THE OCCURRENCE OF INTERCALARY AND UNINTERRUPTED MERISTEMS IN THE INTERNODES OF TROPICAL MONOCOTYLEDONS

The distribution of percent of dividing nuclei, parenchyma cell length, total cell number per internode, and total internode length were determined for successive internodes in the apex and growing vegetative internodes of 23 tropical species in 17 families of monocotyledons. Basal intercalary meristems (IM) were found in representatives of Commelinaceae, Cyperaceae, Flagellariaceae, Poaceae, Restionaceae, and Marantaceae. Uninterrupted meristems (UM) which are confined progressively to the upper region of the internode and are not isolated meristematic regions were found in the Costaceae, Dioscoreaceae, Philesiaceae, Smilacaceae, Agavaceae, Araceae, Arecaceae, Liliaceae, Pandanaceae, and Zingiberaceae. Both IM and UM were found in different species of Orchidaceae. The only morphological trait correlated with meristem type was presence of sheathing leaf bases in all species with IM. Both IM and UM are interpreted as extensions of the primary elongating meristem; the IM is disjunct, and the UM is continuous with it. The phytomer growth unit and the presence of internodal IM's cannot be applied generally to the monocotyledons.     

Dependence of Root Cell Growth and Division on Root Diameter

Primary roots of 98 species from different families of monocotyledonous and dicotyledonous plants and adventitious roots obtained from bulbs and rhizomes of 24 monocot species were studied. Root growth rate, root diameter, length of the meristem and elongation zones, number of meristematic cells in a file of cortical cells, and length of fully elongated cells were evaluated in each species after the onset of steady growth. The mitotic cycle duration and relative cell elongation rate were calculated. In all species, the meristem length was approximately equal to two root diameters. When comparing different species, the rate of root growth increased with a larger root diameter. This was due to an increase in the number of meristematic cells in a row and, to a lesser degree, to a greater length of fully elongated cells. The duration of the mitotic cycle and the relative cell elongation rate did not correlate with the root diameter. It is suggested that the meristem size depends on the level of nutrient inflow from upper tissues, and is thereby controlled during further growth.     

OCCURRENCE OF INTERCALARY AND UNINTERRUPTED MERISTEMS IN EQUISETUM

Quantitative data on cell length and mitotic index in elongating internodes of seven species of Equisetum are presented as evidence for the occurrence of two patterns of internode development in rhizomes of different species. In rhizomes of three species of subgenus Equisetum (E. arvense, E. diffusum, E. telmateia) uninterrupted meristems are present, characterized by acropetal internode maturation. In rhizomes of four species of subgenus Hippochaetae (E. hyemale, E. variegatum, E. scripoides, and E. laevigatum) intercalary meristems are present. The number and locations of intercalary meristems are described for an aerial shoot of E. diffusum. The absence or diminution of intercalary meristems from rhizomes of a variety of vascular plants with intercalary meristems in aerial shoots is discussed from the standpoint of adaptive significance.     

COMPARATIVE FOLIAR HISTOGENESIS IN ACORUS CALAMUS AND ITS BEARING ON THE PHYLLODE THEORY OF MONOCOTYLEDONOUS LEAVES

A comparative histogenetic investigation of the unifacial foliage leaves of Acorus calamus L. (Araceae; Pothoideae) was initiated for the purposes of: (1) re-evaluating the previous sympodial interpretation of unifacial leaf development; (2) comparing the mode of histogenesis with that of the phyllode of Acacia in a re-examination of the phyllode theory of monocotyledonous leaves; and (3) specifying the histogenetic mechanisms responsible for morphological divergence of the leaf of Acorus from dorsiventral leaves of other Araceae. Leaves in Acorus are initiated in an orthodistichous phyllotaxis from alternate positions on the bilaterally symmetrical apical meristem. During each plastochron the shoot apex proceeds through a regular rhythm of expansion and reduction related to leaf and axillary meristem initiation and regeneration. The shoot apex has a three- to four-layered tunica and subjacent corpus with a distinctive cytohistological zonation evident to varying degrees during all phases of the plastochron. Leaf initiation is by periclinal division in the second through fourth layers of the meristem. Following inception early growth of the leaf primordium is erect, involving apical and intercalary growth in length as well as marginal growth in circumference in the sheathing leaf base. Early maturation of the leaf apex into an attenuated tip marks the end of apical growth, and subsequent growth in length is largely basal and intercalary. Marked radial growth is evident early in development and initially is mediated by a very active adaxial meristem; the median flattening of this leaf is related to accentuated activity of this meristematic zone. Differentiation of the secondary midrib begins along the center of the leaf axis and proceeds in an acropetal direction. Correlated with this centralized zone of tissue specialization is the first appearance of procambium in the center of the leaf axis. Subsequent radial expansion of the flattened upper leaf zone is bidirectional, proceeding by intercalary meristematic activity at both sides of the central midrib. Procambial differentiation is continuous and acropetal, and provascular strands are initiated in pairs in both sides of the primordium from derivatives of intercalary meristems in the abaxial and adaxial wings of the leaf. Comparative investigation of foliar histogenesis in different populations of Acorus from Wisconsin and Iowa reveals different degrees of apical and adaxial meristematic activity in primordia of these two collections: leaves with marked adaxial growth exhibit delayed and reduced expression of apical growth, whereas primordia with marked apical growth show, correspondingly, reduced adaxial meristematic activity at equivalent stages of development. Such variations in leaf histogenesis are correlated with marked differences in adult leaf anatomy in the respective populations and explain the reasons for the sympodial interpretation of leaf morphogenesis in Acorus and unifacial organs of other genera by previous investigators. It is concluded that leaf development in Acorus resembles that of the Acacia phyllode, thereby confirming from a developmental viewpoint the homology of these organs. Comparison of development with leaves of other Araceae indicates that the modified form of the leaf of Acorus originates through the accentuation of adaxial and abaxial meristematic activity which is expressed only slightly in the more conventional dorsiventral leaf types in the family.     

STEM ANATOMY AND INTERNODAL DEVELOPMENT IN PHASEOLUS VULGARIS

Internodal length in Phaseolus vulgaris decreases slightly from one through three, then sharply increases in four, and subsequently decreases toward the apex. Two internodes undergo rapid growth at a given time, although they do not mature concurrently. Early growth of the internode takes place throughout, with later growth limited to successively higher levels. Elongation results from both cell division and cell elongation, and the relative importance of these two phenomena is significant in determining final length. The difference in final length between internodes is brought about by rate rather than period of growth. The procambium is relatively homogeneous in structure, whereas the cambium consists of both fusiform and ray initials. During stem development the vascular meristem gradually acquires the characteristics of the cambium. By the time elongation ceases, the meristem has developed all the characteristics of the cambium.     

SHOOT GROWTH AND HISTOGENESIS OF TREES POSSESSING DIVERSE PATTERNS OF SHOOT DEVELOPMENT

Shoot growth and histogenesis were followed in five unrelated tree taxa possessing inherently diverse patterns of shoot development. Following the resumption of growth in spring, each species differs quantitatively in the number of internodes elongating contemporaneously, in rates and duration of internodal elongation and seasonal periodicity of shoot growth. The basic pattern of internode elongation and histogenesis is qualitatively similar in each of the dicotyledonous species observed irrespective of growth habit or final form of the shoot produced. During the intial phase of internode development, growth is essentially uniform throughout young internodes, corresponding to an active period of cell division during which time pith cells increase in size to about one-third their final length. Subsequently, the pattern of cell division shifts progressively upward concomitant with increased elongation and maturation of pith cells in the basal portion of developing internodes. Thereafter, a wave of cell division accompanied by cell elongation continues to proceed acropetally until growth finally ceases in the distal portion of each internode. As long as internode elongation continues, frequently at distances 15–20 cm below the shoot apex, cell divisions still occur in the distal growing portion. As successive portions of each internode mature acropetally, final length of pith cells becomes relatively uniform throughout the internode. During the process of internode growth and development, cell lengths increase only two- to threefold, whereas cell numbers increase ten- to 30-fold, indicating the dominant role of cell division and increases in cell number to final internode length. Morphological patterns of shoot expression associated with differences in internode lengths along the axis of either preformed or neoformed shoots, as well as sylleptic branches, are due to differences in cell number rather than final cell length. Significant variations in final internode lengths along the axis of episodic shoots, caused by either endogenous or exogenous factors, are also attributed to differences in cell number.     

COMPARISON OF SHOOT APEX DYNAMICS AND EARLY LEAF ONTOGENY IN TWO SPECIES OF SARACA (LEGUMINOSAE)

Shoot apices of Saraca indica produce adult leaves that have 4 to 6 pairs of leaflets, whereas those of S. bijuga usually have only 2 pairs. In both species one leaflet pair is found during the juvenile phase. Juvenility lasts many plastochrons in S. bijuga but is restricted to a few in S. indica. The shoot apical meristems of these two taxa are similar in structure, cell number, and cell size; however, the shoot apex of Saraca bijuga is slightly more stratified, having 2–3 tunica layers as opposed to 1–2 in S. indica. For most of the plastochron the apical meristem in both species is situated laterally at the base of the most recently formed leaf. A newly forming primordium and its internode shift the apical meristem upward unilaterally; the meristem passes through a brief apical dome stage and becomes positioned 180° from its origin at the beginning of the plastochron. Hence, there is a true pendulum meristem in both species. In S. bijuga the maximum length of the youngest leaf primordium, just prior to the formation of its successor, is twice that of S. indica. The internodes immediately below the shoot apex and the axillary buds develop more rapidly in S. bijuga than in S. indica. It is suggested that the bijugate leaf of S. bijuga represents a case of neoteny in plants.     

Development of the intercalary meristem in Chorda filum (Laminariales, Phaeophyceae) and other primitive Laminariales

Development of the intercalary meristem in the terete laminarialean species Chorda filum (L.) Stackhouse was studied in culture using light and transmission electron microscopy as well as by tracing elongation and cell divisions in various parts of the sporophyte. Growth of C. filum sporophytes could be classified into three developmental stages: (i) diffuse growth; (ii) basal meristematic growth; and (iii) intercalary meristematic growth. In the diffuse growth stage, elongation and cell division frequency were almost the same in each cell. In the basal meristematic growth stage, elongation and division of cells became localized in the tissues derived from the meristematic initial cell. Cells of the basal meristematic region contained smaller chloroplasts and many small opaque vesicles. In the intercalary meristematic growth stage, there was further elongation and differentiation of cells originating from the meristematic region, and this became more active in adjacent regions below the meristem than in regions above the meristem, causing the relative position of the intercalary meristem to shift towards the tip of the sporophyte. Meristematic cells of C. filum contained well-developed Golgi vesicles around the nucleus (perinuclear Golgi), many secretion vesicles and many small disk-shaped chloroplasts whose thylakoids were not well developed. Sporophytes of three other terete members of Laminariales, Chorda tomentosa Lyngbye, Pseudochorda nagaii (Tokida) Kawai et Kurogi, and Pseudochorda gracilis Kawai et Nabata, show diffuse growth and basal meristematic growth, but no intercalary meristematic growth. This suggests that the common ancestor of the Pseudochordaceae and Chordaceae had basal meristematic growth, and intercalary meristematic growth evolved more recently in C. filum.     

CONTROL OF THE INTERNODAL INTERCALARY MERISTEM OF CYPERUS ALTERNIFOLIUS

In the growing culm of C. alternifolius, surgical removal of parts indicated that the stimulus for the prolonged activity of the internodal intercalary meristem (IM) came from the matured leaves and upper internode and that buds were not involved in maintaining internodal growth. Decapitation of the culm resulted in cessation of internodal extension. Various growth regulators were applied to the decapitated internode, and both the total extension and growth rates were analyzed statistically. Gibberellin A₃ (GA) and benzyladenine (BA) substituted for the excised parts in their effect on internodal extension. Indoleacetic acid (IAA) had little effect. (2-chloroethyl) trimethylammonium chloride (CCC) inhibited internodal growth, and its effects were reversed by GA. IAA was antagonistic to BA but not to GA. BA and GA were somewhat antagonistic. The quantitative effects of growth regulators on epidermal and ground parenchyma cell length and number of interstomatal cells were examined. Extension induced by GA was due to both cell division and cell elongation in the IM. Cells were longer, and fewer stomates differentiated than in the control. In internodes induced to extend by GA + BA cell division, cell length, and stomate differentiation were similar to the control. The results indicate that prolonged internodal IM activity is maintained by cytokinins and gibberellins coming from the matured upper portions of the culm. Changes in the levels of these regulators during growth presumably result in the histological gradient in the internode.     

THE SHELL ZONE: ITS DIFFERENTIATION AND PROBABLE FUNCTION IN SOME DICOTYLEDONS

Thirty-five species belonging to various dicotyledonous families were investigated to study the origin, development, and probable function of the shell zone, which is defined as an arcuate zone of cambiform cells delimiting the early axillary bud meristem. It is present in the majority of the investigated plants and five intergrading patterns of origin are described: (i) from the parenchymatized derivatives of the cells of the peripheral meristem of the shoot apex, adaxial to the bud meristem, (ii) from the peripheral meristem of the shoot apex along with the initiation of the early bud meristem, (iii) from the adaxial cells of the bud meristem, (iv) from the derivatives of the cells of the bud meristem at its base, and (v) partly from the parenchymatized cells of the peripheral meristem adaxial to the bud and partly from the adaxial derivatives of the bud meristem. The shell zone loses its identity at different stages of bud development in various species. Its cells ultimately contribute to the ground meristem, procambium, and pith cells of the axis. In Cuminum cyminum and lpomoea cairica the shell zone contributes in bringing about the axillary position of the bud from its early lateral position. In Solarium melongena, derivatives of the shell zone initiate the internodal elongation between the flower or inflorescence and the shoot apex, ultimately shifting the bud to an extra-axillary position on the internode.     

Internodal elongation and orientation of cellulose microfibrils and microtubules in deepwater rice

Excised stem sections of deepwater rice (Oryza sativa L.) containing the highest internode were used to study the induction of rapid internodal elongation by gibberellin (GA). It has been shown before that this growth response is based on enhanced cell division in the intercalary meristem and on increased cell elongation. In both GA-treated and control stem sections, the basal 5-mm region of the highest internode grows at the fastest rate. During 24 h of GA treatment, the internodal elongation zone expands from 15 to 35 mm. Gibberellin does not promote elongation of internodes from which the intercalary meristem has been excised. The orientation of cellulose microfibrils (CMFs) is a determining factor in cell growth. Elongation is favored when CMFs are oriented transversely to the direction of growth while elongation is limited when CMFs are oriented in the oblique or longitudinal direction. The orientation of CMFs in parenchymal cells of GA-treated and control internodes is transverse throughout the internode, indicating that CMFs do not restrict elongation of these cells. Changes in CMF orientation were observed in epidermal cells, however. In the basal 5-mm zone of the internode, which includes the intercalary meristem, CMFs of the epidermal cell walls are transversely oriented in both GA-treated and control stem sections. In slowly growing control internodes, CMF orientation changes to the oblique as cells are displaced from this basal 5-mm zone to the region above it. In GA-treated rapidly growing internodes, the reorientation of CMFs from the transverse to the oblique is more gradual and extends over the 35-mm length of the elongation zone. The CMFs of older epidermal cells are obliquely oriented in control and GA-treated internodes. The orientation of the CMFs parallels that of the cortical microtubules. This is consistent with the hypothesis that cortical microtubules determine the direction of CMF deposition. We conclude that GA acts on cells that have transversely oriented CMFs but does not promote growth of cells whose CMFs are already obliquely oriented at the start of GA treatment.     

Electron-probe microanalysis of silicon in the epidermis of rice (Oryza sativa L.) internodes

Summary Electron-probe X-ray microanalysis showed that significant amounts of silicon are accumulated in the entire epidermal system of the rice internode except in the stomatal apparatuses. Thus, there is a lack of specific sites for Si deposition from levels just above the base to the tip of the rice internode. In the intercalary meristem region, 1 cm above the base of the internode, point-count data indicate more Si accumulation in the dumb-bell shaped silica cells than in the long epidermal cells. Above this region, Si is accumulated essentially in a uniform pattern in all epidermal cells. Such a pattern for Si accumulation in rice internodes markedly contrasts with that for Avena internodes and may explain, in part, why rice plants have a higher percentage Si (dry weight basis) in their shoots. The adaptive significance of this silicification pattern in rice is discussed.     

Cell-lineage patterns in the shoot apical meristem of the germinating maize embryo

A fate map for the shoot apical meristem of Zea mays L. at the time of germination was constructed by examining somatic sectors (clones) induced by -rays. The shoot apical meristem produced stem, leaves, and reproductive structures above leaf 6 after germination and the analysis here concerns their formation. On 160 adult plants which had produced 17 or 18 leaves, 277 anthocyanin-deficient sectors were scored for size and position. Sectors found on the ear shoot or in the tassel most often extended into the vegetative part of the plant. Sectors ranged from one to six internodes in length and some sectors of more than one internode were observed at all positions on the plant. Single-internode sectors predominated in the basal internodes (7,8,9) while longer sectors were common in the middle and upper internodes. The apparent number of cells which gave rise to a particular internode was variable and sectors were not restricted to the lineage unit: a leaf, the internode below it, and the axillary bud and prophyll at the base of the internode. These observations established two major features of meristem activity: 1) at the time of germination the developmental fate of any cell or group of cells was not fixed, and 2) at the time of germination cells at the same location in a meristem could produce greatly different amounts of tissue in the adult plant. Consequently, the developmental fate of specific cells in the germinating meristem could only be assigned in a general way.Abbreviations ACN apparent cell number - LI, LII, LI-LII sectors restricted to the epidermis, the subepidermis, or encompassing epidermis and subepidermis - PCN progenitor cell     

Elongation of Stem Internodes in the Japanese Morning Glory (Pharbitis nil) in Relation to Auxin Destruction

The relationship between auxin destruction and stem internode elongation was investigated in the vines of the Japanese morning glory (Pharbitis nil Choisy). In young plants an age-dependent gradient was demonstrated in which the decreasing rate of elongation of older internodes correlated with an increasing ability of such tissue to destroy indoleacetic acid. Fragments of tissue from old internodes when incubated with indoleacetic acid (IAA), destroyed the hormone immediately and rapidly; in contrast, young, rapidly elongating internode tissue destroyed IAA only after a lag of several hours. In older plants the gradient was more erratic towards the middle of the plant but old and young tissue behaved as in young plants, i.e., old internodes destroyed IAA rapidly whereas young internodes did not. It appears reasonable to conclude that cessation of elongation in maturing internodes is brought about by developing an internal environment in which auxin is rapidly destroyed.     

PITH DEVELOPMENT IN NORMAL AND SHORT INTERNODE SEEDLINGS OF PRUNUS PERSICA VAR. LOVELL

Holmsen , Theodore W. (U. Florida, Gainesville.) Pith development in normal and short internode seedlings of Prunus persica var. Lovell. Amer. Jour. Bot. 47 (3): 173—175. Illus. 1960.–Investigation of pith cells of mature (no longer elongating) internodes of normal- and short-internode seedlings of Prunus persica var. ‘Lovell’ indicates that the short internode condition results largely from a suppression of cell division. The pith cells of normal seedlings were found to occur in an irregular fashion. The pith cells of short-internode seedlings were found to retain the linear order characteristic of the rib meristem. The relationship of these findings to the accepted long- and short-shoot conditions are discussed.     

SHOOT HISTOGENESIS: SUBAPICAL MERISTEMATIC ACTIVITY IN A CAULESCENT PLANT AND THE ACTION OF GIBBERELLIC ACID AND AMO-1618

Sachs , R. M., A. Lang , C. F. Bretz and Joan Roach . (U. California, Los Angeles.) Shoot histogenesis: subapical meristematic activity in a caulescent plant and the action of gibberellic acid and Amo—1618. Amer. Jour. Bot. 47(4): 260—266. Illus. 1960.–Studies on gibbereilininduced stem formation in rosette plants (Sachs et al., 1959) have shown that a zone of intensive meristematic activity, arising below the existing apical meristem, is almost solely responsible for stem histogenesis, i.e., the formation of the cells constituting the elongate stem. An extensive subapical zone of meristematic activity is also present in caulescent plants, such as Chrysanthemum morifolium, Amo-1618 ([4-hydroxy-5 isopropyl-2 methylphenyl] trimethylammonium chloride, 1-piperidine carboxylate) completely inhibits subapical meristematic activity in chrysanthemum, causing the plants to assume a dwarfed, rosette-like habit of growth. Gibberellic acid, applied either simultaneously, or following the Amo—1618 treatment, completely prevents or reverses the effect of Amo—1618, making the plants retain or resume their normal growth habit. Amo—1618 and gibberellic acid have relatively little effect upon the activity of the apical meristem of Chrysanthemum. Thus, while the apical meristem proper (eu- or promeristem) is the site of shoot organization and the ultimate source of the cells of the entire shoot, the subapical zone of division, termed the subapical meristem, is largely responsible for stem histogenesis in caulescent as well as in rosette plants. Gibberellins, or native, gibberellin-like substances appear to regulate the activity of the subapical meristem and thus to play an important role in shoot development. Amo—1618 and related compounds seem to exert their dwarfing effect in plants by acting as antagonists of gibberellins, at least with respect to the latters' function in regulating the subapical meristematic activity in the shoot.     

INCOMPATIBILITY OF MERISTEMATIC AND FILAMENTOUS GROWTH IN THE FERN GAMETOPHYTE

Gametophytes of Pteridium aquilinum can be maintained in red light as either 1- or 2-dimensional structures. The mode of growth realized in red light is dependent upon the activity of the meristem. An active meristem in a 2-dimensional structure will permit a continued development of that structure. A breakdown in meristematic activity results in filament formation. It is suggested that a group of actively dividing cells in some manner inhibits cell elongation and thus prevents filament formation in red light.     

DEVELOPMENTAL ANATOMY OF JUVENILE AND ADULT SHOOTS OF MARCGRAVIA RECTIFOLIA L

Marcgravia rectifolia L. is a dimorphic vine having distinct juvenile and adult shoots. The juvenile shoot is a climber characterized by an orthotropic growth habit, a flattened stem, adventitious roots, and ovate leaves. The adult shoot, on the other hand, possesses a plagiotropic growth habit, has a cylindrical stem, few or no adventitious roots, and lanceolate leaves. Both phases have distichous phyllotaxy, however the plastochron is shorter for the adult phase than for the juvenile. Internode elongation occurs earlier for adult shoots than for juvenile shoots. Cytological analyses show the flattened stem of the juvenile results from differential production of cells, especially in the pith region. On the other hand, internodes of the adult phase are longer than juvenile internodes, a result of more cells produced rather than longer cells. In juvenile stems a perivascular band of elongated fibers develops, while in adult stems this band consists of brachyosclereids. Both phases undergo secondary growth and have non-storied cambia. Cambial activity begins in the 6th internode of each phase. As secondary growth proceeds, the adult stem produces much more xylem than juvenile stems of the same age. Adventitious roots produced in the juvenile stem are located in vertical rows at the “corners” of flattened stems and are attachment structures aiding the climbing habit of this vine. Phase changes occur regularly in this species. The juvenile phase usually transforms into the adult, however the adult phase can spontaneously revert back into the juvenile phase. The anatomical features and the phase changes are discussed and compared to Hedera helix, a vine whose phase changes have been studied in some detail. It is suggested that the anatomical features of Marcgravia rectifolia L. including its phase changes, may provide an alternative system to study physiological changes similar to those done with Hedera helix.     

The role of nodal roots in prostrate clonal herbs: ‘phalanx’ versus ‘guerrilla’

The influence of nodal rooting on branching was studied in three evolutionarily and morphologically diverse species of prostrate clonal herbs: Tradescantia fluminensis (a monocotyledonous extreme ‘phalanx’ species), Calystegia silvatica (a dicotyledonous extreme ‘guerrilla’ species) and Trifolium repens (a dicotyledonous intermediate species). In all three, branch development from axillary buds is regulated by a positive signal produced by roots together with inhibitory influences from both pre-existing branches and shoot apical buds (apical dominance). Responses to nodal roots are cumulative and increased root activity leads to more vigorous bud outgrowth. In the absence of nodal roots, a single basal root system is unable to maintain continued extension growth of the shoot. We suggest that as individual nodal roots and stem internodes are both short-lived in these nodally-rooting clonal species, the plants’ investment in them is minimal. Thus, in contrast to perennial species lacking nodal roots, individual root systems in prostrate clonal herbs are small and stems have little secondary thickening and development of long-distance transport tissues. Hence the decline in extension growth of the shoot in the absence of nodal roots could be linked to the weak development of long-distance transport tissues in their relatively thin horizontal stems and to resource sharing between primary stems and lateral branches (as suggested by the greater retardation of primary stem growth in the more profusely branched ‘phalanx’ species (Trifolium and Tradescantia) than in the weakly branched ‘guerrilla’ species, Calystegia). These findings are consistent with the view that the long-term persistence of genotypes of nodally-rooting prostrate species is dependent upon them encountering the moist conditions required to facilitate the continual development of new young nodal root systems.