Abstract: The succulent, cylindrical leaves of the C 4 dicot Portulaca grandiflora possess three distinct green cell types: bundle sheath cells (BSC) in radial arrangement around the vascular bundles; mesophyll cells (MC) in an outer layer adjacent to the BSC; and water storage cells (WSC) in the leaf center. Unlike typical Kranz leaf anatomy, the MC do not surround the bundle sheath tissue but occur only in the area between the bundle sheath and the epidermis. Intercellular localization of photosynthetic enzymes was characterized using protoplasts isolated enzymatically from all three green cell types. Like other C 4 plants, P. grandiflora has ribulose 1,5-bisphosphate carboxylase and the decarboxylating enzyme, NADP + -malic enzyme, in the BSC. Unlike other C 4 plants, however, phosphoenolpyruvate carboxylase, pyruvate, Pi dikinase, and NADP + -malate dehydrogenase of the C 4 pathway were present in all three green cell types, indicating that all are capable of fixing CO 2 via phosphoenolpyruvate carboxylase and regenerating phosphoenolpyruvate. Other enzymes were about equally distributed between MC and BSC similar to other C 4 plants. The enzyme profile of the WSC was similar to that of the MC but with reduced activity in most enzymes, except mitochondrion-associated enzymes. Intracellular localization of enzymes was studied in organelles partitioned by differential centrifugation using mechanically ruptured mesophyll and bundle sheath protoplasts. Phosphoenolpyruvate carboxylase was a cytosolic enzyme in both cells; whereas, ribulose 1,5-bisphosphate carboxylase and NADP + -malic enzyme were exclusively compartmentalized in the bundle sheath chloroplasts. NADP + -malate dehydrogenase, pyruvate, Pi dikinase, aspartate aminotransferase, 3-phosphoglycerate kinase, and NADP + -triose-P dehydrogenase were predominantly localized in the chloroplasts while alanine aminotransferase and NAD + -malate dehydrogenase were mainly present in the cytosol of both cell types. Based on enzyme localization, a scheme of C 4 photosynthesis in P. grandiflora is proposed. Well-watered plants of P. grandiflora exhibit a diurnal fluctuation of total titratable acidity, with an amplitude of 61 and 54 microequivalent per gram fresh weight for the leaves and stems, respectively. These changes were in parallel with changes in malic acid concentration in these tissues. Under severe drought conditions, diurnal changes in both titratable acidity and malic acid concentration in both leaves and stems were much reduced. However, another C 4 dicot Amaranthus graecizans (nonsucculent) did not show any diurnal acid fluctuation under the same conditions. These results confirm the suggestion made by Koch and Kennedy (Plant Physiol. 65: 193-197, 1980) that succulent C 4 dicots can exhibit an acid metabolism similar to Crassulacean acid metabolism plants in certain environments.

Abstract: The effect of Zn deficiency on rate of photosynthesis of leaf discs, isolated mesophyll and bundle sheath cells and chloroplasts of maize (Zea mays. L) was studied. The yield of mesophyll and bundle sheath cells obtained by enzymic digestion of the leaf tissues from Zn deficient plants is lower than the identical tissues from normal plants which suggests that Zn deficiency brings about some structural changes in the leaf cell. Photosynthetic oxygen evolution measured in the leaf discs is low due to Zn deficiency. Photosystem‐II dependent Hill reaction and non cyclic photophosphorylation of chloroplasts were also affected by Zn deficiency. Rate of photosynthetic carbon dioxide fixation by both bundle sheath and mesophyll cells obtained from Zn deficient leaf‐tissue waslower than the cells free from Zn deficiency. Addition of various metabolites like NADPH, ATP and PEP to Zn deficient mesophyll cells whowed marked enhancement in 14‐CO2 fixation. However, addition of NADPH, ATP and RuBP to Zn defici...

Abstract: Ultrastructure of the large vascular bundles in expanded and folded leaves of rice plants were observed by an electron microscope, and the results were compared with those of the small vascular bundles previously reported. 1. The large bundles have more developed mestome sheath than the small bundles. The suberized lamellae occur in the outer and inner tangential walls of the mestome sheath cells, but are absent in the middle portion of the radial walls. The lamella occurs between primary and secondary walls of the mestome sheath cells. 2. The protophloem sieve elements and companion cells located abaxially within the large bundles, mature in the elongating zone of the folded leat and begin to degenerate before emergence of the leaf. So far as the same portion of the leaf is concerned, the maturation and degeneration of the protophloem in large bundles are earlier than those in the small bundles. 3. In the border region between phloem and xylem of the large bundles, the parenchyma cells of metaphloem and metaxylem, and the cells of mestome sheath are interconnected by numerous plasmodesmata. According to this observations, the following symplastic pathways for photosynthate are suggested. [figure] 4. The large bundles have many metaxylem parenchyma cells, in which abundant mitochondria and endoplasmic reticulum are contained. The function of the metaxylem parenchyma cells in absorption and transfer of solutes is discussed.

Abstract: Seedlings of Atriplex hortensis were studied to ascertain; 1) in which organ the primary thickening meristem (PTM) first differentiates; 2) the direction of differentiation of the PTM, and 3) the pattern of differentiation of conjunctive tissue. The PTM initially differentiates in pericycle of the primary root base 11 days after emergence of the primary root. It then differentiates in the transition region of the hypocotyl, mostly in cells of pericycle between pairs of vascular bundles. In the upper hypocotyl, PTM differentiates by day 20 in the inner layer qf cortical parenchyma. In the epicotyl, PTM apparently differentiates in the inner layer of cortex, by day 24. Desmogic xylem differentiates from radial files of internal conjunctive tissue cells and desmogic phloem differentiates opposite desmogic xylem strands from newly formed cells of external conjunctive tissue. No interfascicular cambium differentiates in the root, hypocotyl, or epicotvl. ATRIPLEX HORTENSIS L., a member of the Chenopodiaceae, is an example of a dicotyledonous plant that grows in diameter principally by a primary thickening meristem (PTM), instead of a vascular cambium. These two lateral meristems differ in that: 1) PTM differentiates outside of primary phloem whereas vascular cambium differentiates centripetally of primary phloem; 2) PTM produces parenchymatous cells mainly to the inside, internal conjunctive tissue, and fewer cells to the outside, external conjunctive tissue (Stevenson and Popham,

Abstract: . Storage parenchyma, vascular parenchyma and phloem companion cells are found adjacent to sieve tubes in the vascular bundles of cotyledon leaves of mung bean (Vigna radiata L.) seedlings. The paramural bodies of storage parenchyma cells are characterized by flask shaped invaginations of the plasmalemma whereas the plasmalemmasomes of the adjacent vascular parenchyma and companion cells consist of numerous finger-like evaginations which are not enclosed in plasmalemma pockets. Phloem associated transfer cells are not present and it is suggested that the fine tubular plasmalemmasomes may act as the interface between apoplast and symplast in the transport of rapidly mobilized reserves during germination. Tubular structures observed within the protoplasts of storage cells close to vascular tissue were also observed in vascular parenchyma and companion cells between the plasmalemmae and cell walls.

Abstract: The three-dimensional structure of anomalous secondary growth in Stegnosperma is described. The initiation of anomalous increments of vascular tissue is related to the branching pattern in stems and roots of young plants. In more mature stems the relationship between anomalous layers and plant architecture is obscured by the complexity of the system. Tangential connections between vascular bundles in the same anomalous increment and radial connections between successive layers are frequent. These anastomoses do not occur simultaneously in the xylem and phloem, resulting in a complex netlike structure similar to that described for Avicennia, an unrelated genus with anomalous secondary thickening by successive cambia. Anomalous secondary growth in the closely related Amaranthaceae, Chenopodiaceae, and Nyctaginaceae may represent an advanced form of the more slowly developing pattern in Stegnosperma.

Abstract: The relationship of the macrocyclic rust fungus PUccinia poarum with its pycnial-aecial host, Tussilago fapfaPa, and its uredial-telial host, Poa ppatensis, has been investigated, using light microscopy, electron microscopy and micro-autoradiography. Aspects of the morp- hology and ontogeny of spores and sari, which were previously disputed, have been clarified. Monokaryotic hyphae grow more densely in the intercellular spaces of Tussilago leaves than the dikaryotic intercellular hyphae on Poa. Although ultrastructurally sbnilar, monokaryotic hyphae differ from dikaryotic hyphae in their interaction with host cell walls, often growing embedded in wall material which may project into the host cells. The frequency of penetration of Poa mesophyll cells by haustoria of the dikaryon is greater than that of Tussilago cells by the relatively undifferentiated intracellular hyphae of the monokaryon. Intracellular hyphae differ from haustoria in their irregular growth, septation, lack of a neck-band or markedly constricted neck, the deposition of host wall-like material in the external matrix bounded by the invaginated host plasmalemma and in the association of callose reactions \vith intracellular hyphae and adjacent parts of host walls. Monokaryotic and dikaryotic infections differ also in the changes induced in the organization and ultrastructure of their respective host cells. Intracellular hyphae in bundle sheath, xylem parenchyma, transfer cells of phloem parenchyma and companion cells, give the monokaryon of P. poaPUm direct access to nutrients translocated in vascular tissue of Tussilago. Bundle sheath cells of Poa contain unusually long haustoria but· there is no penetration of the endodermis and vascular tissue by the dikaryon. After uptake of tritiated glycerol by infected tissue, microautoradiographic investigation shows radioactivity to be concentrated in sporulation structures, haustoria and, in the case of Poa, in host nuclei. Cells of uninfected tissues differ from those of infected tissue in accumulation of label in chloroplasts. These studies contribute to an understanding of the physiological interaction of P. poaPUm with its alternate hosts.

Abstract: During dormancy induction of Populus deltoides plants, the primary-secondary vascular transition zone advances only in those internodes subtending leaves that expanded prior to bud set. Therefore, most of the vascular cylinder remains primary in the final 2–3 very short internodes beneath the bud. Transitional xylem elements, resembling vascular tracheids, frequently occur in these internodes.

Abstract: Ajaponica X indica rice variety "Tongil" was treated with various low temperatures in phytotrons under natural light to investigate the discoloration processes of leaves experimentally. The results obtained are summarized as follows: 1. Leaf number did not increase at 15°/10°C (day/night), but increased gradually with increase of temperature, while, the length of leaf blades and sheaths which were at grand growth period was markedly shortened under low temperature. The increase in plant height of seedling was inhibited under low temperature over 6 days. 2. Leaf color changed to light green, yellow or brown when treated below 20°/15°C, and brown color spread over whole leaf blade at 15°/10°C. 3. Chlorophyll content of leaf blades began to decrease at the 3rd day under low temperature (17°/12°C), and markedly decreased at the 9th day. 4. Chloroplasts treated by low temperatures contained larger starch grains in size and amount and more lipid globules, and also thylakoid membranes were some-what distended and disordered. 5. Under normal temperatures the mean sectional areas of chloroplasts in meso-phyll cells were about 2.4 times greater than in bundle sheath cells, but under lower temperatures they decreased to 1.7 times mostly due to a grcat expanding of the chloro-plasts in bundle sheath, resulting from a much deposition of starch grains. 6. Under normal temperatures the chloroplasts of the mesophyll cells close to both the adaxial and abaxial epidermis contained less starch grains compared to those at the central portion, but under lower temperatures the chloroplasts at the abaxial side contained more starch grains than those of the adaxial side.

Abstract: Starch formation was studied by means of plant anatomy in segments of maize and pea leaves, deprived of starch, floating overnight in the dark (1) on solutions of various saccharides, (2) on sucrose solutions containing auxin-type growth regulators and (3) on suorose solutions with antibiotios (proteosynthesis inhibitors). A comparison was made of mesophyll tissues of the two species used, of spongy and palisade parenchyma in pea leaves and-especially-of mesophyll and vascular bundle sheath in the leaves of maize. Although the effects of the given treatments were striking, the response of the particular tissues was considerably uniform. If the given sugar appeared as utilizable for starch formation, it was effective equally in all tissues under study. It was not possible to distinguishin situ the different ways of staroh synthesis by means of the application of growth substanoes. No striking differences in the effect of antibiotics upon staroh formation were seen in-normally-starch containing and starch lacking tissues.

Abstract: The leaf anatomy was investigated with respect to the arrangement of cells involved in photosynthesis. The full-grown leaf has one vascular bundle consisting mainly of phioem cells. In similarity to terrestrial C4 plants the vascular bundle is surrounded by mesophyll bundle sheath cells. However, in contrast to C4 plants, these cells do not contain chlorophyll or starch inCeratophyllum. The early products in photosynthesis (10 seconds14C labelling) were analyzed. Although no complete separation of all radioactivity in the plant extracts was reached, it was clear that malate was the major labelled component, indicating C4 activity in the plants. No light saturation could be proven inCeratophyllum in several stages of post-dormancy in a statistically significant way, although a tendency to light saturation was observed at intensities higher than 36 Wm−2. The photosynthetic activity was only slightly depressed by enhancement of the O2 concentration in the medium.

Abstract: Ultrastructural surveys, carried out into an olive necrotic maize mutant (Neuffer E 283 B) grown under a 16 hours photoperiod, have shown remarkable morphological alterations in the plastids. Such alterations, affecting both the mesophyll and the bundle sheath plastids, appear to be photodependent. Quite normal etioplasts are present in dark-grown mutant seedlings. Moreover, light appears also to inhibit the overall growth of mutant plants. 12-day-old mutant plants grown under illumination are 4 cm in comparison with the 10 cm of the wild type, while corresponding dark-grown seedlings of both types are 12 cm high.

Abstract: Electron microscopic studies of Panicum maximum (PCK C4 photosynthetic type) when grown under controlled growth conditions at low nutrient nitrogen (20 ppm N, KNO3) showed interesting second order bundle sheath cell organization: the cells were several times bigger than the mesophyll cells and were packed with starch grains. Under higher nutrient nitrogen (200 ppm N, KNO3) the number of starch grains were drastically reduced, suggesting that at the higher nitrogen levels the fate of fixed carbon is different to that at the lower level of nitrogen supply. This suggests that nitrogen may regulate photosynthetic carbon assimilation after carboxylation. At low nitrogen the carbon is stored as starch at the site of CO2 fixation, the bundle sheath cells. It is argued that the nitrogen may play a role in translocation of metabolites to mesophyll cells, and thus decrease the starch level. These observations are discussed in relation to the photosynthetic characteristics of P. maximum.

Abstract: Thesucculent, cylindrical leaves oftheC4dicot Portulaca grandflora possess three distinct green cell types: bundle sheath cells (BSC) inradial arrangement around thevascular bundles; mesophyll cells (MC)inan outer layer adjacent totheBSC;andwaterstorage cells (WSC)intheleaf center. Unlike typical Kranzleaf anatomy, theMC donotsurround the bundle sheath tissue butoccuronly intheareabetween thebundle sheath andtheepidermis. Intercellular localization ofphotosynthetic enzymes was characterized using protoplasts isolated enzymaticaliy fromali three green cell types. Likeother C4plants, P.grandJlora hasribulose 1,5-bisphosphate carboxylase andthedecarboxylating enzyme, NADP+-malic enzyme, inthe BSC.Unlike other C4plants, however, phosphoenolpyruvate carboxylase, pyruvate, Pidikinase, andNADP -malate dehydrogenase oftheC4pathwaywerepresent inallthree green cell types, indicating thatallare capable offixing CO2viaphosphoenolpyruvate carboxylase andregenerating phosphoenolpyruvate. Other enzymes wereabout equally distributed between MC andBSCsimilar toother C4plants. Theenzymeproffle of theWSC wassimilar tothat oftheMC butwithreduced activity inmost enzymes, except mitochondrion-associated enzymes. Intracellular localization ofenzymes wasstudied inorganelles partitioned bydifferential centrifugation using mechanically ruptured mesophyll andbundle sheath protoplasts. Phosphoenolpyruvate carboxylase wasa cytosolic enzyme inbothcells; whereas, ribulose 1,5-bisphosphate carboxylase andNADP+-malic enzyme wereexclusively compartmentalized in thebundle sheath chloroplasts. NADP+-malate dehydrogenase, pyruvate, Pidikinase, aspartate aminotransferase, 3-phosphoglycerate kinase, and NADP+-triose-P dehydrogenase werepredominantly localized inthechloroplasts while alanine aminotransferase andNAD'-malate dehydrogenase weremainly present inthecytosol ofbothcell types. Basedonenzyme localization, ascheme ofC4photosynthesis inP.grandflora isproposed. Well-watered plants ofP.gradaflora exhibit adiurnal fluctuation of total titratable acidity, with anamplitude of61and54microequivalent per gramfresh weight fortheleaves andstems, respectively. These changes wereinparallel withchanges inmalic acid concentration inthese tissues. Undersevere drought conditions, diurnal changes inbothtitratable acidity andmalcacid concentration inbothleaves andstemsweremuchreduced. However, another C4dicot Amaranthus graecizans (nonsucculent) didnot showanydiurnal acid fluctuation under thesameconditions. These results 'Supported inpart byGrantPCM 7708548fromtheNational Science

Abstract: Microscopical studies on Thlapsi bursa-pastoris Moench (Cruciferae) have been done for its identification including the drug in powdered form. Diagnostic features include anisocytic stomata on both abaxial and adaxial epidermis; stomatal index 16.66 to 25.00; palisade ratio 1 to 1.5; stellate serrately thick-walled trichomes on leaves, each with a wide central circular-pore; sclerenchyma caps above and below vascular bundles in leaves; vessels with bordered pits and spiral thickenings; bast and wood fibres in xylem in roots; pericyclic sclerenchyma localised above primary vascular bundles in stem.

Abstract: Three types of cambial activity, two hitherto unreported, are described for Stylidium. The four species of sect. Rhynchangium of subgenus Nitrangium have woody cylinders in upright stems. In these a cambium formed beneath the endodermis produces a determinate quantity of fibers, vessel elements, and interxylary phloem strands toward the inside but no derivatives toward the outside; this was correctly reported by Van Tieghem and Morot (1884a) but doubted by subsequent workers. The same species have lignotubers in which a cambium produces contorted xylem (mostly vessels) to the inside, phellem toward the outside. In S. glandulosum and S. laricifolium a cambium formed beneath the endodermis produces an indeterminate quantity of xylem (fibers and vessel elements) and interxylary phloem toward the inside, nothing toward the outside. The xylem is rayless and lacks axial xylem parenchyma. These three modes of cambial activity represent innovations within Stylidiaceae. The family has a wholly herbaceous ancestry if one can judge from the total lack of cambial activity in vascular bundles.

Abstract: Leaf blades of 42 grasses (Poaceae) have been examined ultrastructurally for the occurrence of a suberized lamella in walls of parenchymatous bundle sheaths and PCR (= “Kranz”) sheaths in both large and small vascular bundles. The sample includes species from a range of major grass taxa, and represents all photosynthetic types found in the grasses. Three grasses with unusual C4 leaf anatomy were also included:Alloteropsis semialata, Aristida biglandulosa, Arundinella nepalensis. The presence of a suberized lamella in PCR cell walls was perfectly correlated with photosynthetic type. All PEP-carboxykinase type and NADP-malic enzyme type C4 species examined possessed a suberized lamella in outer tangential and radial walls, but with variable presence in inner tangential walls. PCR cells of bothAlloteropsis semialata andArundinella nepalensis also possessed a suberized lamella. A lamella was totally absent from parenchymatous bundle sheath cells of the C3 species examined (5 spp.) and ofPanicum milioides, a C3-C4 intermediate. It was also absent from PCR cells of NAD-malic enzyme type C4 species (14 spp.) andAristida biglandulosa. The results are discussed in relation to the leakage of CO2 from PCR cells, and to differences between C4 types in δ13C values, chloroplast position in PCR cells, and other anatomical characteristics.

Abstract: Studies with purified mesophyll and epidermal protoplasts and bundle sheath strands have shown that the cyanogenic glucoside dhurrin (p-hydroxy-(S)-mandelonitrile-beta-d-glucoside) is localized in the epidermis of sorghum leaves whereas the enzymes involved in its degradation (dhurrin beta-glucosidase and hydroxynitrile lyase) are localized in the mesophyll tissue (Kojima M, JE Poulton, SS Thayer, EE Conn 1979 Plant Physiol 63: 1022-1028). The subcellular localization of these enzymes has now been examined using linear 30 to 55% (w/w) sucrose gradients by fractionation of mesophyll protoplast components. The hydroxynitrile lyase is found in the supernatant fractions suggesting a cytoplasmic (soluble cytoplasm, microsomal or vacuolar location). The dhurrin beta-glucosidase (dhurrinase) is particulate and mostly chloroplast-associated. The dhurrinase activity peak has a shoulder of activity more dense than that of the intact chloroplasts. This shoulder does not coincide with markers of any other cell fraction.In studies of chloroplasts isolated from ruptured mesophyll protoplasts by differential, low-speed centrifugation, the dhurrinase partitions in the same manner as the chloroplast marker triose phosphate dehydrogenase. Chloroplast localization of the beta-glucosidase has also been shown in histochemical studies using 6-bromo-2-naphthyl-beta-d-glucoside substrate coupled with fast Blue B.

Abstract: Assimilates entering the developing rice caryopsis traverse a short-distance pathway between the terminal sieve elements of the pericarp vascular bundle and the aleurone layer. The ultrastructure of this pathway has been studied. Sieve elements in the pericarp vascular bundle are smaller than their companion cells.The sieve elements show few connections with surrounding vascular parenchyma elements but are connected to companion cells by compound plasmodesmata. Companion cells, in turn, are connected to vascular parenchyma elements by numerous compound plasmodesmata present in wall thickenings. Assimilates leaving the sieve element - companion cell complex must laterally traverse cells of the pigment strand before they come into contact with the aleurone layer. The pigment strand cells have modified inner walls made up of a suberin-like material. This material may act as a permeability barrier isolating the apoplast from the symplast of the pigment strand. The walls of the pigment strand cells are traversed by numerous plasmodesmata. Water may be conducted to the endosperm through the isolated cell-wall system of the pigment strand while assimilates possibly move via plasmodesmata. High frequencies of plasmodesmata occur at the junction between the pigment strand and the nucellus and also between adjacent cells of the nucellus. By contrast, plasmodesmata are absent between the nucellus and the aleurone layer and also between the nucellus and the seed coat. A predominantly circumferential and symplastic transport pathway is likely between the pigment strand and nucellus. In view of the total absence of plasmodesmata between the nucellus and the aleurone layer assimilates entering the endosperm may have to cross the plasmalemma of the nucellus. It is possible that constraints to the flow of assimilates may occur in the short-distance pathway between the terminal sieve element - companion cell complexes and the endosperm, and this is discussed.

Abstract: Rye plants were cold hardened by growth at 4/2 C day/night (D/N); unhardened control plants remained at 25/20 C D/N. Leaves from hardened plants contained 33% less water on a dry weight basis than those from unhardened plants. The osmolar concentration of expressed sap from hardened leaves was 1.5 times greater than that of unhardened leaves. The number of cell layers in fresh leaves remained constant, but leaves from hardened plants were about 1.5 times thicker than those from unhardened plants Increased leaf thickness was caused by increased mesophyll cell size. The structure of the vascular bundle and both stomatal frequency and distribution patterns of the upper and lower epidermal surfaces were altered.

Abstract: Plastidic polyphenol oxidase (PPO) was localized in various plastid types ofSorghum bicolor (L.) Moench using cytochemical and biochemical franctionation techniques. PPO was found to be present in the mesophyll plastids yet absent from the bundle sheath and guard cell plastids. Mechanical fractionation of mesophyll and bundle sheath plastids, with subsequent electrophoretic or spectrophotometric assay of the preparations, also indicated that PPO was absent from the bundle sheath but present in the mesophyll fraction. A developmental study revealed that, although all leaf plastids near the basal meristem were ultrastructurally similar, the mesophyll and bundle sheath plastids were already differentiated with respect to PPO activity.

Abstract: Assimilates entering the developing rice caryopsis traverse a short-distance pathway between the terminal sieve elements of the pericarp vascular bundle and the aleurone layer. The ultrastructure of this pathway has been studied. Sieve elements in the pericarp vascular bundle are smaller than their companion cells.The sieve elements show few connections with surrounding vascular parenchyma elements but are connected to companion cells by compound plasmodesmata. Companion cells, in turn, are connected to vascular parenchyma elements by numerous compound plasmodesmata present in wall thickenings. Assimilates leaving the sieve element — companion cell complex must laterally traverse cells of the pigment strand before they come into contact with the aleurone layer. The pigment strand cells have modified inner walls made up of a suberin-like material. This material may act as a permeability barrier isolating the apoplast from the symplast of the pigment strand. The walls of the pigment strand cells are traversed by numerous plasmodesmata. Water may be conducted to the endosperm through the isolated cell-wall system of the pigment strand while assimilates possibly move via plasmodesmata. High frequencies of plasmodesmata occur at the junction between the pigment strand and the nucellus and also between adjacent cells of the nucellus. By contrast, plasmodesmata are absent between the nucellus and the aleurone layer and also between the nucellus and the seed coat. A predominantly circumferential and symplastic transport pathway is likely between the pigment strand and nucellus. In view of the total absence of plasmodesmata between the nucellus and the aleurone layer assimilates entering the endosperm may have to cross the plasmalemma of the nucellus. It is possible that constraints to the flow of assimilates may occur in the short-distance pathway between the terminal sieve element — companion cell complexes and the endosperm, and this is discussed.