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Cells of the root epidermis ofPisonia grandis R. Br. at the interface with the mycorrhizal fungus are modified as transfer cells. The length of wall profile in transverse section is increased 1.7-fold by the wall ingrowths, on average, over the outer tangential wall and the outer third of the radial walls; this corresponds to a 1.3—fold increase in wall profile length over the whole cell. These increases in length of wall profile approximate—slightly underestimating-the amplification of surface area of the epidermal cells that results from the ingrowths. The surface area between the symbionts in thePisonia mycorrhiza is less amplified than in classical ectomycorrhizas with a Hartig net: this may be functionally adequate because of the extremely high nutrient status of theP. grandis habitat.

The development of external glands on traps and stolons ofU. monanthos has been studied using transmission electron microscopy. During early differentiation of the epidermis some cells remain narrow and develop a protuberance which subsequently divides into a terminal and a pedestal cell, with the remainder of the original cell forming the basal epidermal cell of the gland. The lateral wall of the pedestal cell soon becomes densely impregnated throughout its thickness, and this is followed by the formation of discontinuous cuticular deposits within the primary wall of the terminal cell. The outer wall of the terminal cell then usually undergoes extensive secondary wall thickening beginning with the formation of ingrowths which for a period characterize the cell as a transfer cell. Later, at the stage when traps begin capturing prey, these ingrowths are overlain by further layers of secondary wall material. Concomitantly, in the pedestal cell, wall ingrowths become fully differentiated on the outer transverse wall and persist throughout the remaining life of the gland. The function of external glands during early ontogeny is discussed. At the stage when the terminal cell is differentiated as a transfer cell it is suggested that the gland is mainly responsible for absorbing solutes from the external medium. Once traps commence capturing prey the gland may become modified for a role in water secretion, facilitated by the differentiation of the pedestal cell as a transfer cell, and by the formation of a thick outer wall in the terminal cell.

Plasmatubules are tubular evaginations of the plasmalemma. They have previously been found at sites where high solute flux between apoplast and symplast occurs for a short period and where wall proliferations of the transfer cell type have not been developed (Harris et al. 1982, Planta 156, 461-465). In this paper we describe the distribution of plasmatubules in transfer cells of the leaf minor veins of Pisum sativum L. Transfer cells are found in these veins associated both with phloem sieve elements and with xylem vessels. Plasmatubules were found, in both types of transfer cell and it is suggested that the specific distribution of the plasmatubules may reflect further membrane amplification within the transfer cell for uptake of solute from apoplast into symplast.

Histological changes in the developing rice endoperm and nucellus were observed with attention to the translocation pathway of reserve substances. Results were discussed by comparison with those of two-rowed barley which reported in the previous papers. 1. On the young endosperm (9-10 days after anthesis), some irregular shaped cells were found in the midway area between peripheral and center of the lateral portions. However, they were remarkably less in number than those of two-rowed barley (Fig. 1). 2. In the aleurone and subaleurone cells of lateral and ventral portions, zigzag modification of the wall as found in barley was could not observed throughout the developing stage. 3. Anticlinal cell walls of nucellar epidermis increased in area about several tens times by expanding of the cell as the endosperm grew fat, and they increased more by folding. A large number of plasmodesmata were formed in the anticlinal cell walls. This may be a proof that the nucellar epidermis developed as the conductive tissue from dorsal vascular bundle to the ventral side (Figs. 2∼10). 4. Wall-ingrowth structure that symbolizes the figure of transfer cell was found in some young dorsal aleurone cells (Fig. 12). However, the transfer cells were small in number in rice, and this differed from two-rowed barley in which many transfer cells were formed in the dorsal portion as reported previously. 5. Dorsal aleurone tissue was composed of 6-8 cell-layers in rice. On the 9-10th day after anthesis, some "unusual cells" were observed sporadically in the young dorsal aleurone tissue. They were stained with toluidine blue darker than ordinary aleurone cells, and were mutually connected with their characteristic uneven cell walls to form a channel across the aleurone tissue (Figs. 13, 14). They degenerated and were depressed gradually in contrast with the normal aleurone cells developed (Figs. 15-17), and that they were found as rather thick traces even in mature aleurone tissue (Fig. 18). The "unusual cells" might be cause of the irregular arrangement of the dorsal aleurone cells. The channels made of connection of the "unusual cells" may play an important role as the translocation pathways through thick aleurone layers which cells were fully packed with aleurone grains etc.

Mature leaves of Polytrichum commune Hedw. were examined in regard to structural features and possible solute transport mechanisms. The leaf bundle is organized into several parenchyma layers which are termed passage cells, deuters, and socci. The deuters (leptoids of some authors) are large-diametered parenchymatic elements which exhibit papillate wall ingrowths characteristic of transfer cells. The deuter cytoplasm is typical of transfer cells, including a large number of mitochondria, numerous polysomes, and a peripheral network of ER associated with the wall ingrowths. Deuters show little ultrastructural similarity to leptoids (sieve elements of bryophytes) of the stem. Socci parenchyma and passage cells have a distinct inner wall layer which is convoluted in places, and therefore, these cells as well as the deuters might be involved in intensive short-distance fluxes of solutes. A possible translocation mechanism involving an active, carrier-mediated loading of leaf parenchyma is discussed in relation to these structural features. IN A SEARCH for the mechanism of photoassimilate conduction in the leaf of the moss, Polytrichum commune, Reinhart and Thomas (1981) concluded on the basis of solute uptake saturation kinetics and inhibition studies that the mechanism of translocation involves an active, carrier-mediated loading of sucrose from apoplastic free space into the food-conducting cells. This process would be analogous to vein loading, i.e., transport and loading of sucrose into the sieve elements of phloem against a concentration gradient. Apart from the above study and several oth

Four types of wall formation occur in the endosperm of chickweed Stellaria media (L.) Cyrill. Freely growing walls form at the edges of the micropylar region of the embryo sac during the early heart stage of embryogenesis; Golgi and dilated endoplasmic reticulum may be sources of precursors for these anastomosing walls which form independently of a mitotic spindle Other walls form by means of cell plates and in a manner similar to wall regeneration reported in protoplasts. Also, wall ingrowths of the transfer cell type form on the embryo sac wall in the micropylar region and protrude into the endosperm.

A B S T R A C T Micropylar wall extensions in the egg of Plumbago capensis arise as small pegs of periodic acid-Schiff's-positive material soon after the egg is organized. These inward projections of egg wall increase in size, becoming extensive near anthesis. Some branching of the filiform apparatus occurs. In mature embryo sacs the micropylar portion, with the lowermost portion of the egg included, becomes entrenched in the nucellus. The possible significance of this "gametic transfer cell" is discussed in relation to the apparent absence of synergids from the reduced embryo sac of Plumbago.

A study of the fine structure of minor veins of mature leaves of 975 species and 242 families of Angiosperms shows that transfer cells are widespread amongst herbaceous Dicotyledons, are much rarer in woody Dicotyledons, and are virtually absent from the Monocotyledons. The evolutionary significance of the distribution of the cells amongst and within orders, families and minor groupings is discussed. Four types of transfer cell are recognized in minor veins, all possessing irregular ingrowths of wall material protruding into their protoplasts, and all being regarded as modified parenchyma of the minor vein. Two types occur in phloem. One (the A-cell), with ingrowths distributed right round its periphery, is associated specifically with the sieve elements. The other (the B-cell) occurs more generally throughout the phloem and has zones of wall ingrowths oriented towards sieve elements and their associated companion cells or A-cells. Two other types (C- and D-cells) occur in xylem parenchyma and bundle sheath respectively, and have ingrowths only on walls in contact with or in close proximity to vessels or tracheids. Each species has a characteristic combination of types of transfer cell. The variations encountered in the survey are classified. Consistent differences in the frequency and form of ingrowths are to be found between the different types of transfer cell of a single species, and between different species in respect to a particular type of transfer cell. The functional significance of transfer cells in minor veins is discussed in relation to the loading and unloading of the conducting elements and to the retrieval of extra-cytoplasmic solutes from the mesophyll and the transpiration stream.

The distribution and time course of development of transfer cells in the hypocotyl region of lettuce (Lactuca sativa L.) and groundsel (Senecio vulgaris L.) are examined by light microscopy of serial sections through a sequence of ages of hypocotyls. Investments of xylem transfer cells occur in departing traces to the cotyledons and, later, in the traces to foliage leaves; phloem transfer cells are widely distributed but particularly prominent in those bands of protophloem in the plumule vasculature which lie alongside xylem of the cotyledonary traces. Both classes of transfer cell are well endowed with wall ingrowths before differentiation of xylem and perforation of stomata occurs in the plumule. Autoradiographic evidence is obtained of a transport pathway from cotyledonary trace xylem elements to xylem transfer cell to plumule, and analyses of xylem sap collected from above or below the zones of transfer cells in the hypocotyl show that certain materials can be removed from the xylem sap by transfer cells as it moves towards the cotyledons. From these findings it is concluded that the seedling transfer cells play an important role in nutrition of the young plumule, particularly before the latter has become adequately connected with the vascular systems of cotyledons and root. Experiments on the experimental modification of transfer cell development in the hypocotyl suggest that both photosynthetic fixation of carbon dioxide and a transpirational loss of water by a cotyledon must take place before the presumptive xylem transfer cells in its traces can develop normal sets of wall ingrowths. Discussion is extended to the general role of transfer cells in the nodal regions of stems. Possible functions envisaged are, the general nutrition of young tissues of the apical region, the abstraction of assimilates for local storage, the transfer of assimilates to axillary buds released from apical dominance, and the interchange of assimilates between adjacent vascular traces running through the node.

Plant cells possessing ingrowths of wall material, and hence having protoplasts with unusually high surface-to-volume ratios, may be found in a wide variety of anatomical situations and in most of the major taxa of multicellular plants. They are termed here “transfer cells”. Their function relates to any of four categories of trans membrane flux: 1. Absorption of solutes from the external environment (e.g. epidermis of submerged leaves), 2. Secretion of solutes to the external environment (e.g. nectaries and other glands), 3. Absorption of solutes from an internal, extracytoplasmic compartment (e.g. vascular parenchyma, haustorial-type connections, embryo sacs and embryos), 4. Secretion of solutes into an internal extracytoplasmic compartment (e.g. tapetum of anther, pericycle of root nodule). An overall assessment of their occurrence, structure, development and role in the plant is presented taking account of published information and new observations.

Summary In seedling nodes of Linum usitatissimum numerous transfer cells (modified cells with irregular wall ingrowths into the cytoplasm) occur adjacent to the leaf trace xylem. The presence of such cells at nodes where buds are absent (or rudimentary) is relevant to a recent theory concerning xylem transfer cell function.

The term transfer cell applies to cells that are thought to be particularly active in transport of inorganic or organic solutes across the plasmalemma. Structurally the most significant feature is the differentiation of cell-wall protuberances on the inner surface of the wall. These wall ingrowths — which often form a considerable wall labyrinth — are surrounded by the plasmalemma, which is therefore largely increased. Transfer cells are regarded as analogues of the microvilli in many animal cells, which are specialized in transport processes. It is widely held that in both systems the amplified plasmalemma surface leads to a largely increased number of pumping sites per cell, thus promoting active transport at the symplast/apoplast border [1]. The occurrence of transfer cells has been demonstrated to be common in all plant organs. In roots, transfer cells are common in the xylem parenchyma of leguminous nodules [7] as well as in rhizomes [8]. However, until 1976 no transfer cells had been described that could be directly related to the absorption of nutrients by the root and their transport to the shoot.

Transfer cell is a term describing cells with an increased plasmalemma surface area due to the formation of a wall labyrinth. It is presumed that this corresponds to the role of such cells in transport processes, either in secretion (e. g., glands) or absorption (e. g., xylem parenchyma cells). Although the main function of root epidermal cells is absorption of nutrients, transfer-cell characteristics have not been reported previously for this tissue.

Phloem tissue as a whole, according to the introduction of this term by Nageli (1858), consists of sieve elements, sclerenchymatic elements and parenchymatic elements. Companion cells and transfer cells, the two cell types to be considered more in detail in the following, belong to the latter group. Parenchymatic elements of phloem as a general term, however, comprise a wide variety of nucleate cells which are referred to as related or unrelated to the enucleate sieve elements. By definition of the phloem those are characteristic constituents in seed plants and ferns, but they are also present in the leptom of bryophytes.

Abscisic acid (ABA) treatment of secondary protonema of Physcomitrium pyriforme Brid in the presence of sucrose does not prevent cell division but results in shorter cells with vesicular cytoplasm and an accumulation of lipid. When transferred to sucrose medium without ABA and with low irradiance isodiametric intercalary cells are cut off which give rise to apogamous sporophytes either directly or after the formation of a small amount of callus. The organization of the cells leading up to the apogamous sporophyte is described. The cells initiating the sporophyte develop dense cytoplasm and the walls become labyrinthine and callosed, but they do not form any recognizable placenta. It is proposed that labyrinthine walls are a consequence of a perturbation of cell wall metabolism as growth changes from gametophytic to sporophytic. The use of the term “transfer cell” for this kind of cell is questioned and the need for a causal approach to the investigation of labyrinthine walls is stressed.

Helianthus annuus L. responds to iron deficiency by forming a thickened cortex and abundant root hairs in a zone near the root apex that corresponds to the primary developmental stage. Cytological investigations revealed that within 24 to 48 h of iron deficiency most of the peripheral cells differentiate into transfer cells. The wall labyrinth is always situated on the peripheral walls that face the external medium. The cytoplasm of these cells is characterized by numerous mitochondria, extensive rough endoplasmic reticulum, and large leucoplasts containing protein bodies. These observations are discussed in relation to the fact that Helianthus, as an "iron efficient" plant, responds physiologically to iron deficiency by extrusion of H(+), production of reducing substances, and a steep increase in the uptake efficiency of Fe.

There are two possible pathways for solute movement through plant tissues, the cell wall pathway and the cytoplasmic pathway. Movement may take place extracellularly through channels in the cell walls, thus bypassing the protoplasts and obviating the need to cross membrane barriers. Solutes in these cell-wall channels can be freely exchanged with an external solution and therefore this pathway is often referred to as the free space, the whole cell-wall continuity being termed the apoplast or apoplasm. In parallel with the free-space pathway is a route through the cytoplasm of the protoplasts, which are linked through the plasmodesmata to form a three-dimensional cytoplasmic continuity known as the symplast or symplasm. Solutes entering the symplast must first cross the plasma membrane, imparting a high degree of selectivity upon this pathway.

The development of the placental complex including the gametophyte vaginula and the bulbous foot of the sporophyte in the liverwort Targionia hypophylla L. (Marchantiales) was studied by transmission electron microscopy. The vaginula and foot are separated by an intervening space and each consist of parenchymatous cells without intercellular spaces. Transfer cells begin to differentiate at the gametophyte-sporophyte interface just prior the onset of meiosis. While a single epidermal transfer-cell layer has developed in the foot by the end of meiosis, a multilayered pattern of transfer cells is formed in the vaginula. Gametophyte transfer cells have wall labyrinths which decrease in complexity with distance from the foot, lack plasmodesmata, and show signs of degeneration in the proximity of the foot. During meiosis, amyloplasts of both vaginula and foot lack starch and develop some thylakoid grana. In the subsequent stage of spore maturation, obliteration of the wall labyrinth occurs in both gametophyte and sporophyte transfer cells. The developmental pattern of the placental complex in Targionia is discussed in relation to that of mosses.

The formation and structure of a syncytium induced by the potato cyst-nematode (Heterodera rostochiensis Woll.) in potato roots is described. At the permanent feeding site of the nematode larva, usually in the root cortex, the larva pierces a cell with its mouth stylet and injects saliva. Cell wall dissolution occurs to incorporate neighbouring cells into a syncytium. A column of cells is incorporated towards the vascular tissue. Centripetal advance is limited by the lignifled xylem, then syncytial spread continues laterally along xylem parenchyma and pericycle cells. Wall protuberances form on syncytial walls adjacent to conducting elements. This indicates the syncytium is a multinucleate transfer cell, and by ingesting syncytial contents the larva is the nutrient sink. As syncytial expansion occurs, sieve elements are crushed and probably cease to function, hence protuberance development continues only against xylem elements. Cell alterations on incorporation into the syncytium involve expansion, loss of cell vacuole, nuclear hypertrophy and a proliferation of cytoplasmic organelles free to move through wall gaps into the communal cytoplasm. ‘Boundary formations’ and microtubules are associated with the growing ends of protuber ances, and appear to be involved in their synthesis. Fibrillar material, possibly cellulose microfibrils, occurs between the plasrnalemma and the membrane of the ‘boundary formation’, and the forming protuberance. To induce the formation of the syncytium, the larva controls the differentiation of unspecialized cells to cells with a specific physiological function. The occurrence of wall protuberances suggests that transfer cells form as a response to solute flow.

The patterns of gene expression in giant cells induced in balsam roots by the root-knot nematode Meloidogyne javanica, balsam root tips, leaves and the infective nematode were examined by SDS microgel electrophoresis. The pattern of protein and glycoprotein bands obtained for giant cells and root tips was almost identical, but differed from that of leaf tissues. The giant cell protein pattern, the first reported for a transfer cell, was consistent over the time course of examination (24–41 days after infection). It is concluded that giant cells do not act as specialized gland cells, but that their metabolism is a stimulated version of meristematic cell metabolism. Results from localization of peroxidases on gels suggest that adult nematode peroxidase activity originates from giant cells by ingestion, and a role for this nematode peroxidase in giant cell induction or maintenance is unlikely. Initial experiments in which nematode galls were fed with 35S, followed by electrophoresis and fluorography, suggest that this approach is potentially useful in studying giant cell metabolism. Since adult nematodes contain more protein than the giant cells they feed from, the importance of micro-techniques to examine this association is stressed.

Recently Zee and O'Brien (1971a, 1971b) have described the distribution of the vascular tissue and transfer cell in the nodes of the spikelet axis of wheat and millet. From those reports it was clear that the distribution of the vascular system and its associated transfer cells could be quite variable. In this communication another variation found in the nodal region of the spikelet of the rice plant will be described.