THE ROLE OF LIGHT IN HISTOGENESIS AND DIFFERENTIATION IN THE SHOOT OF PISUM SATIVUM. I. THE APICAL REGION

Thomson , B. F., and P. M. Miller . (Connecticut Coll., New London.) The role of light in histogenesis and differentiation in the shoot of Pisum sativum. I. The apical region. Amer. Jour. Bot. 49(3): 303–310. Illus. 1962.—Seedlings of Pisum sativum grown under constant conditions and kept in total darkness or exposed daily to red or white light were harvested at the same plastochron age and examined histologically to determine what specific aspects of histogenesis and differentiation are affected by light. The tissue organization of the shoot apex is the same in all light conditions to a point below the 2 youngest leaf primordia. The first detectable difference is a slight thickening of the internode in light due to more and larger cells. The first effect on longitudinal growth appears below the fourth youngest primordium and consists of an increase of internode length in light-grown plants. This is associated with a greater distance between the apex and the first mature protoxylem. The distance from apex to the first pith, provascular strands, and protophloem and the distances between the 4 youngest leaf primordia are not affected by light.     

THE ROLE OF LIGHT IN HISTOGENESIS AND DIFFERENTIATION IN THE SHOOT OF PISUM SATIVUM. II. THE LEAF

Thomson , Betty F., and Pauline Monz Miller . (Connecticut Coll., New London.) The role of light in histogenesis and differentiation in the shoot of Pisum sativum, II. The leaf. Amer. Jour. Bot. 49(4): 383–387. Illus. 1962.—Development of the form and anatomy of leaves was studied in plants of Pisum sativum grown in vermiculite under constant conditions and exposed daily to red or white light or kept in continuous darkness. The red light used had an intensity in the morphogenetically active red region of the spectrum of 70–75% that of the white light. Light had no effect on the manner of initiation or early development of leaf primordia. Quantitative data from older leaves showed that light has no effect on the pattern of later development but does affect the rate and extent of development. Under all light conditions, the length of the leaflet is closely correlated with the state of its internal anatomy. “Mature” etiolated leaves duplicate young stages of light-grown leaves. Mature leaves grown in red light duplicate not-quite-mature leaves grown in white light. The difference between white-light and red-light leaves is attributed here to light intensity and resembles that between sun and shade leaves.     

SEASONAL DEVELOPMENT OF THE SECONDARY PHLOEM IN PINUS

Functional sieve cells are present at all times in the secondary phloem of Pinus banksiana Lamb., P. resinosa Ait., and P. strobus L. With regard to a given year's growth increment, all but the last-formed sieve cells (2-4 layers) cease functioning the same season they are derived from the cambium. The former overwinter and remain functional until new sieve cells differentiate in spring. Toward the end of March undifferentiated cells in the outer margin of the cambial zone begin to differentiate into sieve cells. About a week later, cambial activity (cell division) commences. All early phloem is produced by early May before new xylem differentiation begins. Most sieve cells are differentiated by late August, but a few not until late September. Cessation of function begins in late May or June with formation of definitive callose on sieve areas of the sieve cells which overwintered and continues slowly to sieve cells of the current season's early phloem. By mid-December all but the last-formed sieve cells (i.e., those which will overwinter in a functional state) are devoid of contents. Phloem differentiation precedes xylem differentiation by approximately 1 1/2 months. Xylem and phloem production cease more or less simultaneously in August, xylem and phloem differentiation in September.     

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.     

XYLARY UNION BETWEEN THE NEW SHOOT AND OLD STEM DURING TERMINAL BUD BREAK IN POPULUS DELTOIDES

Bursting terminal buds and subtending stems of Populus deltoides Bartr. ex Marsh. plants were examined at several stages to determine the pattern of xylary union between the 1st- and 2nd-yr growth increments. Metaxylem vessels differentiated first in traces serving the basalmost leaf in the bud. As successively younger leaves began growth, metaxylem vessels differentiated in their traces. The site of metaxylem reactivation was in the second or third internode beneath the bud (i.e., in the 1st-yr stem), and subsequent differentiation progressed bidirectionally in each set of traces as the leaves they served expanded. Traces leading either to abscised leaf positions on the 1st-yr stem or to bud-scale leaf positions were reactivated by the tangential “spread” of activity from adjacent traces serving expanding leaves. The new elements were all secondary xylem vessels, as were those of the basipetal trace components, although they were functionally continuous with the metaxylem vessels that differentiated acropetally. Xylem fibers were initiated in the same position and differentiated in the same sequence as vessels. However, fiber differentiation lagged behind that of vessels. Whereas vessel differentiation was associated with leaf expansion, fiber differentiation was associated with leaf maturation. As each leaf matured in sequence, the primary-secondary transition zone advanced acropetally to the bud base and then in the new shoot until it attained a positional relation with leaf maturation comparable to that of 1st-yr plants.     

ANATOMY OF NODES VS. INTERNODES IN COLEUS: THE NODAL CAMBIUM

Secondary growth begins in the nodal regions before the internodal regions in Coleus, so that longitudinally discontinuous vascular cambia are formed in the 6th through the 9th or 10th nodes, where the internodal cambium becomes continuous between nodal cambia. The nodal cambia are identifiable by radial seriation in interfascicular regions, typical cytology of fusiform initials, and the presence of a ray system. Anatomical features distinct from the primary plant body are shared by the nodal and internodal cambia. Branching of primary vascular strands, restricted to procambium and phloem, is virtually confined to nodal regions. In secondary growth, vascular branching of xylem and phloem occurs in both nodes and internodes. Xylem strand branches are formed only from derivatives of vascular cambia. It is proposed that the cambium provides the secondary plant body an efficient channel for lateral auxin transport, by which branching across interfascicular regions is facilitated.     

PHOTOCONTROL OF THE ORIENTATION OF CELL DIVISION IN ADIANTUM. I. EFFECTS OF THE DARK AND RED PERIODS IN THE APICAL CELL OF GAMETOPHYTES

When protonemata of Adiantum capillus-veneris L. which had been grown filamentously under continuous red light were transferred to continuous white light, the apical cell divided transversely twice, but the 3rd division was longitudinal. An intervening period of darkness lasting from 0 to 90 hr either between the 1st and the 2nd cell division or between the 2nd and the 3rd one did not affect the number of protonemata in which the 3rd cell division was longitudinal. The insertion of red light instead of darkness greatly decreased the percentage of 1st longitudinal divisions occurring at the 3rd division, and increased the number of transverse divisions. Fifty percent reduction of induction of 1st longitudinal division was caused by ca. 50 hr exposure to red light between 1st and 2nd division and by ca. 20 hr between 2nd and 3rd division, and total loss was induced by an exposure of ca. 100 hr or longer to red light in the former and by ca. 40 hr longer in the latter. Thus, by using an appropriate intervening dark period or exposure to red light, the orientation and timing of cell division could be controlled in apical cell of the fern protonemata.     

Anatomical analysis of growth and developmental patterns in the internode of deepwater rice

Submergence of the stem induces rapid internodal elongation in deepwater rice (Oryza sativa L. cv. Habiganj Aman II). A comparative anatomical study of internodes isolated from airgrown and partially submerged rice plants was undertaken to localize and characterize regions of growth and differentiation in rice stems. Longitudinal sections were examined by light and scanning-electron microscopy. Based on cell-size analysis, three zones of internodal development were recognized: a zone of cell division and elongation at the base of the internode, designated the intercalary meristem (IM); a zone of cell elongation without concomitant cell division; and a zone of cell differentiation where neither cell division nor elongation occur. The primary effects of submergence on internodal development were a threefold increase in the number of cells per cell file resulting from a decrease in the cell-cycle time from 24 to 7 h within the IM; an expansion of the cell-elongation zone from 5 to 15 mm leading to a threefold greater final cell length; and a suppression of tissue differentiation as indicated by reduced chlorophyll content and a lack of secondary wall formation in xylem and cortical sclerenchyma. These data indicate that growth of deepwater-rice internoes involves a balance between elongation and differentiation of the stem. Submergence shifts this balance in favor of growth.Abbreviations GA gibberellin - IM intercalary meristem     

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.     

VASCULAR CONNECTION BETWEEN LATERAL ROOTS AND STEM IN THE OPHIOGLOSSACEAE

The vascular connection between lateral roots and stem in the Ophioglossaceae and in two leptosporangiate fern species was examined. Two types of connections were found: “gradual” connections, which resemble leaf traces in ontogeny and morphology, and “abrupt” connections, which resemble the connections between lateral roots and their parent roots. Gradual root-stem connections occur in the genera Ophioglossum and Helminthostachys and in Woodwardia virginica. They are initiated in shoot apices distal to the level where cauline xylem elements mature. They resemble leaf traces in being provascular (procambial) strands that connect the cauline stele with the future vasculature of lateral appendages. As with leaf traces, gradual connections are part of the provascular and, later, protoxylem continuity between stems and lateral appendages. Gradual connections have many features in common with leaf traces, and the term root trace is applicable to them. The order of radial maturation of the primary xylem in gradual connections varies in different parts of the connections. It is endarch near the intersection with the cauline stele and exarch where the connections intersect root steles. Gradual connections resemble the transition regions of certain seed plants where protoxylem is also continuous from stem to root and the order of maturation is found to change continuously from stem to root. Abrupt connections occur in Botrychium and Osmunda cinnamomea. They develop in shoot apices at levels where cauline xylem is mature or maturing. The mature xylem does not dedifferentiate, so provascular and protoxylem continuity of the kind found in root traces does not occur. Also, reorientation of the order of maturation does not occur in abrupt connections. Xylem connectors are found in the region where radially oriented elements of the connections abut the longitudinally oriented cauline elements. Abrupt connections resemble the connection of secondary roots with their parent root systems since xylem connectors and the lack of continuity are also features found in these vascular systems. The resemblance of the vascular pattern of the fern root trace to the transition region of seed plants suggests that the radicle is more closely comparable to the cladogenous roots of pteridophytes than hitherto supposed.     

The rate of cell division in the shoot apical meristem during photoperiodic induction and transition to flowering

Cell division contributing to longitudinal growth of the shoot apex was investigated inChenopodium rubrum in segments marked by the axils of leaf primordia. Plants treated with two short days (16h of darkness and 8h of light) were compared with two non-induced controls (cultivated in continuous light or treated by alternations of 8 h of darkness and 4 h of light for two days). During the short-day treatments the rate of cell division contributing to the longitudinal growth decreases in all segments of the shoot apex irrespective of whether the darkness was given in inductive or non-inductive photoperiods. The rate of cell division contributing to longitudinal growth increases in the upper internodes of the shoot apex after the termination of the photoperiodic treatment and transfer of the plants to continuous light. However, cell division remains inhibited in the lowest segment of the shoot apex. This inhibition in the differentiating parts of the shoot apical meristem is a direct consequence of photoperiodic induction. It is supposed that this inhibition is related to evocation similarly as the well-known phenomenon of stimulation of cell division in the apical dome.     

RADIAL GROWTH IN THE CYCADALES

The development of radial growth which leads to the pachycaulous form was investigated in eight of the 10 genera of the Cycadales; i.e., Ceratozamia, Cycas, Dioon, Encephalartos, Macrozamia, Microcycas, Stangeria, and Zamia. In all taxa, development of radial growth is essentially the same: a primary thickening meristem is differentiated in the stelar region of the cotyledonary node of the seedling at germination and produces derivatives mainly centrifugally. This primary thickening meristem (PTM) then differentiates acropetally and becomes continuous with the peripheral zone of the shoot apex. At first the PTM is a vertical cylinder, but as the seedling continues to grow into an adult plant, the PTM shows a more horizontal orientation (like an open umbrella) and produces the broad cortex. Secondary growth is by a vascular cambium which produces secondary xylem to the inside and secondary phloem to the outside. The broad pith originates from derivatives of the rib meristem of the massive shoot apex. The seedling and young plant is composed of a shortened shoot (i.e., no internodes) produced by the PTM and rib meristem, and a large fleshy primary root which results from a diffuse growth pattern. Individual cells in both the pith and cortex of the root divide. Their derivatives divide at right angles to the original division plane. Thus, quartets and even octets of cells are recognizable and can be traced to individual parent cells.     

THE EFFECTS OF AUXINS AND KINETIN ON XYLEM DIFFERENTIATION IN THE PEA EPICOTYL

Sorokin , Helen P., S. N. Mathur , and Kenneth V. Thimann . (Harvard U., Cambridge, Mass.) The effects of auxins and kinetin on xylem differentiation in the pea epicotyl. Amer. Jour. Bot. 49(5): 444–454. Illus. 1962.—Treatment of isolated segments from the second internode of etiolated ‘Alaska’ pea epicotyls with indoleacetic acid or 2,4-D results in: (1) activation of fascicular cambium, and initiation of some interfascicular cambium, resulting in abundant production of secondary xylem, and in formation of hyperplastic tissue; (2) partial or even total occlusion of proto- and metaxylem. The secondary xylem formed consists of short vessel members with scalariformly reticulate or pitted walls, which often lack vertical connection with each other, being interrupted by unlignified cells. When IAA is used, the hyperplastic growth mainly takes the form of root primordia, whereas 2,4-D initiates the formation of callus, but not of root primordia. The growth of this callus causes a characteristic split at the base of the internode. Treatment with kinetin, alone or in combination with the auxin, changes the above structure markedly. It leads to the initiation, over the entire circumference of the core of the internode, of a still more active cambium, which forms several layers of secondary xylem; this consists mainly of long vessel members with pitted walls. Hyperplastic growth is completely absent, and the xylem does not become occluded. Thus the effect of kinetin is to make the xylem more normal and to alter the epicotyl structure from herbaceous to more-or-less woody.     

GROWTH PERIODICITY AND THE SHOOT TIP OF ABIES CONCOLOR

Parke , Robert V. (Colorado State U., Fort Collins.) Growth periodicity and the shoot tip of Abies concolor. Amer. Jour. Bot. 46(2) : 110-118. Illus. 1959.—The terminal shoot of Abies concolor shows a marked seasonal activity or growth periodicity of the meristmatic regions. Hence, the annual developmental sequence may be divided into three growth phases: the rest phase, during which the fully formed telescoped shoot remains in a state of suspended growth; the first growth phase, during which the telescoped shoot elongates rapidly and gives rise to numerous cataphylls; and, the second growth phase, during which shoot elongation is completed and a new unelongated axis bearing many needle primordia is formed. The shoot tip of Abies consists of 4 clearly definable cytohistological zones; the apical initials, the sub-terminal mother cells, the peripheral zone, and the zone of central tissue. The shape of the shoot tip and the volume of its various cytohistological zones change markedly during the annual growth sequence, but the basic zonation pattern remains the same.     

THE SHOOT-GROWTH RHYTHM OF A TROPICAL TREE,THEOBROMA CACAO

The shoot-growth rhythm of tropical trees is a little understood phenomenon. Correlations of tree growth with environment, determined from field studies in the tropics, have been largely inconclusive, and few studies have been done under controlled environmental conditions. As an initial part of a project to study shoot-growth rhythms in tropical trees this paper describes the rhythm in Theobroma cacao L. An individual shoot passes through alternate periods of growth and dormancy. The growth period is characterized by the expansion of leaves and elongation of the shoot. During dormancy the length of the shoot remains constant, and no new leaves expand. Shoot-growth rhythm was divided into phases. Dissection of shoot tips from the various phases shows that the total number of leaves and leaf primordia in the shoot apex remains constant during the dormant period and does not increase until the onset of the growth period. This indicates that activity of the apical meristem as well as leaf expansion and shoot elongation are rhythmic. We found that the rhythm of shoot growth persists under controlled environmental conditions and that growth under these conditions is asynchronous, as it appears to be in the field. Our data strongly suggest endogeneity.     

THE CONTROL OF THE ORIENTATION OF CELL DIVISIONS IN FERN GAMETOPHYTES

Time-lapse observations of filamentous fern gametophytes were used to evaluate whether the plane of cell division is referable to the plane of minimal surface area before and during the transition to two-dimensional growth. Cell dimensions of the apical cell were related to the length/width ratios associated with minimal area in the transverse plane vs. longitudinal plane, by modeling the apical cell as a hemisphere subtended by a cylinder. Our working hypothesis predicts that filamentous growth is perpetuated by an apical cell geometry that makes the transverse division plane the orientation of minimal surface area, whereas the transition to two-dimensional growth (longitudinal division of the apical cell) occurs once the longitudinal plane becomes the position of minimal surface area. The predictions of this hypothesis are fulfilled regardless of variations in light intensity and light quality, the presence of regulators of metabolism, or whether the experimental perturbation causes a corresponding selective inhibition of the transition to two-dimensional growth. Thus, the control of the plane of cell division in this system seems to depend on thermodynamic considerations of surface area. Furthermore, we favor the conclusion that the role of the genome in the transition to two-dimensional growth involves its influence on apical cell dimensions rather than the induction of specific genes for specific morphogenetic mRNAs.     

SOME ASPECTS OF VASCULAR DIFFERENTIATION IN ROOTS OF CASSIA

Hayat, Mohammed Arif (North Dakota State U., Fargo), and Charles Heimsch. Some aspects of vascular differentiation in roots of Cassia. Amer. Jour. Bot. 50(10): 965–971. Illus. 1963.—Vascular development, with emphasis on the differentiation of the protophloem, was studied in tips of primary roots of 18 species of Cassia. Variations in levels of protophloem sieve tube maturation were observed among roots of different species as well as among those of different length in the same species. In general, protophloem matured at greater distances from the apex in roots with the larger diameters. Compared with woody species, herbaceous species exhibited greater uniformity in levels of protophloem maturation, and this was correlated with greater uniformity in root diameter. Roots were either triarch or tetrarch. In some species with tetrarch roots, a change to a triarch pattern occurred during early growth. Structural changes in the differentiating root tip which involve the loss of a xylem arm and subsequent fusion of phloem strands are described.     

EMBRYOGENESIS AND SEEDLING DEVELOPMENT IN CASSIPOUREA ELLIPTICA (SW.) POIR. (RHIZOPHORACEAE)

Embryogenesis in Cassipourea elliptica (Sw.) Poir, begins with a first division of the zygote which may be oriented transversely, obliquely, or rarely longitudinally. The orientation of the second division is also variable. Though the differentiation of suspensor and embryo proper occurs early, some derivatives of the terminal cell sometimes contribute to the suspensor. Provascular tissue “differentiates” after the initiation of the cotyledons. The radicle apical meristem originates subterminally, 5–10 cell layers from the juncture of the embryo proper and the suspensor. After germination, during early seedling establishment, radicle apical organization is of an unspecialized, columellate type. Vascular differentiation occurs before germination, and there are two loci of initial xylem differentiation: one in the hypocotyl and another in the median trace of the cotyledons. After germination, additional xylem differentiates de novo (without lateral or longitudinal continuity with already-mature vessels) inside the arcs of phloem in the hypocotyl, a pattern reported in few angiosperms. The cotyledonary node is one-trace, unilacunar.     

Light enhances differentiation of the vascular system in the fruit of Actinidia deliciosa

Light is recognized as crucial in determining high quality of fleshy fruits, for example, kiwifruit [Actinidia deliciosa var. deliciosa (A. Chev.) C. F. Liang et A. R. Ferguson]. Among the possible mechanisms through which light improves the quality of kiwifruit berry, there may be a direct morphogenic role on the differentiation of the fruit's vascular system, though this has not yet been investigated. The present study's aim was to determine (1) whether light positively affects the differentiation of the vascular system of the fruit and/or the pedicel, and, if so, (2) which component (xylem, phloem, or both) is more affected, and (3) in which period of the berry's development the improvement of the vascular differentiation (if any) occurs. To this end, fruit morphogenesis of kiwifruit was studied in two developmental environments (i.e., in full sunlight and in paper bags that reduced the full sunlight to 10%), and in two phases of fruit development (i.e., 1 and 5 months [harvest] after anthesis). During the growth period, the type of environment did not affect the differentiation pattern of the vascular system in the three types of bundles present in the fruit. However, in comparison with shade, light improved the vasculature in the fruit pericarp and pedicel, inducing a consistently higher extent of the xylary component in the main bundles of the fruit and pedicel, principally due to an increase in the number of xylem elements. The phloic component was also increased by light, but to a much lesser extent than that of the xylary. During the entire period of development, light-grown fruits contained higher concentrations of calcium and magnesium, as compared with shade-grown fruits. In conclusion, in the berry of Actinidia deliciosa, light enhances the differentiation of the vascular system, in particular the xylary component. The hypothesis that fruit quality is improved through a more efficient translocation of specific mineral nutrients (e.g., calcium) via the xylem is presented.     

THE PRODUCTION AND DIFFERENTIATION OF SECONDARY XYLEM IN XANTHIUM PENSYLVANICUM

The removal of leaves and buds from the shoot of Xanthium seedlings caused the cessation of xylem fiber differentiation in all internodes while allowing the production of cambial derivatives to continue toward both the xylem and the phloem. The potential xylem fibers developed into parenchymatous cells with thin cell walls. The vessels developed normally except for their small size. Cambial derivatives and vessels were produced linearly with time in intact plants (6.1 cells per file/day and 9.7 vessels per vascular bundle/day) and in decapitated plants (2.2 cells per file/day and 5.5 vessels per vascular bundle/day). Fiber production was linear with time in intact plants (163 fibers per vascular bundle/day) and did not occur in decapitated plants. When a single leaf was allowed to develop from a lateral bud of a decapitated plant, xylem fiber differentiation was restored for a period of time corresponding to the period of rapid expansion of the leaf blade. When the leaf passed the phase of rapid expansion, it no longer had an inductive effect on xylem fiber differentiation.