershoots were composed of a primary apex, two mature internodes,and three branchlet whorls. A 10 min loading of the isolatedupper shoot with H¹⁴CO₃^– resulted in a greater accumulationof ¹⁴C in the apical complex and branchlets than in the internodes,while a subsequent 50 min chase with unlabelled solution inthe light resulted in a greater accumulation of ¹⁴C in internodesthan in other parts of the shoot. In the isolated internode-branchlet complex, when the apex wasnot detached, the amount of ¹⁴C transported from branchletsto internodes was about fives times that transported from internodesto branchlets. Removal of the apex resulted in a decrease intransport from branchlets to internodes and an increase in transportin the opposite direction. In an attempt to explain the mechanism of the polar transportof photosynthetically fixed carbon between branchlets and internodes,photosynthetic activities of both types of cells were investigated.Detached branchlets have higher photosynthetic ¹⁴C-fixationactivities than those of internodes. Chlorophyll contents, measuredin terms of surface area, in internodes and branchlets werealmost identical. The ribulose-l,5-bisphosphate carboxylase(RuBPCase) activity of branchlets was 1.6 times that of internodes,and the rate of ferricyanide-dependent evolution of oxygen inbranchlets was 1.4 times that in internodes. Key words: Chara, internode, branchlet, polar transport, photosynthesis     

Shoot-tip abscission and adventitious abscission of internodes in mulberry (Morus alba)

In mulberry ( Morus alba L. cv. Shin-ichinose), shoot-tip abscission following the cessation of apical growth could be induced in different internodes, depending on the vigour of the shoot and its apex and other internal and external factors. In the lateral, short shoots of 1-year-old stems of low-pruned trees, the apical growth cessation and shoot-tip abscission (May–June) resulted primarily from the dominance of the upper, long shoots and intense competition among laterals along the stem. Decapitation of the laterals, before abortion of their apices took place (early May), readily caused adventitious abscission of the distal internode. Similar decapitation-induced, adventitious abscission of the distal internode of the upper, long shoots of 1-year-old stems of pruned trees also occurred (May–September), demonstrating that the abscission itself is not directly associated with photoperiod. In May and June, decapitation induced abscission primarily in parallel with or after sprouting of lateral buds and shoot elongation, while in July, August and September, the abscission was induced by decapitation and independently of sprouting. Shoot (stem) orientation positively affected the abscission, which is related to gravimorphic effects on buds and shoots on the lower and lateral sides of the horizontally trained stem. These results suggest that the vigour of shoots and apices is an important determinant of growth and apex abscission in mulberry.     

The Shoot Apex and its Dichotomous Branching in the Nypa Palm

Seedlings of the palm Nypa fruticans van Wurmb are viviparous,the plumule becoming exserted before the fruit is ripe and possiblyassisting in fruit detachment. Established seedlings have horizontalaxes, this growth orientation being maintained throughout thelife of the palm which may be described as ‘rhizomatous’.Inflorescences are axillary in adult plants, but their distributionis irregular. The shoot apex is small and very asymmetricalsince it is more or less displaced into a lateral position byprogressive enlargement of each leaf primordium during thisleaf's first plastochrone. The plastochrone interval is apparentlya long one so that leaves have a wide developmental gap betweenthem. This results in a leaf base which is more or less cylindricalbut with a groove to accommodate the next youngest leaf. All the available evidence suggests that vegetative branchingis by equal dichotomy of the shoot apex at wide intervals. Thedichotomy is marked superficially by a leaf, enclosing the twonew shoots, which has two grooves. The twin shoots are insertedin the lateral, not the dorsiventral plane of this enclosingleaf. The daughter shoots are always at precisely the same stageof development and they always have mirror-image symmetry withrelation to each other, the phyllotaxis of the parent shootbeing maintained without obvious interruption. There is no anatomicalevidence for abortion of the original apex and its replacementby two new ones. The vascular system is divided equally betweeneach daughter shoot without interruption, suggesting stronglythat there is continuity of growth from the undivided to thedivided condition.     

Auxin Relations of the Woody Shoot: The Distribution of Diffusible Auxin in Shoots of Apple and Plum Rootstock Varieties

Surveys have been made of diffusible auxin in the stem tissuesof growing shoots of apple and plum rootstock varieties. Usingagar plates as carriers auxin was collected from the lower surfaceof isolated stem sections and assayed by the Avena curvaturemethod. The stool and layer shoots studied grew for severalmonths producing many leaves and reaching considerable lengths.The data provide information on selected internodes and showthe auxin status of the shoot at various times during growth,and the auxin gradients down the stem at these various times.Free auxin content of the shoot apex was consistently less thanthat of the internodes below. In 1946 auxin content declinedthroughout growth with a steady positioning of the auxin peakin the upper shoot. In 1947, following a period of drought,when growth almost ceased, a secondary auxin peak occurred positionedin lower internodes distant from the apex. This seasonal contrastwas reflected in the auxin relations of the individual internode,and was observed both in apple and plum. The nature of the auxindecline below the peak region, and the total disappearance offree auxin from the shoot as growth subsides, is discussed.The reappearance of free auxin in mature internodes, which hasnot been transmitted from the stem apex, implies either derivationfrom a stored state or the ability of the internode to produceits own auxin.     

THE TENDRIL OF PARTHENOCISSUS INSERTA: DETERMINATION AND DEVELOPMENT

Tendrils on long shoots of Parthenocissus inserta occur in a regular pattern opposite the alternate distichous leaves at two successive nodes of each three nodes. Ontogenetic study shows that the tendril is initiated at the flank of the shoot apex during the second plastochron in an essentially axillary position. It is carried upward with growth of the internode above the axillant leaf and ultimately is situated opposite the next younger leaf. In a rhythmic pattern a different group of appendages is produced by the shoot apex at each node in the sequence of three. In acropetal order these are: at the tendrilless node the leaf subtends an axillary bud complex which in turn subtends a tendril; the leaf at the lower tendril-bearing node directly subtends a tendril, and the leaf at the upper tendril-bearing node subtends an axillary bud. Tendril primordia were not induced to develop as foliaceous shoots when cultured in vitro or in decapitation experiments, indicating that the meristem which becomes a tendril is determined early in its inception. Although built on a shoot pattern, the tendril is regarded as an organ sui generis with a possible relationship to the inflorescence. The morphological nature of the tendril is discussed in the light of theories postulated in the literature.     

Bud Content and its Relation to Shoot Size and Structure in Nothofagus pumilio(Poepp. et Endl.) Krasser (Nothofagaceae)

Buds of shoots from the trunk, main branches, secondary branchesand short branches of 10–21 year-old Nothofagus pumiliotrees were dissected and their contents recorded. The numberof differentiated nodes in buds was compared with the numberof nodes of sibling shoots developed at equivalent positionsduring the following growing season. Axillary buds generallyhad four cataphylls, irrespective of bud position in the tree,whereas terminal buds had up to two cataphylls. There were morenodes in terminal buds, and the most distal axillary buds, oftrunk shoots than in more proximal buds of trunk shoots, andin all buds of shoots at all other positions. The highest numberof nodes in the embryonic shoot of a bud varied between 15 and20. All shoots had proximal lateral buds containing an embryonicshoot with seven nodes, four with cataphylls and three withgreen leaf primordia. The largest trunk, and main branch, shootswere made up of a preformed portion and a neoformed portion;all other shoots were entirely preformed. In N. pumilio, theacropetally-increasing size of the sibling shoots derived froma particular parent shoot resulted from differences in: (1)the number of differentiated organs in the buds; (2) the probabilityof differentiation of additional organs during sibling shootextension; (3) sibling shoot length; (4) sibling shoot diameter;and (5) the death of the apex and the most distal leaves ofeach sibling shoot. Copyright 2000 Annals of Botany Company Axis differentiation, branching, bud structure, leaf primordia, neoformation, Nothofagus pumilio, preformation, size gradient     

Stem types of Phragmites communis Trin.

The three usual stem types of Phragmites communis Trin. canbe arranged in a maturation sequence. The horizontal rhizomeis the juvenile type, responsible for renewing the stand; thevertical rhizome is intermediate and bears the aerial shoots;and the aerial shoot is the mature type, photosynthesizing,potentially flowering, and short lived, but not normally capableof renewing the population. The difference between horizontaland vertical rhizomes appears to be determined internally, andthat between vertical rhizomes and aerial shoots, by the environment.     

Surgical Experiments on the Differentiation of Vascular Tissue in the Shoot Apex of Carrot (Daucus carota L.)

Surgical techniques were applied to the shoot apex of carrot(Daucus carota L.) to test the interpretation that provasculartissue is the initial stage of vascular differentiation andto localize the sources of the influences that control its differentiation.If the apex is isolated laterally by vertical incisions leavingit at the summit of a plug of pith tissue, vascular differentiationproceeds normally and an independent vascular system is formedin the pith plug. If all leaf primordia are systematically suppressed,provascular tissue continues to differentiate as an acropetalextension of the pre-existing vascular system but no furtherdifferentiation occurs. When the apex is isolated laterallyand all leaf primordia are suppressed, provascular tissue continuesto be formed acropetally and is extended basipetally into thepith plug by redifferentiation of pith cells, but no furtherdifferentiation occurs. This tissue reacts positively to histochemicaltests for esterase indicating its vascular nature. If only oneleaf primordium is allowed to develop on an isolated shoot apex,its vascular system develops normally and extends basipetallyinto the pith plug, but there is no extension of provasculartissue into the pith plug. These results support the interpretationthat the initial stage of vascular differentiation is controlledby the apical meristem but that further maturation of vasculartissue depends upon influences from developing leaf primordia.Copyright 2000 Annals of Botany Company Provascular tissue, differentiation, carrot (Daucus carota L.), shoot apex, surgical techniques, leaf primordia     

Delivery rates of abscisic acid in xylem sap of Ricinus communis L. plants subjected to part-drying of the soil

Abscisic acid (ABA) moving from roots to shoots in the transpirationstream is a potential hormonal message integrating perceptionof a root stress with adaptive changes in the shoot. A twinroot system was used to study ways of estimating effects ofdroughting the upper roots of Ricinus communis L. on abscisicacid (ABA) transport to the shoot in the transpiration stream.Droughted plants transpired more slowly than controls. Droughtingalso increased concentrations of ABA up to I I-fold in sap inducedto flow from the roots of freshly decapitated plants at ratesof whole plant transpiration. However, because of dilution effectsarising from the different sap flows in control and droughtedplants, these changes in ABA concentration in the xylem sapdid not accurately reflect amounts of ABA transported. To overcomethis problem, delivery rates were calculated by multiplyingconcentration with sap flow rate to generate ABA delivery interms of µmol s–1 per plant. Droughting for 24 hor more increased ABA delivery from roots to shoots by 5-fold.Since droughting can alter the relative sizes of the roots andshoots and also the root:shoot ratio these delivery rates wererefined in several ways to reflect both the amount of root generatingthe ABA message and the size of the recipient shoot system. Key words: Abscisic acid, Ricinus communis L., soil drying, xylem sap     

Influence of Bud Position and Plant Ontogeny on the Morphology of Branch Shoots in Pea (Pisum sativum L. cv. Alaska)

The morphology of axillary shoots of pea plants (Pisum sativumL. cv. Alaska) was analysed as a function of the position ofthe bud on the plant axis and the stage of plant developmentwhen the buds began to grow. Buds from the three most basalnodes were stimulated to develop by decapitating the main shootwhen buds were still growing (4 d plants), shortly after budsbecame dormant (7 d plants) or after the initiation of floweringon the main shoot (post-flowering plants, about 21 d after sowing).Branch shoots were scored for node of floral initiation (NFI),shoot length, and node of multiple leaflets (NML), a measureof leaf complexity. Shoots that developed spontaneously fromupper nodes (nodes 5-9) on intact post-flowering plants werescored for NFI. NFI for basal buds on 4 and 7 d plants variedas a function of nodal position and ranged from 5 to 6·7nodes. NFI on these plants was not influenced by bud size orwhether a bud was growing or dormant when the plant was decapitated.NFI for shoots derived from basal buds on decapitated post-floweringplants and upper nodes on intact post-flowering plants was about4. Reduced NFI on post-flowering plants may be due to depletionof a cotyledon-derived floral inhibitor. Basal axillary shootson 4 d plants were about 20% longer than those on 7 d plantsand about five times longer than those on post-flowering plants.These differences may be due to depletion of gibberellic acidsfrom the cotyledons. NFI and NML for the main shoot and forbasal axillary shoots were similar under some experimental conditionsbut different under other conditions, so it is likely that eachdevelopmental transition is regulated independently.Copyright1995, 1999 Academic Press Apical dominance, bud development, garden pea, initiation of flowering, Pisum sativum L., shoot morphology     

Studies on the Fragmented Shoot Apex of Grapevine: IV. SEPARATION OF PHENOTYPES IN A PERICLINAL CHIMERA IN VITRO

Adventitious shoots were induced on apical fragments of thegrapevine, cv. Meunier, a presumptive periclinal chimera. Thiscultivar resembles Pinot noir in essential characteristics,but is phenotypically distinguishable by the dense white matof hairs on the apex and expanding leaves (tomentose phenotype).Plants derived from apical culture were of three types: (i)phenotypically resembling Meunier, (ii) bearing tomentose leaveswith comparatively hairless sectors, and (iii) with completelynon-tomentose shoots phenotypically similar to Pinot noir. These results establish that adventitious buds of grapevineproduced by this method are multicellular in origin and arenot derived from single cells. Furthermore, it is concludedthat fragmented shoot apex culture of grapevine periclinal chimerasdisturbs the integrity of apical tissue allowing separationof component genotypes. Fragmented apex culture of such chimerasis not recommended where trueness-to-type is of prime importance.However, circumstances under which this technique may be usefullyapplied to separate chimeral genotypes are discussed. Key words: Vitis vinifera, Grapevine, In vitro, Chimera separation     

Leaf Production and Morphology in the Artichoke Gall of Yew (Taxus baccata L.)

Comparative leaf production rates and leaf morphology studiesfor galled and normal shoots of yew trees have been obtainedthroughout the life cycle of the causative agent, the gall midge,Taxomyia taxi. Normal and galled shoot leaf numbers have beenrelated to those of their parent shoots. It was found that whereasthe annual leaf production of normal shoots was positively relatedto that of the parent shoots, galled shoot leaf production remainedconstant regardless of parent shoot vigour showing leaf stimulationby the midge to be a special case. The midge larva appears tobe determining the rate of leaf production in galled shoots.In galls leaf production continues throughout the winter monthswith no dormant period. From morphological evidence, alternationof leaves and cataphylls is continued in galled buds.     

The Growth of the Shoot Apex of Chenopodium rubrum L. Treated with a Florigenic Extract from Flowering Tobacco Plants: Preliminary Anatomical Observations

Anatomical changes in the shoot apex of Chenopodium rubrum L.treated with an extract from flowering tobacco plants and cultivatedin non-inductive conditions are described. They are comparedwith the anatomy of non-treated vegetative apices and with apicesof plants induced with a short day. Treatment with the extractresulted in both activation of cell division in the upper partof the apex and in apex elongation. Acceleration of leaf primordiainitiation and stimulation of branching took place. The effectcorresponds to the sequence of changes in photoperiodically-inducedplants but is more pronounced. Elongation following 10^–4M GA₃ treatment was of a differentnature; there was only a slight stimulation in the upper partof the apex in contrast with a strong stimulation of growthin length in the lower internodes. These preliminary resultssuggest a similarity between apical changes evoked by a stimulusproduced by short days and an exogenously applied floral stimulus.The changes differed from those caused by exogenous phytohormones. Key words: Chenopodium rubrum, florigen, shoot apex     

A Morphometric Analysis of the Redistribution of Organelles in Columella Cells of Horizontally-oriented Roots of Zea mays

In order to determine what structural changes in graviperceptivecells are associated with the onset of root gravicurvature,the redistribution of organelles in columella cells of horizontally-oriented,graviresponding roots of Zea mays has been quantified. Rootgravicurvature began by 15 min after reorientation, and didnot involve significant changes in the (i) volume of individualcolumella cells or amyloplasts, (ii) relative volume of anycellular organelle, (iii) number of amyloplasts per columellacell, or (iv) surface area or cellular location of endoplasmicreticulum. Sedimentation of amyloplasts began within 1 to 2min after reorientation, and was characterized by an intenselystaining area of cytoplasm adjacent to the sedimenting amyloplasts.By 5 min after reorientation, amyloplasts were located in thelower distal corner of columella cells, and, by 15 min afterreorientation, overlaid the entire length of the lower cellwall. No consistent contact between amyloplasts and any cellularstructure was detected at any stage of gravicurvature. Centrally-locatednuclei initially migrated upward in columella cells of horizontally-orientedroots, after which they moved to the proximal ends of the cellsby 15 min after reorientation. No significant pattern of redistributionof vacuoles, mitochondra, dictyosomes, or hyaloplasm was detectedthat correlated with the onset of gravicurvature. These resultsindicate that amyloplasts and nuclei are the only organelieswhose movements correlate positively with the onset of gravicurvatureby primary roots of this cultivar of Zea mays. Zea mays, root gravitropism, ultrastructure, morphometry, graviperception     

Aspects of Morphogenesis in a Dorsiventral Fern, Pteridium aquilinum (L.) Kuhn: With one Plate and ten Figures in the Text

The formation of the horizontal dorsiventral rhizome of Pteridiumaquilinum from the erect radial axis of the young sporelingis described. The shoot apex and the inception of leaves andbuds at the apical meristem have been investigated, and theirinception is shown to be essentially similar in long and shortshoots, later differences being due to differing rates of growthand internodal elongation in the two types of shoot.