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The development of secondary xylem has been studied historically from an anatomical point of view, but recent developments in cell and molecular biology have revitalized this field. An integrated view of cell structure with physiology is emerging for each stage of the developing xylem cells’ lives, from birth in the cambium to programmed cell death. High-quality structural information is essential in building this type of integrated view, but conventional electron microscopy of developing wood cells has been problematic. The importance of adequately preserving cells of the secondary vascular system is illustrated with examples from dormant cambium, cell division, and secondary cell wall deposition. In many cases, contemporary gene expression studies can be viewed in the context of both new structural information and pioneering live cell studies done in the early 1900s to increase our understanding of secondary xylem development.

The distribution and the ontogenesis of lateral roots have been investigated in the Mediterranean woody species Quercus suber L. (cork oak). Lateral roots arose in protoxylem-based ranks and a tendency to clumping was observed. Three stages are distinguished in lateral root primordium development. Lateral root primordia are derived mainly from pericycle cells. The endodermis contributed to the initial lateral root development, forming an endodermal cover that sloughs off with lateral root emergence. The unemerged lateral roots show an incipient layered root meristem; this meristem can be classified as a closed type meristem. Primary vascular connection takes place with the xylem strand opposite the lateral root primordium and the two adjacent phloem strands. Primary vascular connector elements are derived from pericyclic derivative cells. Vascular parenchyma cells contribute mainly in the development of the cambium and the subsequent secondary xylem and phloem connector elements. The secondary vascular elements of the lateral root and parent root differentiate in continuity. Vascular connection is discussed in relation to the root vascular plexus described in monocotyledonous and in some herbaceous dicotyledonous plants. An endodermis with suberized lamellae is continuous between the lateral and parent root in emerged lateral roots.Key words: lateral root, development pattern, apical lateral root meristem, root vascular connection, Quercus suber L.

Six-week-old Pinus contorta seedlings were inoculated with spores of the western gall rust fungus Endocronartium harknessii. Of 660 seedlings inoculated, 174 showed early symptoms and were sampled for histological observation in the 7 weeks following inoculation. Pigmentation of epidermal cells was the first externally visible response to infection, generally occurring 14–28 days after inoculation. Developing symptoms were extremely variable, ranging from little or no visible response to severe necrosis. The appearance of external symptoms was largely due to changes occurring in underlying infected cortical cells such as the production of phenolic compounds, necrophylactic periderm, or necrotic tissue. Intercellular lignin deposition was observed in infected tissue and was thought to be associated with necrophylactic periderm formation. Suppression of normal exophylactic periderm development was evident in virtually all infected tissue zones. Infection of the vascular cambium occurred as early as 21 days after inoculation and resulted in the subsequent formation of distorted xylem tracheids typical of gall tissue.

On three dates during September–December, the histochemistry and dormancy status of cambial zone cells were investigated in 6- and 19-year-old balsam fir (Abies balsamea) trees at the top of the crown (1-year-old cambium) and near the base of the bole (5- and 13-year-old cambium, respectively). The extent of dormancy on the date of collection and after 4 weeks in controlled-environment conditions favorable for growth indicated that a changeover between the dormancy stages of rest and quiescence occurred in all ages of the cambium during the experimental period. In trees of both ages, cambial dormancy began later, and the number of fusiform cells in the cambial zone was greater at the bottom of the tree than at the top. The staining intensities for total cytoplasmic RNA, protein, and insoluble carbohydrates in the fusiform cambial zone cells increased during the rest–quiescence transition at the top and bottom of the 6-year-old trees and at the top of the 19-year-old trees. However, no significant increase in RNA staining occurred at the bottom of the 19-year-old trees.

The maple cambium miner, Phytobia setosa (Loew), attacks Acer spp., producing ray flecks which result in degrade in face veneer and furniture wood. Samples from infested sugar maple, Acer saccharum Marsh, trees demonstrated that while mines passed close to the vascular cambium the initial cells were not affected. Thus, although it is called a cambium miner it does not mine the cambium. Mines filled with parenchyma cells which proliferated from severed vascular rays. These cells, when mature, stored starch. In heavily infested trees the starch storage area in the xylem may thus be measurably increased. The zone of newly differentiating xylem provides the insect with the path of least resistance; variation in the condition of secondary xylem may account for the variability in host susceptibility.

Fusiform initial reorientation, deduced from changes in tracheid arrangement, was investigated in helical bridges left after Abies balsamea stems were girdled. The first responses included an increase in the number of S- and Z-oriented anticlinal divisions without any change in their ratio, a decrease in fusiform initial length, and the formation of traumatic resin canals. Subsequently, normal tracheid differentiation resumed, the frequency of anticlinal divisions further increased, and there was an increase in the number of oriented intrusive growth events. The orientations of the anticlinal wall and of the intrusive growth were predominantly in the direction that eventually would result in the fusiform initials being aligned parallel to the bridge angle. The application of exogenous indol-3-ylacetic acid to the upper edge of a helical bridge inhibited fusiform initial reorientation, mainly by decreasing the frequency of oriented intrusive growth events. However, immediately below the application point, exogenous indol-3-ylacetic acid also induced the fusiform initials to begin temporarily to realign so as to be perpendicular to the bridge angle. The transport of a pulse of [1-14C]indol-3-ylacetic acid applied to the shoot apex was inhibited 1 day after girdling in both helical and longitudinal bridges, particularly the former, whereas it was inhibited only in helical bridges at the end of a subsequent 11-week growing period. The degree of inhibition in the helical bridges was greater the day after girdling than at the end of the growing period, during which interval fusiform initial reorientation occurred at the bottom of the bridge width. The results support the conclusion that the orientation of fusiform initials is parallel to the direction of indol-3-ylacetic acid transport.

Germinated and attached seedlings of the phanerogamic parasite Conopholis americana (L.) Wallroth are described for the first time. The morphogenetic changes which characterize the transition from seedling to preflowering tubercle, as well as the anatomy of the mature tubercle, including endogenous floral buds, are also described. Conopholis seedlings were found attached only to mycorrhizal oak roots. The primary haustorium attached to and penetrated the fungal sheath, pushing aside epidermal and cortical cells of the host as it grew toward the host stele. Cells of the endophyte contacted host xylem and differentiated directly into reticulately thickened tracheary elements. Concomitantly a cambial are of parasite origin was established within the young tubercle, aligned with the vascular cambium of the host root. The cambium of the tubercle gave rise to xylem centripetally, which differentiated into tracheary elements juxtaposed to host secondary vessels, and cortical parenchyma centrifugally. Sieve elements could not be unequivocally demonstrated in seedlings or tubercles. Tissues and cell types of the parasite could be distinguished from those of the host by both morphological and histochemical criteria. The parasite to host transition zone was characterized by direct tracheary element connections and delimited by densely cytoplasmic parasite parenchyma cells that had greatly enlarged centrally located nuclei.

This study examines the microscopic anatomy and seasonal changes of the secondary phloem, cambium, and a portion of the xylem of grand fir trees (Abies grandis [Dougl.] Lindl.) infested with the balsam woolly aphid (Adelges piceae Ratz.) as compared with tissues of non-infested trees.The reactivation of the vascular cambium and production of astrosclereids and resin cells are about the same in infested and non-infested trees.The infested trees exhibit sieve cells that are shorter in length, having a tangential dimension about the same as normal cells, and produce more tangential bands of phloem parenchyma cells, more fiber sclereids, biseriate rays, and lipoidal-filled ray cells, abnormally shaped ray parenchyma cell nuclei, giant cortical parenchyma cells, and traumatic resin ducts in the xylem.

Radial files of tracheids having changing orientations were traced through serial tangential sections to deduce events in fusiform cambial cells (FCCs) associated with spiral grain formation. Within an induction bridge of phloem and cambium sloping diagonally across a stem girdle in Pinus contorta Dougl. ssp. latifolia, repetitive, homologously sloping, pseudo-transverse anticlinal divisions in FCCs were a primary event contributing to FCC orientation changes. Imperfect periclinal divisions (IPDs) in FCCs also contributed. In Picea glauca (Moench.) Voss, IPDs and FCC elongation interacted to change tracheid orientation during natural spiral grain development. In both natural and induced reorientations, small numbers of adjoining FCCs changed their orientation abruptly while the greater population of FCCs changed orientation less rapidly. These sites of abrupt reorientation are termed "microdomains."not available

The vascular cambium was reoriented in square 5-mm panels of bark at 90° and 180° from the normal position in three varieties of Malus baccata and Malus sylvestris (ornamental crabapple), Cornus stolonifera, Sorbus aucuparia, and Thuja occidentalis, Douglasii pyramidalis. Panels grafted in the normal orientation served as controls. The cambium in the panels continued to function normally, and where it was reoriented, it retained the imposed orientation. The secondary vascular tissues that were produced likewise retained the imposed orientation, a feature which was particularly striking in the case of transversely oriented panels. In early growth after the grafting, the secondary xylem showed some histological abnormalities particularly in abnormally oriented panels, but these diminished in later-formed xylem in the case of normally and transversely oriented panels. It is concluded that the cambial initials possess a high degree of autonomy which is reflected in their resistance to realignment which would correspond to the lines of flow of nutrients and water in an axis into which they are grafted.

In the roots of Hieracium florentinum plants grown in hydroponic nutrient cultures, vascular parenchyma cells adjacent to both xylem and phloem conducting elements develop wall ingrowths and become transfer cells. Xylem transfer cells occur around the protoxylem elements and secondary xylem elements at the base of the junction of a lateral root with the main root stele and along the xylem elements of the lateral root for some distance into the lateral root. Phloem transfer cells occur adjacent to sieve elements in the phloem regions of the main root stele which have connections with the lateral root phloem and adjacent to sieve elements in the lateral root. Transfer cells were absent in the vascular parenchyma of the main root stele not associated with lateral roots.

Ophiostoma ulmi (the Dutch elm disease pathogen) was inoculated into 13 nonhost tree and shrub species. Four classes were recognized based on their susceptibility to this pathogen. In Prunus pensylvanica (class I), bubbles formed in vessel members within 3 days after inoculation, and gels formed 5 days later. At day 5, the vascular cambium covering the colonized zone became altered. A barrier zone was formed in 60% of the inoculated twigs. When twigs wilted, the vessels around the twig were occluded, and barrier zone formation was partial. In Populus balsamifera (class II), tyloses formed in vessels of invaded xylem within 8 days after inoculation followed by accumulation of compounds suspected to be phenolics in parenchyma cells. Barrier zone formation occurred in 66% of the inoculated twigs. In Sorbus americana (class III), a pronounced dark discoloration developed rapidly in the invaded xylem. Sparse gels and O. ulmi cells were present in vessel members. Gel formation was limited to vessels that were adjacent to parenchyma cells. Pit membranes of bordered and half-bordered pit pairs became thicker and more darkly stained than in controls. These observations suggest that the first steps of pathogenesis are induced by the action of harmful metabolites of O. ulmi. Few noticeable changes occurred in the species of class IV.

Incipient and advanced tumors were present in the hypocotyl and the first three internodes of severely stunted, 10-node plants of Gossypium hirsutum × G. gossypioides hybrid. Tumors, free of bacteria or fungi, were initiated at or near the vascular cambium as small clumps of hypertrophied parenchyma. Disorganization of segments of the vascular cambium occurred in association with tumor initiation. Subsequent growth of tumors caused disorganization of the phloem and formation of wedge-shaped masses of tumorous parenchyma in the xylem. Isolated tumors occurred in the pith of 14-node plants.The stem xylem of tumorous 10-node plants had increased levels of the flavanols catechin and gallocatechin, normally present only in a few scattered parenchyma cells in nontumorous xylem tissue. Several terpenoid aldehydes, predominantly hemigossypol and methoxyhemigossypol, occurred in tumorous xylem tissue, but were absent in nontumorous tissues. Histochemical reagents showed that the flavanols and terpenoid aldehydes were associated with incipient and advanced stages of tumors in the stem xylem.

A light microscopic study of the inner bark tissues of 11 temperate hardwoods showed seasonal differences in the degree of vacuolization and in the type and amount of storage material within phloem parenchyma and vascular cambium cells. Throughout the summer these cells were highly vacuolated, and starch-containing plastids were often present. During the overwintering period the typical large, central vacuoles were replaced by small, spherical organelles approximately 0.5–7.0 μm in diameter. The contents of the spherical organelles stained positively for protein. These organelles are considered to be analogous to protein bodies of seeds and to play a role in the storage of nitrogen. This study confirms and extends previous reports of protein bodies within parenchymatous cells of overwintering temperate trees. It further demonstrates that the organelles occur seasonally and that the formation of protein bodies for overwintering storage is a common phenomenon.

The vascular cambium and the growth ring it produces are continuous from trunk to branch, but the cells formed by the cambium in the upper junction of branch and trunk are oriented at approximately right angles to the normal orientation in the trunk and branch. Branch tissues begin to develop before trunk tissues early in the growing season. Maturation of branch tissues proceeds basipetally. The branch xylem is oriented downward at the branch base and encircles it to form a collar. The collar tissues meet on the trunk below the branch. The branch collar is enveloped later in the growing season by a collar of trunk xylem. Xylem in the trunk collar meet above and below the branch. Conduction into and out of the branch follows the pathway of the branch collar. The branch is structurally attached to the trunk by a series of trunk collars that envelop the branch collars every growing season. When the trunk collar was injured or removed by branch pruning, the trunk xylem above and below the cut was rapidly and extensively infected and decay developed. When pruning cuts did not injure or remove the trunk collar, no infections developed in the trunk xylem. Dye movement and the patterns of spread of bacterial and fungal pathogens also suggested that there was no local direct conduction between trunk xylem above a branch and within a branch.

The results are presented of investigations on the effect of 2,4,5-T (butyl ester of 2,4,5-trichlorophenoxy acetic acid) and dalapon (sodium salt of 2,2-dichloropropionic acid) on tracheid differentiation and mitotic divisions of vascular cambium and phellogen in 1- and 2-year-old shoots of spruce seedlings treated with herbicides in concentrations of 10, 100, 1000, 5000, and 50000 mg l−1 before and after bud differentiation.The investigations showed that 2,4,5-T at 100 and 1000 mg l−1 can periodically stimulate mitotic activity of the cambium, resulting in an increase of tracheid number. Increased mitotic activity of phellogen, which resulted in an increase in number and expansion of cortical and peridermal cells, caused by 2,4,5-T, was also observed. Dosages of 5000 mg l−1 and higher of 2,4,5-T supressed mitotic divisions.No stimulatory effects of dalapon, applied on seedlings either before or after bud differentiation, were observed either in the cambium or in the cortex and the periderm. High dalapon concentrations (over 5000 mg l−1) retarded mitotic divisions in the cambial zone.

Anatomical changes in the vascular cambium and secondary xylem of the first internode of soybeans were observed 15 days after an aqueous spray of 4-(2,4-dichlorophenoxy)butyric acid (2,4-DB) was applied at 0.56 kg acid equivalent per hectare. The cambium became a meristem of initials, which were intermediate in morphology between normal ray and fusiform initials, and the system of ray initials was lost. The xylem lost its rays and became a tissue in which narrow vessel members abnormally outnumbered the axial parenchyma cells, libriform fibers, and gelatinous fibers which were common in normal wood. The herbicide inhibited the normal expansion of vessel-member primordia.

Phytolacca dioica (L.) is characterized by anomalous secondary thickening by means of supernumerary cambia. After a period of primary growth and the formation of an initial (normal) vascular cambium, supernumerary cambia are initiated outside of the primary vascular cylinder. The initiation of the first supernumerary cambium takes place through approximately the number of nodes equal to the denominator of the phyllotactic fraction characterizing a given axis. At each node a segment of supernumerary cambium is initiated opposite the leaf traces supplying the leaf inserted at that node. The segments of differentiated cambium are preceded by regions of obliquely and anticlinally dividing cells. In the single juvenile axis studied supernumerary cambial segments also appear above the node to the cathodic side of the entering leaf traces, and opposite the medullary bundle immediately anodic to these traces. Vascular connections among the primary and supernumerary vascular cylinders occur between leaf insertions on the same orthostichy. The levels at which these connections occur vary among stems. The switch from ordinary to anomalous secondary growth may be caused by a change in tissue response to stimuli produced by leaves.

The ratio of ray and ray cell initials to fusiform initials of the vascular cambium relative to radial growth rate as reflected in the secondary xylem was studied in stems of Acer saccharum Marsh. Ray initials increased in size as they aged, slowly when growth rate was low, rapidly when it was high, but there was little fluctuation in the number of rays per unit of tangential area; as the cambium increased in circumference, the older, larger rays diverged and new small rays arose in intervening areas, thus maintaining a uniform unit area population independent of growth rate. However, since ray size increased rapidly when growth rate was high, the unit area population of ray cells rose abruptly with accelerating growth rate: the relative volume of xylem ray tissue rose from 8.6 to 12.7% of the total xylem volume when annual ring width increased from 1 to 7 mm. When fast growth was not maintained, the unit area population of ray cells declined slowly as the large rays diverged.

Many of the large columnar cacti Trichocereus chilensis near Santiago are infected by Tristerix aphyllus. This is one of the most highly reduced seed plants known: as it is an endoparasite, inflorescences are the only parts of the plant ever to emerge from the host, all the rest exists as an endophytic haustorial system; roots, vegetative stems, and leaves are not produced. After infection, the parasite spreads through the thick cortex of the host, reaching the vascular cambium and conducting tissues. It continues to grow intrusively throughout all tissues of the host shoot. In its invasive stage it occurs as a "myceliumlike" mass of uniseriate filaments that grow between host cells but only rarely enter them. Later growth is by longitudinal cell division that produces irregular parenchymatous strands. Eventually, xylem and phloem differentiate in the endophytic strands; the phloem is like that of other seed plants, but the xylem is almost pure parenchyma, with only occasional, idioblastic tracheary elements. Strands close to the epidermis of the host are able to produce adventitious flower buds that emerge through either soft regions in the epidermis (the areoles) or through accidental breaks in it. The flower stalk may persist, forming a small perennial inflorescence that has well-developed xylem, phloem, and periderm but is without leaves. Host cells appear healthy and normal, with no sign of damage caused by the presence of the parasite.

A root undergoing secondary growth exhibits massive girth increases owing to the activity of the vascular cambium. To determine the effects of this growth on endodermal structure and function, cleared cross sections from areas of primary and secondary growth in broad bean, sunflower, and garden balsam roots were examined. Casparian bands were detected by staining the sections with Chelidonium majus root extract and viewing them with ultraviolet (uv) light using an epifluorescence microscope. Endodermal diameter and numbers of endodermal cells increased dramatically in the area of secondary growth. The distance spanned by the Casparian band (i.e., the tangential width of the endodermal cells) between radial walls also increased except in sunflower. The permeability of the endodermis to an apoplastic fluorescent dye in roots undergoing secondary growth was tested by treatment with Calcofluor white M2R. In all species examined, the Casparian band continued to function as an effective apoplastic barrier during early stages of secondary growth despite the increase in endodermal cell size and number. Eventual decortication with concurrent disruption of the endodermis resulted in massive penetration of Calcofluor white M2R into the xylem.

The primary phloem system in the rhizome of Hieracium floribundum has transfer cells that have developed from companion cells and parenchyma cells, which are adjacent to sieve elements. In both cell types changes occur in the cytoplasmic organelles at the time of wall ingrowth formation. Dicytosomes and polyribosomes become more numerous and 'boundary formations' and other multivesiculated structures appear. Few microtubules were found in the cytoplasm at this time. After the wall ingrowths become obvious, the transfer cells develop numerous mitochondria and an enlarged nucleus. The phloem transfer cells become vacuolated with age and the wall ingrowths become less numerous. This may be associated with a change in the translocation pattern in the phloem after the inception of vascular cambium activity. Parenchyma cells in the secondary phloem usually become rather vacuolated and develop few wall ingrowths.

DNA content was measured cytophotometrically in Feulgen-stained nuclei of fusiform cambial initials from 1- (twig) and 19-year-old (stem base) cambia of balsam fir (Abies balsamea) trees throughout the season. Telophase (2C) and prophase (4C) DNA contents varied with both cambial age and time of year. Maximum DNA contents were observed at the beginning of cambial activity in April. They were lower in the 1-year-old cambium (42.3 (2C) and 84.3 (4C) pg DNA) than in the 19-year-old cambium (59.4 (2C) and 111.1 (4C) pg DNA). The DNA levels decreased during the cambial growing season, attaining minimal values in September of 28.3 (2C) and 53.5 (4C) pg and 37.9 (2C) and 74.5 (4C) pg in the 1- and 19-year-old cambia, respectively. In the autumn, cells accumulated in the G1 phase of the cell cycle. DNA synthesis then took place increasing the 2C levels from the minimum values to the maximum values. This DNA synthesis in 1-year-old cambia started before the beginning of the transition from the resting to the quiescent stage of dormancy.

In the early growth of the stem, branches, and roots, the vascular elements are relatively short and the frequency of anticlinal division involved in cambial cell multiplication is high. As growth sheaths are added in the stem, length of cell increases and rate of multiplicative division declines. A similar trend occurs upward through the lower quarter to half the height of the stem. In the root system, the later growth of vertical roots is characterized by shortness of cell and high frequency of anticlinal division, and conversely, that of horizontal roots by great length of cell and low rate of anticlinal division. Although a general negative relationship exists between rate of anticlinal division and cell length throughout much of the tree, these features sometimes vary independently, and length of cell seems to be more closely related to amount of yearly radial accretion. Through the middle to late growth of the stem a negative relationship obtains between length of cell and width of annual ring, cell length maximating at a ring width of 1–2 mm. At this stage, frequency of division may fluctuate only narrowly over a considerable range of ring width. A continued decline in ring width to less than 0.5 mm, such as may occur on senescence, is accompanied by decreased cell length and accelerated anticlinal division. Length of the cell plate in anticlinal division, relative to that of the dividing cell, is greater in the early growth of the stem and branches and throughout horizontal roots than elsewhere in the tree. Most of the anticlinal divisions are pseudotransverse. The proportion of lateral divisions ranges from about 1% in the late growth of stems to 11% in horizontal roots.

The anatomy of roots and haustoria of Pholisma depressum is detailed. Lignified xylem consists of vessel elements only, and is associated with some xylem parenchyma. Sieve tube members exist in both the stem and the root. The pilot root is characterized by a restriction of cambial activity to the primary phloem regions, thus resulting in 3–5 bundle-like structures in older roots. From the protoxylem points between these bundles vascular flanges extend radially outward. The steles of lateral roots are extensions of these vascular flanges. A root cap is recognizable but very thin.The endophyte is a lobed organ with sinuous and irregular vascular strands lacking any phloem-like tissues. Strands, occasionally uniseriate, invade adjacent host tissues, usually in phloem-producing regions of the host cambium. These strands establish xylem-to-xylem contact, but are parallel to host xylem elements. They may subsequently become partially encapsuled by the latter. No radial sinkers exist.The nuclei of the endophyte show a more or less permanent type of heteropycnosis, the significance of which is unknown.A comparison of the parasitism of Lennoaceae and Orobanchaceae yields a number of significant differences in the structure and development of roots and haustoria.

The monostelic seed fern Schopfiastrum decussatum Andrews is described from a specimen collected at a Middle Pennsylvanian petrifaction locality in southern Illinois. The specimen measures 24 cm long and is about 1.1 cm in diameter. Two petioles are attached to the axis and abundant foliar material is also present. Leaf arrangement is alternate and distichous. The stem consists of an exarch protostele surrounded by a prominent zone of secondary xylem. Secondary phloem and a vascular cambium are also preserved. The cortex is characterized by an undulating outer epidermal zone consisting of alternating ridges and furrows; internally this zone is delimited by conspicuous lacunae. Sclerenchyma bands occur in the outer cortex, with prominent resin canals present in the inner cortex. The fronds are represented by dichotomizing rachides, primary pinnae, and laminar pinnules. Features of the plant are compared to those of other Carboniferous pteridosperms, and a reconstruction of Schopfiastrum is included.

In coniferous stems the rate of multiplication of fusiform initials in the vascular cambium by means of pseudotransverse division often fluctuates considerably around the circumference. In fluted stems the frequency of these divisions is appreciably higher and the ratio of survival of the newly formed initials relative to the rate of production lower in the depressions than in the adjoining convex portions of the perimeter. The probability that compression is a factor tending to accelerate the frequency of anticlinal division is indicated by the decided increase in rate in areas of the stem subjected to radial pressure. Higher frequencies of pseudotransverse division are accompanied by reduction in mean cell length of the cambial cells and hence of the derived wood elements.

The development of the vascular pattern was studied in Doxantha unguis-cati as a prelude to correlating the vascular system to the anomalous cambial activity observed in the mature stem. The shoot apex of this liana has a two-layered mantle covering a corpus of a mother cell zone and a flanking meristem. Just below the shoot apex a cylinder of procambium differentiates. The first primary phloem can be recognized in the third internode and the first primary xylem in the fourth or fifth internodes. As the internodes elongate so do the procambial cells. The cells of the pith, on the other hand, are shorter than the procambial cells and accommodate to the extensive internode elongation by cell division. The vascular system of the young shoot includes not only bundles, but also individual strands of phloem. The stem has multilacunar nodes from which five bundles and two phloem strands depart into each of the oppositely paired leaves. Within the stem there are four continuous bundles to which all other bundles connect. The location of these four bundles corresponds to the formation of the characteristic anomalous cambium observed in the secondary growth of the older stem. The relationship between the vascular pattern of D. unguis-cati to other vascular patterns is also discussed.

The distribution of xylem and phloem transfer cells in the node of bamboo has been described. Bundles supplying the leaf sheath contain numerous xylem transfer cells while those supplying the internode above the node and the axillary bud contain groups of phloem transfer cells which appear as swollen or knotted regions in the node. The significance of the presence of transfer cells in the nodes and the possible function of them are discussed.

Preliminary studies of naturally infected American elm trees show that abnormalities occur in the vascular cambium and the secondary phloem. Abnormalities include deposition of callose on sieve plates, septation of fusiform initials, lack of fiber differentiation, and accumulation of lipoidal substances in the vascular cambium. The sequence of these abnormalities and their importance in the death of the tree are yet to be determined.