Histochemical Studies of Sclereid Induction in the Shoot of Rauwolfia Species: I. CYTOCHROME OXIDASE

Results of histochemical tests for cytochrome oxidase activityin four species of Rauwolfia have been reported. The cells beneaththe terminal shoot meristem constitute the pith. Histochemicaltests showed intensified enzymatic activity in those cells ofthe pith which would differentiate as sclereid initials. Similarcytochrome oxidase activity also occurred in the sclereid initialsand the developing sclereids. The cytochrome oxidase activitywas associated with two types of particulate formation, thegranular and rod-shaped. The ground parenchymatous cells ofthe pith and the leaf-base showed very little enzymatic activityof cytochrome oxidase. The characteristic indophenol blue colorationdue to cytochrome oxidase activity did not appear in controlsections. Physiological changes correlated with morphogeneticexpression of some pith cells demonstrate that the physiologicalchanges occur before the initiation of sclereids in the morphologicallyhomogeneous parenchymatous cells of the pith. Intensified cytochromeoxidase activity was also recorded in the meristematic tissuesof shoot apex, procambium, axillary buds and the laticiferouscells of Rauwolfia.     

Anatomy of Gymnosperms Endemic to China,II. Taiwania flousiana Gaussen (Taxodiaceae)

Taiwania Hayata contains two species: T.flousiana Gaussen and T. cryptomerioides Hayata, both endemic to China. T. flousiana was investigated with both light and scanning electron microscopes in respect to shoot apex, external and internal surfaces of leaf cuticle, primary leaf, juvenal and mature leaves, young stem, secondary phloem and wood of stem, etc, It is shown that the shoot apex consists of the following five regions: (1) the apical initials; (2) the protoderm, (3) the subapical moher cells;. (4) the peripheral meristem, and (5) the pith mother cells. The periclinal and anticlinal division of the apical initials takes place with approximately equal frequency. The juvenal leaf is nearly triangular or crescent-shaped in cross section and belongs to the leaf type II. The mature leaf is quadrangular in cross section (the leaf type I). There are a progressive series of changes in size and shape of the leaf cross section. The stoma of the mature leaf is amphicyclic and occasionally tricyclic. The crystals in the juvenal leaf cuticle are more abundant than those in the mature leaf cuticle. The transfusion tissue conforms to the Cupressus type. The structure of juvenal leaf is the nearest to that in Cunninghamia unicanaliculata D. Y. Wang et H. L. Liu, while the mature leaf is similar to that of the Cryptomeria. Sclerenchymatous cells of the hypodermis in the young stem comprise simple layers and are arranged discontinuously. No primary fibers are found in the primary phloem. Medullary sheath is present between the primary xylem and the pith. There are some sclereids in the pith. The secondary phloem of the stem consists of regularly alternate tangential layers of cells in such a sequence: sieve cells, phloem parenchyma cells, sieve cells, phloem fibers, sieve cells. The phloem fiber may be divided into thick-walled and thin-walled phloem fiber. The crystals of calcium oxalate in the radial walls of sieve cells are abundant. Homogeneous phloem rays are uniseriate or partly biseriate, 1-48 (2-13) cells high, and of 26-31 strips per square mm. Growth rings of the wood in Taiwania are distinct. The bordered pits on the radial walls of early wood tracheids are usually uniseriate, occasionally paired and opposite pitting. Wood parenchyma is present, and its cells contain brown resin substances. Their end walls are smooth, lacking nodular thickenings. Wood rays are homogeneous. Cross-field pits are cupressoid. Resin canals are absent. Based on the anatomy of Taiwania and comparison with the other genera of Taxodiaceae, the authors consider the establishment of Taiwaniaceae not reasonable, but rather support the view that the genus is better placed between Cuninghamia and Arthrotaxis in Taxodiaceae.     

Noteworthy idioblastic sclereids in the stems of Eulychnia (Cactaceae)

Stems of Eulychnia (a genus of six to nine species of candelabriform or arborescent cacti) have a parenchymatic cortex with two distinct regions. The outer chlorenchymatic layer is characterized by a conspicuous parallel striping, whereas the inner cortex region devoid of chlorophyll has a coarsely granular aspect. Stem samples from nine accessions, collected in the field or taken from cultivation, were studied from resin-embedded microtome sections and maceration. Two different forms of lignified sclereids were found dispersed in the cortex and the pith. The sclereids of the outer palisade-like cortex layer are distinctly elongated and strictly oriented at right angle to the stem surface, whereas those of the inner cortex and pith are globular or subglobular and conspicuously enlarged compared with the surrounding parenchyma cells. The ontogeny of the sclereids was studied from stem samples of different ages. Formation of the secondary cell walls starts only after cell growth is completed. A screening of numerous South American cacti for the presence of idioblastic sclereids showed that these structures are unique for the genus Eulychnia. Finally, functional aspects of the sclereids are shortly discussed. It is assumed that the sclereids contribute to the mechanical support and reinforcement of the plants.     

The comparative stem and root anatomy of Goniothalamus andersonii, G. macrophyllus, G. malayanus and G. velutinus (Annonaceae) from the peat swamps of Sarawak

The stem and root anatomy of G. andersonii, G. macrophyllus and G. velutinus from the peat-swamps of Sarawak is compared. Sufficient anatomical differences exist to differentiate the four species. The main distinguishing anatomical features between the stems are the cork cells, the nature and relative quantity of cortical sclereids and the presence or absence of lignified pith parenchyma. In differentiating between the roots of G. andersonii, G. macrophyllus and G. malayanus. the principal features are the cork cells and the relative quantity of cortical sclereids. In addition, the medullary rays of G. macrophyllus root in transverse sections are noticeably broader in the xylem towards the phloem than those of the other species.     

木姜子油细胞的发育解剖学研究

利用薄切片法对木姜子茎叶油细胞的发育以及油细胞分布的研究结果表明：油细胞最早发生于第一叶原基以及茎端皮层和髓的基本分生组织中,在未出现油细胞以痛,上述器官的基本分生组织和原分生组织,难以区分油细胞的原始细胞与周围细胞。当油细胞原始细胞呈现出体积较大,液泡化程度较低,细胞核大而明显的特征才明显可辩,以后经过液泡融合,油细胞成熟和油细胞细胞质解体阶段而成为一贮油的囊,油细胞中未出现杯形构造。叶和茎中,     

A SEASONAL STUDY OF THE VEGETATIVE SHOOT APEX AND THE PATTERN OF PITH DEVELOPMENT IN HIBISCUS SYRIACUS

Tolbert , Robert J. (West Virginia U., Morgantown.) A seasonal study of the vegetative shoot apex and the pattern of pith development in Hibiscus syriacus. Amer. Jour. Bot. 48(3): 249–255. Illus. 1961.—The shoot apex of Hibiscus syriacus L. is described as having a cytohistological zonation superimposed on a tunica-corpus configuration. The apex is flat-topped or may have a saddle-back or concave appearance as seen in median longitudinal section. The metrameristem, consisting of the tunica and corpus initials, is comprised of large, light-staining, vacuolate cells that have thick cell walls and exhibit much dark-staining intercellular substance. Surrounding the metrameristem is the flanking meristem, which is responsible for the outer layers of the shoot, and from which the leaf primordia arise. The pith rib meristem lies below the metrameristem and consists of files of cells that are responsible for the pith. There are no major seasonal changes in the structure of the apex during the yearly cycle. The pith displays a long-shoot type of development with the cells remaining in distinct files during the first flush of growth in the spring. As growth slows and internode elongation is gradually reduced, the pith displays the characteristic short-shoot type of development, consisting of a spongy tissue of rounded cells with many intercellular spaces and no distinct files of cells. A crown is differentiated across the top of the pith at the end of the growth period. This consists of a band of cells with thick, dark-staining cell walls, which separates by the apex from the last year's growth. In contrast to many gymnosperms, this crown is dispersed by renewed cell activity the following spring.     

Ripening-related changes in the cell walls of Spanish pear (Pyrus communis)

Unripe Spanish pears ( Pyras commanis L. ev. Blanquilla ) were ripened at 18°C for 5 and 10 days. Softening of the cortical tissues was associated with swelling of parenchyma cell walls from 1 to more than 5 μm in 10 day ripe pears, by which time the pears were over ripe. However, there was little indication of cell separation and the middle lamella could be detected between most cell walls. Furthermore, cell separation was constrained by regions rich in plasmodesmata where wall swelling was prevented. Parenchyma cells in the 500 μm of tissue underlying the epidermis did not undergo ripening-related changes to the same extent as those of the cortex. These cells, in combination with a sub-epidermal layer of lignified sclereid clusters, constituted a relatively tough and protective skin. Ripening of the cortical tissues was associated with a depletion of alcohol-insoluble pectic polysaccharides, as indicated by the decrease in arabinose and uronic acid. Analysis of alcohol-insoluble cell wall preparations enriched in either parenchyma or sclereid cell walls indicated that this change was predominantly associated with the parenchyma walls. Such changes were less prominent in the peel. The decrease in pectic polysaccharides was accompanied by an increase in their solubility. During ripening, the sclereid clusters of the cortex continued in develop, as indicated by an increase in their size and yield of cell wall xylose and glucose. Cortical parenchyma cells radiating from the sclereids were firmly attached to the lignified cells. This was due to lignification extending from the sclereids into the primary walls of the parenchyma cells. We conclude that dissolution of pectic polysaccharides is one of several factors which determine softening during ripening of Spanish pears.     

ON THE DEVELOPMENT OF MACROSCLEREIDS IN SEED COATS OF PISUM SATIVUM L.

Macrosclereid differentiation was investigated by light and electron microscopy in pea testae during the transformation of protodermal precursors to the mature sclereids. The protodermal cells divide anticlinally and elongate into the macrosclereid layer during seed coat development. Young sclereids have elongate nuclei, plastids become somewhat granal during cellular maturation, vacuolation appears to be an autolytic process, and the cells have dense arrays of endoplasmic reticulum and ribosomes. Considerable dictyosome activity and microtubule development is observed as the secondary wall is produced. Many coated vesicles are associated with and fuse with the plasmalemma. During development, the outer tangential wall area of the macrosclereids acquires a definite cuticle and subcuticular layer. Also, at this time the sclereid walls under the subcuticular layer display semicircular microfibril orientation. The sclereid walls adjacent to the hypodermis become multilayered. As the macrosclereids near maturity, the “light line” becomes discernable in the light microscope at the junction of the cellulosic tips of the macrosclereids and the subcuticular layer. This “light line” is prominent using interference optics and is an osmiophilic layer in the electron microscope. This layer may represent the suberin “caps” reported by earlier workers.     

ONTOGENY OF THE PRIMARY THICKENING MERISTEM IN SEEDLINGS OF BOUGAINVILLEA SPECTABILIS

Anomalous secondary thickening occurs in the main axis of Bougainvillea spectabilis as a result of a primary thickening meristem which differentiates in pericycle. The primary thickening meristem first appears in the base of the primary root about 6 days after germination and differentiates acropetally as the root elongates. It begins differentiating from the base of the hypocotyl toward the shoot apex about 33 days after germination. The primary thickening meristem is first observable at the base of the first internode about 60 days after germination. It then becomes a cylinder in the main axis of the seedling. No stelar cambial cylinder forms in the primary root, hypocotyl, or stem because vascular cambium differentiation occurs neither in the pericycle opposite xylem points in the primary root nor in interfascicular parenchyma in the hypocotyl or stem. The primary vascular system of the stem appears anomalous because an inner and an outer ring of vascular bundles differentiate in the stele. Bundles of the inner ring anastomose in internodes, whereas those of the outer ring do not. Desmogen strands each of which is composed of phloem, xylem with both tracheids and vessels, and a desmogic cambium, differentiate from prodesmogen strands in conjunctive tissue. The parenchymatous cells surrounding desmogen strands then differentiate into elongated simple-pitted fibers and thick-walled fusiform cells that are about the same length as the primary thickening meristem initials.     

SHOOT APEX ORGANIZATION AND ORIGIN OF THE RHIZOME-BORNE ROOTS AND THEIR ASSOCIATED GAPS IN DENNSTAEDTIA CICUTARIA

The shoot apex of Dennstaedtia cicutaria consists of three zones—a zone of surface initials, a zone of subsurface initials, and a cup-shaped zone that is subdivided into a peripheral region and central region. A diffuse primary thickening meristem, which is continuous with the peripheral region of the cup-shaped zone, gives rise to a broad cortex. The roots occurring on the rhizomes are initiated very near the shoot apex in the outer derivatives of the primary thickening meristem. The roots that occur on the leaf bases also differentiate from cortical cells. Eventually, those cortical cells situated between the newly formed root apical cell and the rhizome procambium (or leaf trace) differentiate into the procambium of the root trace, thus establishing procambial continuity with that of the rhizome or leaf trace. Parenchymatous root gaps are formed in the rhizome stele and leaf traces when a few of their procambial cells located directly above the juncture of the root trace procambium differentiate into parenchyma. As the rhizome procambium or leaf trace continues to elongate, the parenchyma cells of the gap randomly divide and enlarge, thus extending the gap.     

Myb genes from Hordeum vulgare: tissue-specific expression of chimeric Myb promoter/Gus genes in transgenic tobacco

The structures of the three Myb -related genes Hv1 , Hv5 and Hv33 from barley were determined. They contain a single intron located in the second repeat unit of the Myb -related domain. By analogy to the animal MYB oncoproteins this conserved region of the gene product was shown by filter-binding experiments to exhibit nucleic acid-binding activity. Tobacco plants transgenic for chimeric Myb promoter/ Gus genes express the enzyme in a developmentally controlled and tissue-specific manner. During germination and early stages of plant growth, GUS activity is seen in the root cap and adjacent meristematic tissue. At later stages of plant development, GUS activity is predominantly observed in the shoot apical meristem, the roots and the nodal regions of the stem. Within the stem at stages of secondary growth, Myb promoters are active in defined cell types. In the internode low GUS activity is displayed by the innermost cell layer of the cortex, the starch sheath, that surrounds the vascular cylinder of secondary xylem and phloem tissue, as well as in pith rays originating from vascular cambium initials. In the nodal region Myb promoter-controlled Gus expression is mainly confined to the abaxial starch sheath of the leaf trace, to the branch traces and to internal strands of primary phloem. It is suggested that in addition to their activity in meristematically active plant tissues Myb genes are expressed in conductive tissues that are closely associated with vascular bundles.     

EMBRYOGENY AND HISTOGENESIS IN NERIUM OLEANDER II. ORIGIN AND DEVELOPMENT OF THE NON-ARTICULATED LATICIFER

Mahlberg , P. G. (U. Pittsburgh, Pittsburgh, Pa.) Embryogeny and histogenesis in Nerium oleander. II. Origin and development of the non-articulated Iaticifer. Amer. Jour. Bot. 48(1): 90–99. Illus. 1961.—Laticifer initials, collectively considered as a laticifer system, are differentiated in the globular embryo from meristematic cells which occupy a position within the potential procambial tissue. A total of usually 28 initials, in Nerium oleander, arise as an irregular ring of cells directly below the embryonic shoot apex, during initiation of the cotyledonary primordia. No anastomoses occur between laticifer initials. During subsequent development of the embryo, the laticifer initials grow in a bi-directional manner and penetrate into the root, cotyledons and toward the shoot apex. Upon enlargement the initials bifurcate repeatedly, many branches penetrate into the cotyledons, others grow into the cortex of the hypocotyl or penetrate between cells of the procambium. Repeated nuclear divisions within each initial result in the formation of a multinucleated protoplast in this cell type. The tips of laticifers occupy intercellular spaces during their growth; they do not penetrate into or through adjacent cells. A plexus of laticifer branches is formed within the cotyledonary node of the mature embryo. No new initials are formed during subsequent growth of the plant, rather certain branches from the cotyledonary nodal plexus penetrate into the enlarging shoot system. The nature of their growth habit and branching suggests that the tips of laticifer initials exhibit an intrusive form of growth.     

Vascular Differentiation in the Shoot Apex of Matteuccia struthiopteris

Initial vascular differentiation is generally considered tooccur in procambium. In ferns, however, a provascular tissueimmediately subjacent to the promeristem has been suggestedas an initial stage within which the procambium is subsequentlyformed. In contrast to this interpretation, a zonation conceptapplied in ferns recognizes a promeristem consisting of severallayers of cells in which no differentiation takes place. Thisstudy demonstrates that the shoot apex of Matteuccia struthiopterishas one cell layer of promeristem. Immediately subjacent tothe promeristem is the provascular tissue surrounding a centralgroup of pith mother cells. The developmental continuity betweenthe provascular tissue and the mature vascular tissue, and betweenthe pith mother cells and the pith, through transitional stages,indicates that the initial differentiation of vascular tissueand pith takes place in this prestelar tissue. The continuityof vascular differentiation in the area confronting young leavesor incipient leaf positions is interrupted by the formationof leaf gap initials. Developing leaves thus begin to exertinfluence on the vascular system at the prestelar stage. Smallprotoxylem elements with helical cell wall thickening, and distinctiveprotophloem elements are present in the leaf traces, but endabruptly near the junction regions of leaf traces to the meristele.Copyright1994, 1999 Academic Press Initial vascular differentiation, provascular tissue, pith mother cells, shoot apex, Matteuccia struthiopteris     

SCLEREID DISTRIBUTION IN THE LEAVES OF PSEUDOTSUGA UNDER NATURAL AND EXPERIMENTAL CONDITIONS

Al -talib , Khalil H., and John G. Torrey . (U. California, Berkeley.) Sclereid distribution in the leaves of Pseudotsuga under natural and experimental conditions. Amer. Jour. Bot. 48(1): 71–79. Illus. 1961.—A study of the distribution of sclereids in cleared leaves taken from 1-, 2-, and 4-year-old shoots of an adult tree of Pseudotsuga menziesii (Mirb.) Franco showed a repeated pattern of sclereid distribution along the shoot axis with many sclereids in the basal leaves grading into few or no sclereids in the terminal leaves of each year's growth. Attempts were made to influence sclereid distribution by bud defoliation of attached branches with and without auxin treatment and by testing the effects of growth-regulating substances on sclereid formation in leaves of excised buds of Pseudotsuga cultured in vitro. Whereas removal of the basal ¾ of the leaves at the time of bud unfolding had no effect on bud, leaf or sclereid development, removal of the leaves of the upper half or complete defoliation led to premature expansion of next year's terminal bud with leaves developing in part from presumptive bud-scale primordia. Indoleacetic acid at 0.5% in lanolin paste applied to the defoliated region prevented this premature bud expansion. Defoliation of the basal half did not affect sclereid formation in the terminal leaves. Sclereid development in leaves of prematurely expanded buds on defoliated branches was normal except in the few cases where bud expansion occurred in the presence of low-auxin concentrations. Then, sclereid development was inhibited. Sclereid formation in leaves of excised buds grown in nutrient culture was generally much less frequent than in intact branches, and auxin treatment still further reduced the frequency of sclereids. It was concluded that sclereid initiation and differentiation in the intact plant may well be under the control of hormonal factors in the plant, one of which may be auxin.     

Developmental anatomy of vascular cambium and periderm ofBotrypus virginianus and its bearing on the systematic position of ophioglossaceae

The developmental anatomy of the vascular cambium and periderm ofBotrypus virginianus was studied, and its bearing on the systematic position of Ophioglossacease is discussed. The cambial zone including cambium is initiated in a procambial ring of the stem before primary vascular tissue is well differentiated. The presumed cambium is composed of fusiform and ray initials. The cambium is extremely unequally bifacial, producing secondary xylem centripetally, and quite a small number of parenchymatous cells but no secondary phloem centrifugally. The cambial activity persists long, although it is very low in the mature part of the stem. It seems that the circumferential increase of the cambium is accommodated by an increase in the number of cambial initials. Secondary xylem is nonstoried and composed of tracheids with circular-bordered pits with evenly thick pit membranes, and uniseriate or partly biseriate radial rays. It makes up the bulk of the stem xylem. Periderm is formed almost entirely around the stem, simultaneous with its increment due to the secondary xylem. The combination of these anatomical features of secondary tissue supports the idea that Ophioglossaceae are living progymnosperms.     

毛青藤茎的发育解剖学研究

毛青藤茎顶端的原生分生组织由原套和原体组成,其行生细胞形成初生分生组织──原表皮、原形成层和基本分生组织。初生分生组织的衍生细胞分化形成茎的初生结构,包括表皮、皮层、维管束和髓。随着茎的继续发育,维管形成层开始活动,由束中形成层产生次生韧皮部和次生木质部分子,而束间形成层仅产生薄壁组织细胞形成宽的射线。在原生韧皮部筛管分化成熟的过程中,韧应部外方仍保留1—2层原形成层细胞,以后,它们分裂为多层纤维原始细胞,在次生结构形成时,这些细胞的细胞壁加厚,形成初生初皮纤维。茎始终未产生用皮。     

Comparative stem anatomy of Mirabilis (Nyctaginaceae)

Mirabilis, a primarily American genus of 50?C60 species almost restricted to the New World, is the most diverse within Nyctaginaceae. It not only has the greatest number of species, but also many life forms, with annual herbaceous, suffrutescent and shrubby species and with prostrate, decumbent to erect stems that are sometimes clambering. Stem anatomy has been studied only for M. jalapa, and its characteristics extrapolated to the entire genus. In this study we evaluated the taxonomic significance of stem anatomical characters from 24 species of Mirabilis, as well as their potential relation to habit evolution. Qualitative and quantitative characters of transverse and longitudinal sections were evaluated using light and scanning electron microscopy. Stem anatomy varies in several features. The phloem is arranged in short tangential spreading bands or in large tangential bands forming semi-complete rings; the conjunctive tissue is fibrous, with thin-walled sclereids and fibres, or parenchymatous; the vessels are solitary or grouped in radial multiples or clusters; the xylem fibres are very thin-walled or thin- to thick-walled with simple to minutely bordered pits; most species are raylessness; the pith has parenchyma or thin-walled sclereids or brachysclereids. The distribution of anatomical characters in Mirabilis does not correspond with the current infrageneric classification. We suggest that some stem anatomical characters are correlated with habit and that the vascular cylinder and pith characters are related to an increase in mechanical strength. This study provides new information and novel characters about the stem anatomy not only of Mirabilis, but of the family.     

ANATOMY OF SEEDLING ROOTS OF PSEUDOTSUGA MENZIESII

The seedling root system of Pseudotsuga menziesii (Mirb.) Franco consists of the primary root, active long laterals, long laterals that become mycorrhizal, and short roots that may or may not become mycorrhizal. Numerous adventitious roots arise from the pericycle in young roots and from the vascular cambium and pericycle in older roots following pruning. All actively growing apices have a single plate of initials, a complex zonation of mother cells, and a similar pattern of primary tissue differentiation. Short roots and mycorrhizal short roots have 2 plates of initials, one producing the stele and the other the root cap and cortex, and differentiation occurs close to the apex. Primary and adventitious roots are usually triarch, while long laterals are usually diarch as are all short roots. The latter lack secondary xylem, but mycorrhizal short roots may produce a small amount of secondary phloem.     

The role of plant hormones in higher plant cellular differentiation. II. Experiments with the vascular cambium, and sclereid and tracheid differentiation in the pine, Pinus contorta

In sterile-cultured explants of stems of the pine Pinus contorta Dougl., fusiform cambial cells differentiated entirely into axial parenchyma cells when exogenous indol-3yl-acetic acid (IAA) was omitted. The normal appearance of the cambial zone was maintained when IAA was included in the medium. The IAA-maintained stability of cambial structure suggests physiological rather than epigenetic control over vascular cambium structure. IAA was essential for the occurrence of callus growth in stem explants. Callus growth was similar in appearance and extent in winter- and summer-explanted material. Tracheids differentiated in explants only when actively differentiating tracheids were already present at the moment of explanting, suggesting the absence of factors necessary for tracheid differentiation in over-wintering tissues. Sclereid differentiation, which normally does not occur in phloem or xylem development in P. contorta, occurred in callus derived from active cambial explants. The sclereids were identical to sclereids which differentiated in pith of intact stems. The possibility that sclereid and tracheid differentiation may be fundamentally similar types of gene expression is discussed. Growth of P. contorta trees in continuous darkness resulted in extensive compression-wood tracheid differentiation in the upright main stem. Normal-wood tracheids differentiated in similar trees grown in light. More tracheids differentiated in light than in darkness. This apparently is the first report of induction of compression-wood tracheid differentiation in the absence of hormone treatment or tilting of trees. Different types and numbers of tracheids differentiated at different position in two-year-old disbudded defoliated stem cuttings of P. contorta in response to apically supplied IAA. No evidence for new tracheid differentiation was seen in control cuttings; however, the results suggest that neither cambial cell division nor tracheid differentiation were actually initiated by IAA. Directed transport of additional regulatory factors toward areas of high IAA concentration is formulated as a hypothesis to explain these observations. Gibberellic acid, (S)-abscisic acid and IAA inhibited tracheid differentiation when individually supplied to basal ends of P. contorta cuttings predisposed to differentiate new tracheids. Experiments with single intact needles on Pinus cembroides var. monophylla cuttings confirmed a previous interpretation that the mature pine needle, rather than the short-shoot apical meristem at its base, promotes tracheid differentiation in the stem.