SHOOT HISTOGENESIS: THE EARLY EFFECTS OF GIBBERELLIN UPON STEM ELONGATION IN TWO ROSETTE PLANTS

Sachs , Roy M., Charles F. Bretz , and Anton Lang . (U. California, Los Angeles.) Shoot histogenesis: The early effects of gibberellin upon stem elongation in two rosette plants. Amer. Jour. Bot. 46(5): 376–384. Illus. 1959.—Within 24 hr. after the application of gibberellic acid (GA) to vegetative plants of biennial Hyoscyamus and of the long-day plant Samolus, a considerable increase in mitotic activity was observed in the pith, cortical, and vascular tissues of the rosette axis immediately below the apical meristem. As the treatment continued, the zone of cell division increased commensurate with the increase in length of the stem; the new cell divisions formed transverse walls predominantly and thus contributed to stem elongation. The cell contribution from the apical meristem was but a small fraction of the total produced by the subapical tissues, suggesting that the induced subapical mitotic activity is the main site of tissue development in the shoot. There was no evidence for cell elongation for at least 72 hr. after application of GA, and, hence, the initial increase in stem length was due solely to an increase in cell number. With regard to the general problem of shoot histogenesis, our data for the rosette plants and those for Xanthium and Chrysanthemum showing extensive cell division far below the apical meristem, are in full agreement with the studies by Bindloss (1942) with tomato, and support her conclusion that “. . . it is no longer possible to think that the chief center of cell division is in a relatively short zone 60 to 100 microns from the stem tip . . . and that cell division activity in the promeristem is not solely responsible for stem length.” On the contrary, the mitotic activity in the subapical regions is undoubtedly responsible for the major part of the cells found in the stem.     

Characterization of Top Leader Elongation in Nordmann Fir (Abies nordmanniana)

Our understanding of the developmental changes that occur during top leader elongation in gymnosperms lags behind that in angiosperms. We developed a semiquantitative method for determining epidermal cell size, by measuring the Feret diameter after cell wall staining of stem epidermal peels. This method allowed a large number of cells to be measured at various locations in the top leader of the Christmas tree Abies nordmanniana. Further, we have identified the growth rate of individual sections of the top leader, and the relationship between cell length and needle arrangement throughout the top leader. At bud break, all stem units begin to elongate simultaneously, but growth ceases from the base upwards during top leader elongation. Long top leaders were characterized by having up to three times as long cells at the base compared to short top leaders, whereas the cell lengths were similar in the apical region independent of the given plant growth capacity. In the basal sector, the level of auxin was much higher, whereas the levels of cytokinins were lower than in the apical sector, causing the auxin/cytokinin ratio to change from about 3 in the apical sector to more than 20 in the basal part. The Fibonacci number changed in the apical sector due to an increased cell number in the stem units and therefore longer distance between the needles. We conclude that the general growth pattern during top leader elongation in A. nordmanniana is similar to angiosperms but differs at the cellular level.

     

STEM ELONGATION OF XANTHIUM PLANTS PRESENTED IN TERMS OF RELATIVE ELEMENTAL RATES

Vegetative Xanthium plants grown under noninductive conditions were marked along the stem with India ink and photographed during three successive days. The relative elemental rates of stem elongation [d(dX/dt)/dX] were estimated for 18 plants between 15 and 18 plastochrons. On the average, only the 8.0 cm terminal part of the stem was elongating in this group of plants. Young internodes were elongating at constant relative elemental rates ([d(dX/dt)/dX] was about 0.2 days^–1); nodal portions of the stem beteween two young internodes were not elongating. Internodes longer than 2 cm displayed an acropetal pattern of elongation in which the basal part of an internode stopped elongating and matured first and the apical portion last. The pattern of elongation of the stem could be best approximated to a set of cascading waterfalls with declining plateaus in the direction of the water flow. The acropetal pattern of individual internode elongation observed in Xanthium was similar to those reported for Helianthus and Phaseolus internode growth.     

PROBABILITY OF DIVISION OF CELLS IN THE EPIDERMIS OF THE PHLEUM ROOT

Erickson , Ralph O. (U. Pennsylvania, Philadelphia.) Probability of division of cells in the epidermis of the Phleum root. Amer. Jour. Bot. 48(3): 268–274. Illus. 1961.—Photomicrographic records of the growth of a Phleum root, made at R. H. Goodwin's laboratory, in which individual epidermal cells can be identified, have been analyzed to provide estimates of the probability that cells at various distances from the apex will divide. In the apical part of the meristem, from 0μ to about 100μ from the apex, all cells divide (prob. = 1.0). From about 100μ to 275μ, the probability of division falls progressively to 0.0. The relationship of these estimates to rates of cell division and elongation in the same root is discussed.     

A CALAMITEAN SHOOT APEX FROM THE PENNSYLVANIAN OF IOWA

Melchior , Robert C., and John W. Hall . (U. Minnesota, Minneapolis.) A calamitean shoot apex from the Pennsylvanian of Iowa. Amer. Jour. Bot. 48(9): 811–815. Illus. 1961.—A shoot apex of a calamitean stem is described from the Des Moines Series, Middle Pennsylvanian. Internodal elongation of the 7 preserved internodes follows a sigmoid curve. A large apical cell has produced derivatives in a fashion apparently comparable to those in Equisetum arvense, except for the number of cells in the first leaf primordium ring and, possibly, the intercalary meristem. Pith meristem developed close to the apical cell. Data from internodal cell elongation of hypodermal cells of the cortex are presented which demonstrate intercalary internodal growth; no intercalary meristems are preserved and the existence of intercalary meristems which might have produced a jointed stem like that of Equisetum is only inferred.     

LONG-TERM DEVELOPMENTAL CHANGES IN XANTHIUM INDUCED BY GIBBERELLIC ACID

One application of gibberellic acid (GA₃) to Xanthium shoots resulted in an initial large stimulation, followed by inhibition, of internode elongation. After presumed translocation of the hormone from the locus of its application to the stem apex several morphological changes were observed. There was a significant increase in number of mitotic figures in the apical meristem and a twofold increase in volume of the apical dome. With time, the rate of leaf production was accelerated about 1.8 times. The phyllotaxis of leaf primordia initiated under the influence of GA:, changed from a (2, 3) contact parastichy pattern in control shoot to a (3, 5) pattern. Final petiole length was smaller than the control, and the absolute rate of lamina expansion decreased under prolonged treatment. Gibberellic acid had a pronounced effect on leaf morphology. GA_a induced the development of lanceolate leaves instead of typical deltoid leaves. The reduction in leaf area coincided with a 32% reduction in the average area of epidermal cells. Plastochron changes were correlated with anatomical and morphological changes during the course of leaf development.     

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.     

AN ANALYSIS OF LEAF ELONGATION IN XANTHIUM PENSYLVANICUM PRESENTED IN RELATIVE ELEMENTAL RATES

Maksymowych , Roman . (Villanova U., Villanova, Pa.) An analysis of leaf elongation in Xanthium pensylvanicum presented in relative elemental rates . Amer. Jour. Bot. 49(1): 7–13. Illus. 1962.—Xanthium plants were grown vegetatively, and leaves, whose developmental stages were specified by a previously described leaf plastochron index (L.P.I.), were marked with India ink along the midrib and photographed during 3 successive days. The relative elemental rates of elongation, d(dX/dpl)/dX were estimated during the whole course of development. The pattern of elongation was not constant but was changing with increasing plastochron age of the leaf. The elements of a young leaf of L.P.I. 0.75 elongated with a constant relative rate. In older leaves, the d(dX/dpl)/dX values were progressively declining toward the tip of the lamina. After L.P.I. 6.3 the only increment in length was due to the elongation of the elements of the petiole. The pattern of growth distribution is discussed in terms of relative elemental rates with respect to cell division and cell elongation in various portions of the lamina and is correlated with the basipetal trend of tissue differentiation in the developing Xanthium leaf.     

The interaction of plant hormones and rotation around a horizontal axis on the growth and Flowering ofXanthium pensylvanicum

Summary Xanthium pensylvanicum plants were rotated around a horizontal axis at 0.25 rpm. The stem growth, and flowering response were studied in rotatedXanthium treated with gibberellic acid (GA₃), indoleacetic acid (IAA). Leaf growth of rotatedXanthium was also studied.In both light and dark, the length and the number of nodes in stems of rotatedXanthium were significantly less than the stationary controls. Application of 10 g and 100 g of GA₃ increased the stem length over that of the untreated controls and furthermore, GA₃ application decreased the dwarfing effect of rotation such that stems of rotatedXanthium treated with GA₃ were not significantly different from stationary GA₃ treated plants.Leaf laminae that were in the late cell division and leaf expansion stage at the beginning of the rotational treatment were not significantly different in size from the controls after 21 days of rotation. However, laminae that were in the cell division and earlier stages of development during rotational treatment were significantly smaller than the controls by the 17th day of rotational treatment. As the successive younger leaves expanded, the rotated laminae were significantly smaller than stationary laminae at the progressively earlier stage.In rotatedXanthium plants, floral enhancement did not occur when GA₃ was applied just prior to rotational treatment. Foliar application of IAA to both rotated and stationary plants did not significantly affect the floral response.With 2 Figures in the TextSupported in part by a grant toK. C. Hamner from the National Science Foundation and by USAF Grant AFOSR 61-81/French.     

Growth of the Stem of Agropyron repens (L.) Beauv

The growth rate of the stem of Agropyron repens (L.) Beauv.begins to decline when the sixth foliage leaf has expanded butthe relative growth rate declines throughout the period betweenthe production of one and ten mature leaves. On an absolutetime scale there is a progressive decline in growth rate ofsuccessively formed stem (node-internode) units. On a plastochronscale the relative growth rate of successive stem units declineswithin the apical region but increases behind the apex. Thedecline in the apical region is related to a decrease in therate of cell division and in the later formed stem units thereis no significant increase in cell number from the time of theirformation by the apex until the internode is initiated duringtheir fourth plastochron. These changes are related to concurrentchanges in the size of the shoot apex and in rates of leaf growth.     

Apical Dominance Control in Ipomoea nil: The Influence of the Shoot Apex, Leaves and Stem

The outgrowth of lateral buds is known to be controlled by theupper shoot tissues, which include the apex, the young leavesand the upper stem. An analysis of the influence of these plantparts on axillary bud elongation in Ipomoea nil was carriedout by various treatments on these specific tissues. A restriction of elongation in the main shoot due to eitherdecapitation or shoot inversion resulted in the release of apicaldominance A non-linear type of compensating growth relationshipwas observed between the 13 cm apical growing region of thestem and the lateral buds. It was determined by decapitation,defoliation and AgNO₃ treatments that both the 13 cm stem-growthregion and the young leaves (1–5 cm in length) had a muchgreater inhibitory influence on the outgrowth of specified lateralbuds than did the stem apex (consisting of the terminal 0.5cm of the shoot). The specified lateral buds which were analyzedfor outgrowth were located a number of nodes below the shootapex. The intervening nodes were debudded. Although the importanceof young leaves in the control of apical dominance has beenpreviously recognized, the most significant result from thepresent study with Ipomoea was the strong influence of the 13cm apical growth region of the stem on the out growth of thelateral buds. Apical dominance, Ipomoea nil L., Pharbitis nil, growth region, lateral bud outgrowth, decapitation, defoliation, shoot inversion     

QUANTITATIVE STUDIES OF THE ROOT APICAL MERISTEM OF EQUISETUM SCIRPOIDES

An investigation was made of the meristematic activity of the apical cell, its immediate derivatives (merophytes), and of other selected cell populations of the root of Equisetum scirpoides Michx. The plane of the first division of a derivative of the apical cell is radiallongitudinal, which provides evidence that merophytes immediately adjacent to the apical cell cannot be the ultimate root initials. The apical cell is as active mitotically in roots 20–40 mm long as it is in roots that are 0.25–1 mm in length. The mitotic activity of the apical cell and of other cell populations was determined from the mitotic index, and from determination of the durations of the cell cycle and of mitosis of the apical cell by using the colchicine method of metaphase accumulation. Microspectrophotometric measurements of DNA content indicated that there was no consistent increase in DNA (endopolyploidy) in the apical cell or in the other meristematic cells as roots increased in length. Conclusion: there is no evidence that the apical cell becomes quiescent or undergoes endopolyploidy as a root increases in length.     

The seminal root primordia in barley and the participation of their non-meristematic cells in root construction

In addition to the primary seminal primordium, the so-called secondary seminal root primordia are also initiated in a barley embryo. The primary root primordium is developmentally most advanced. It is formed by root meristem covered with the root cap, and by a histologically determined region with completed cell division. On germination, the restoration of growth processes begins in this non-meristematic region of root primordium by cell elongation, with the exception of the zone adjacent to the scutellar node, the cells of which do not elongate but continue differentiating. In the root primordia initiated later, the zone with completed cell division is relatively shorter, in the youngest primordia the non-meristematic cells may be lacking. The root meristem is reactivated after the primary root primordium has broken through the sheath-like coleorrhiza and emerges from the caryopsis as the primary root. The character of root meristem indicates a reduced water content at the embryonic development of root primordium. With progressing growth the root apex becomes thinner, the meristematic region becomes longer, and the differences in the extent of cell division between individual cell types increase. — The primary root base is formed of cells pre-existing in the seminal root primordium. Upon desiccation of caryopsis in maturation, and subsequent quiescent period, their development was temporarily broken, proceeding with the onset of germination. The length of this postembryonically non-dividing basal zone is different in individual cell types. The column of central metaxylem characteristic of the smallest number of cell cycles, has, under the given conditions, a mean length of about 22 mm, whereas the pericycle, as the tissue with most prolonged cell division, has a mean length of about 6 mm. In the seminal root primordia initiated later the non-dividing areas are relatively shorter. The basal region of seminal roots thus differs in its ontogenesis from the increase which is formed “de novo” by the action of root meristem upon seed germination.     

A kinematic analysis of tepal growth in Lilium longiflorum

Time-lapse marking experiments indicate that the growth of tepals in Lilium longiforum Thunb. from 3.7 mm to maturity is triphasic. Phase I (tepal lengths 3.7–10 mm) is characterized by spatial and temporal variation in growth rate and, in the epidermis, a random distribution of mitoses with an acropetal increase in cell area. During phase II (10–90 mm) cell elongation and (later) cell division is restricted largely to basal regions. Cell division ceases when tepals are less than one-third of their mature length of 155 mm. Phase III (90–155 mm) is characterized by the gradual transition from basal to apical growth, and a modification of epidermal cell shape. A sharp peak in growth at the extreme tip of the tepal coincides with anthesis.Abbreviations LRGR local relative growth rate - RER relative elemental rate of growth     

AN ANALYSIS OF STAMEN FILAMENT GROWTH OF NIGELLA HISPANICA

The post-meiotic stamen filament of Nigella hispanica L. under greenhouse conditions grows in length from 1 mm to approximately 10 mm at maturity in 16 days. Analysis of the filament epidermis suggests that the intercalary meristem is diffuse along the filament with a mid-point of activity near the center of the filament. The point of maximal activity, while initially central, is variable as cell division nears completion. Measurement of cell lengths along filaments suggests that an elongation gradient from base to tip is operative in filaments 1 mm and longer. Average cell lengths of epidermal cells increase faster than do those of terminal cells. Once average cell length begins to increase in any region of the epidermis it continues to do so until flower maturity. At maturity the longest epidermal cells are near the filament base and the shortest cells are at the tip. The differences between cell division and cell elongation patterns suggest that these two processes are controlled by different sites or substances. A comparison is made between the development of the Nigella filament and other determinate organs having intercalary meristems.     

PETIOLE DEVELOPMENT AND XYLEM DIFFERENTIATION IN XANTHIUM REPRESENTED BY THE PLASTOCHRON INDEX

Petiole development and formation of xylem vessels have been investigated in Xanthium leaves from early ontogeny to maturity. Kinetics of growth was presented in terms of absolute and relative elemental rates of elongation. The process of vascularization was assessed by the number of differentiated xylem vessels. The leaf plastochron index (LPI) developed by Erickson and Michelini (1957) was used for designating the various stages of development. An exponential increase in petiole length was observed between the LPIs –3 and +4 indicating a constant relative rate of 0.20 or 20% increase per day. After cessation of lamina elongation at LPI 8, petiole elongation continued for an additional 5 day period, to LPI 9.5. Relative elemental rate analysis revealed that the basipetal pattern of elongation was maintained throughout the leaf development. At a specific plastochron age, the only growth was due to the petiole elongation. Leaves which ceased elongating had not completed their internal development, since the process of xylem formation continued for several plastochrons, or about 8 days. The highest rate of xylem formation was ten vessels per day at LPI 5. On the average, about five xylem vessels differentiated per day in the middle portion of a Xanthium petiole. Mature petioles contained an average of 218 xylem vessels. About 12 canals of schizogenous origin preceeded the development of the vascular tissue.     

INTERCALARY MERISTEMATIC ACTIVITY IN THE SPOROPHYTE OF FUNARIA (MUSCI)

Marking procedures were combined with anatomical techniques to establish that in Funaria (1) the apical region does not act as an apical meristem contributing to seta growth, and (2) the subapical region contains an intercalary meristem the derivatives of which account for the elongation of the seta. In sporophytes that are 8 mm long there is a distinctive difference in the pattern of cell division in the apical and subapical regions. Large, undivided endothecial cells exist in the apical region, and a central strand of elongated cells occupies the analogous position in the subapical region. The apical region is earmarked to form the operculum and spore sac and part of the apophysis. There is an ontogenetic continuity between the seta and the lower portion of the apophysis, but the uppermost cells of the subapical region do not contribute to seta formation. Instead, these cells and those at the base of the apical region form a transitional zone between apical and subapical influences, and they account for most of the stomates that develop on the apophysis.     

The role of nodal regions in growth of Xanthium (Compositae) stem

Vegetative Xanthium plants grown under noninductive conditions were marked with India ink along the stem and photographed at three consecutive time intervals. Relative elemental rates (d[dX/dt]/dX) of stem elongation were estimated from displacement of marks during stem elongation. Young internodes elongated with constant relative elemental rates of 0.2 day^-1. Older internodes displayed an acropetal pattern of elongation in which the basal segments of an internode stopped elongating first and the apical portion last. Nodal regions elongated with very small relative elemental rates of 0.05 day^-1. Rates decreased as the age of the internodes and nodes increased and stopped shortly after Leaf Plastochron Index 9.     

MITOTIC ACTIVITY IN THE ROOT APICAL MERISTEM OF AZOLLA FILICULOIDES LAM., WITH SPECIAL REFERENCE TO THE APICAL CELL

The meristematic activity of the apical cell and its derivatives (merophytes) in the unbranched, determinate roots of Azolla filiculoides Lam. was investigated. The plane of division of the apical cell indicates that it is the initial of each merophyte. The division plane of each newly formed merophyte is strictly periclinal to the root surface and provides confirmation that the immediate derivatives of the apical cell cannot be the ultimate root initials. The frequency of cell division as determined by the mitotic index, and by the duration of the cell cycle as determined by the colchicine method, confirmed the meristematic activity of the apical cell. As roots increase in length, the duration of the cell cycle in the total meristem increases, with the apical cell possessing the longest cell cycle, whereas the immediate derivatives maintain approximately the same cycle duration as in shorter roots. In determinate Azolla roots, cell division appears to play a major role up to a certain root length, then increase in length is produced mainly by cell elongation.     

ORGANOGRAPHY,BRANCHING, AND THE PROBLEM OF LEAF VERSUS BUD DIFFERENTIATION IN THE VINING EPIPHYTIC FERN GENUS MICROGRAMMA

Investigation of the development and organography of the shoot systems of Microgramma vacciniifolia and M. squamulosa was undertaken for the purpose of determining: (1) the features of shoot growth that are responsible for the distinctive vining character of these epiphytic ferns; and (2) the mode of origin of branches and their contrast with leaf initiation. Shoots of both species are dorsiventral and plagiotropic (i.e., parallel to the substrate) in habit. Since the shoot apical meristem is radial in transectional symmetry, shoot dorsiventrality in Microgramma is a postgenital or secondary developmental event, and its inception is related to the initiation of lateral appendages. Leaves and buds arise in a distichous phyllotaxis and occupy opposite and alternating positions on the dorsal surfaces and flanks of the rhizome. Endogenous roots are initiated in two rows from the ventral surface of the stem, in the vicinity of the rhizome meristem; however, they do not emerge from the rhizome until some distance behind the tip and do not elongate until the region of substrate contact. We conclude that the vining nature of this fern rhizome is a result of precocious internodal elongation and the concomitant delay of leaf and bud expansion in the region of stem elongation. In addition, observation of branch origin confirms previous suggestions that branching in Microgramma is strictly lateral and extra-axillary and not a dichotomous derivative as proposed by some workers. Leaf and bud primordia differ not only in the nature of their respective vascular supplies but also in their actual course of initiation. In the case of the leaf, the primordium is precociously emergent and exhibits a lenticular apical cell at its summit when it is only one plastochron removed from the flanks of the apical meristem. By contrast, initials of the bud primordium divide less actively and remain in a sunken position for at least 5–6 plastochrons; only when the bud apex becomes expanded and emergent does a tetrahedral apical cell become recognizable at the tip of the bud promeristem. Because of the distinctive pattern of branch and leaf origin, as well as the lack of adventitious and phyllogenous origin of branch primordia, we suggest that the shoot of Microgramma is a useful test organism for the re-examination of the problem of leaf and bud determination in the ferns.