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In the multinucleate cells induced in Allium cepa L. meristems, the nuclei surrounded by the largest cytoplasm environment complete replication earlier (advanced nuclei), but have a longer G2, than the others (delayed nuclei). Thus, all nuclei break down the nuclear envelope and start metaphase simultaneously. The present report shows that this synchronization relies on a checkpoint mechanism. When completion of replication was prevented in the delayed nuclei (due to in vivo 5-aminouracil feeding initiated when the advanced nuclei were already in G2), the metaphase was also further delayed in the advanced ones. In turn, some of the delayed nuclei overrode the G2 checkpoint (adaptation) and entered into mitosis with broken chromatids (Del Campo et al., 1997). Anoxic UVA (313 nm) irradiation apparently prevents the binding of regulatory proteins to Br-DNA. The present report shows that late replicating sequences are the targets of the checkpoint signal produced by the still replicating nuclei. This signal delays metaphase in the advanced nuclei, whose DNA is already fully replicated. Thus, when the already replicated sequences of late replicating DNA was modified in the advanced nuclei by bromosubstitution followed by anoxic UVA irradiation, they entered into mitosis without any delay, ignoring the inhibitory signals produced by the still replicating nuclei.

Treatment of Allium cepa meristematic cells in metaphase with the topoisomerase II inhibitor ICRF-193, results in bridging of the sister chromatids at anaphase. Separation of the sisters in experimentally generated acentric chromosomal fragments was also inhibited by ICRF-193, indicating that some non-centromeric catenations also persist in metaphase chromosomes. Thus, catenations must be resolved by DNA topoisomerase II at the metaphase-to-anaphase transition to allow segregation of sisters. A passive mechanism could maintain catenations holding sisters until the onset of anaphase. At this point the opposite tension exerted on sister chromatids could render the decatenation reaction physically more favorable than catenation. But this possibility was dismissed as acentric chromosome fragments were able to separate their sister chromatids at anaphase. A timing mechanism (a common trigger for two processes taking different times to be completed) could passively couple the resolution of the last remaining catenations to the moment of anaphase onset. This possibility was also discarded as cells arrested in metaphase with microtubule-destabilising drugs still displayed anaphase bridges when released in the presence of ICRF-193. It is possible that a checkpoint mechanism prevents the release of the last catenations linking sisters until the onset of anaphase. To test whether cells are competent to fully resolve catenations before anaphase onset, we generated multinucleate plant cells. In this system, the nuclei within a single multinucleate cell displayed differences in chromosome condensation at metaphase, but initiated anaphase synchronously. When multinucleates were treated with ICRF-193 at the metaphase-toanaphase transition, tangled and untangled anaphases were observed within the same cell. This can only occur if cells are competent to disentangle sister chromatids before the onset of anaphase, but are prevented from doing so by a checkpoint mechanism.

Multinucleate plant cells with genetically balanced nuclei can be generated by inhibiting cytokinesis in sequential telophases. These cells can be used to relate the effect of changes in the distribution of nuclei in the cytoplasm to the control of the timing of cell cycle transitions. Which mitotic cell cycle events are sensitive to differences in the amount of cytoplasm surrounding each chromosomal complement has not been determined. To address this, we maximized the cell size by transiently inhibiting replication, while cell growth was not affected. The nuclei of 93% of the elongated cells reached prophase asynchronously compared to 46% of normal-sized multinucleate cells. The asynchronous prophases of normal-sized cells became synchronous at the time of nuclear-envelope breakdown, and the ensuing metaphase plate formation and anaphase onset and progression occurred synchronously. The elongated multinucleate cells were also very efficient in synchronizing the prophases at nuclear-envelope breakdown, in the prophase-to-prometaphase transition. However, 2.4% of these cells broke down the nuclear envelope asynchronously, though they became synchronous at the metaphase-to-anaphase transition. The kinetochore-microtubular cycle, responsible for coordinating the metaphase-to-anaphase transition and for the rate of sister segregation to opposite spindle poles during anaphase, remained strictly controlled and synchronous in the different mitoses of a single cell, independently of differences in the amount of cytoplasm surrounding each mitosis or its ploidy. Moreover, the degree of chromosome condensation varied considerably within the different mitotic spindles, being higher in the mitoses with the largest surrounding cytoplasm.

Condensed sister chromatids possess a protein scaffold or axial core to which loops of chromatin are attached. The sister cores are believed to be dynamic frameworks that function in the organization and condensation of chromatids. Chromosome structural proteins are implicated in the establishment of sister chromatid cohesion and in the maintenance of epigenetic phenomena. Both processes of templating are tightly linked to DNA replication itself. It is a question whether the structural basis of sister chromatid cores is templated during S phase. As cells proceed through the cell cycle, chromatid cores undergo changes in their protein composition. Cytologically, cores are first visualized at the start of prometaphase. Still, core assembly can be induced in G1 and G2 when interphase cells are fused with mitotic cells. In this study, we asked if chromatid cores are similarly able to assemble in S-phase cells. We find that the ability to assemble cores is transiently lost during local replication, then regained in chromosome regions shortly after they have been replicated. We propose that core templating occurs coincident with DNA replication and that the competence for the assembly of the sister chromatid cores is acquired shortly after passage of replication forks.

The herbicide carbetamide [(R)-1-(ethylcarbamoyl) ethylphenylcarbamate], in the 0.4 to 0.8 mM range, efficiently induced multipolar mitoses in Allium cepa L. The frequency of multipolar anaphases rose earlier and reached higher values when both concentration and time of treatment increased, up to a maximum of 90% after 1 h of treatment. To identify the physiological target, the kinetics of induction of multipolar mitoses were followed during recovery from very short treatments (5, 10, and 15 min). Tubulin immunodetection showed that phenylcarbamate immediately disrupts the cohesion between the different bundles of microtubule minus ends which converge at the pole. The spindle was rendered multipolar about three times more efficiently in metaphase than in anaphase. The observations do not support any effect of the herbicide on the tubulin polymerization-depolymerization cycle, and suggest that the minus ends of the microtubules remained stabilized in carbetamide. Thus, the density of kinetochore microtubules and their lengths were unmodified in the individual chromosomes which became detached from both spindle poles in response to the herbicide. Extra microtubule-organizing centres for the assembly of both preprophase band and phragmoplast (the tubulin arrays which characterize the microtubular cycle responsible for cytokinesis in plant cells) were also rapidly induced.

Indirect immunodetection of tubulin showed that the herbicide carbetamide activated silent signals left by the preprophase band (PPB) and by old phragmoplasts. Thus, after half an hour of treatment, 5.3% of anaphases in Allium cepa L. meristems showed spindle microtubules pointing to sites of the longitudinal cell membranes which, under control conditions, would only start attracting microtubules from the growing phragmoplast at late telophase. After 2 h, 12.8% of the telophases showed not only the expected phragmoplast between the two sister nuclei, but one or two additional phragmoplasts, at one or both cell tips, the sites of the phragmoplasts from the telophases of previous cycles. A few binucleate cells, obtained by aborting phragmoplast formation by a short caffeine treatment, developed three phragmoplasts in their next mitosis (bimitosis) in the presence of carbetamide: one between each sister pair of telophasic nuclei plus an extra one. The latter also occupied the site of the phragmoplast of the telophase of the previous cycle.

Autotetraploid (4n = 32) cells were induced in Allium cepa L. root meristems by successively treating with a multipolarizing agent in anaphase (carbetamide) and an inhibitor of cell plate formation in telophase (caffeine). This treatment produced cells with their 32 chromosomes distributed in more than two nuclei. During G1, one of the nuclei in the resulting trinucleate cells had a DNA content which was equivalent to an average of 16 chromosomes, while the other 16 chromosomes were randomly distributed in two aneuploid nuclei. In the set of 16 chromosomes forming the onion diploid complement, there are 4 NOR-chromosomes and 5 chromosomes carrying DNA domains providing a nucleus with the competence to replicate, as previously shown. Expected probabilities derived from the different possible models for cosegregation of both kinds of chromosomes in the two aneuploid nuclei of the trinucleate cells were estimated by a computer simulation. These expected values were compared with the recorded frequencies of aneuploid nuclei which were able to organize a nucleolus and to respond to inducers of replication. The present data are compatible with the existence of sequences providing a nucleus with the competence to replicate in three out of the four NOR-bearing chromosomes, as well as in two other chromosomes of this diploid complement. The scarcity of chromosomes bearing early origins able to initiate replication in a nucleus is a striking feature of this huge onion genome.

Multinucleate tetraploid cells with unbalanced chromosomal distribution in aneuploid nuclei were obtained in Allium cepa L. root meristems. For this, their natural diploid cells were treated with a multipolarizing agent (1h carbetamide) followed by an inhibitor of cytokinesis (1h caffeine). Data from these multinucleate cells with aneuploid nuclei suggest that only four out of the thirty-two chromosomes of their autotetraploid complement possess DNA sequences making the nucleus competent to respond to inducers of replication and mitosis. Direct observation of cells where a single replicated chromosome had reached mitosis showed that this chromosome was the one bearing the nucleolar organizer. Six specific chromosomes would confer competence to the nucleus to respond to inducers of replication but not to those producing chromosome condensation. Another four different chromosomes would confer the nucleus with the ability to respond to mitotic inducers but not to replication inducers. The rest of the chromosomal complement seemed to lack any of the DNA sequences needed for these two important cycle transitions. In a nutshell, certain DNA sequences distributed in a few chromosomes of the onion complement are an intranuclear requirement to initiate replication and mitosis in these plant cells.

The acrosome of rodents, as well as the acrosomal granule,the head cap and the elastic materials fixed in glutaraldehyde and embedded in EPON, show a high degree of affinity for aqueous phosphotungstic acid (PTA). Under the electron microscope the reactive material shows up in strong contrast when fine sections have been floated on aequous 3-5 % PTA. In thick PTA-treated sections, the PTA can be easily located by light microscopy by subsequent staining with methyl green pyronin or UNNA's blue.

After using a silver staining procedure two deeply stained structures appear in spermatogonial, first and second meiotic metaphase chromosomes of grasshoppers. First of all, rounded structures of about 0.4–0.6 um in diameter, which represent kinetochores. Moreover a longitudinal axes, which runs the whole length of each chromatid and which seems to represent a core or scaffold-like structures. This axis is always in close relation with kinetochores. The homologous axes of chromosomes in first meiotic metaphase are very close together at the point were chiasmata take place.

Positive staining of kinetochores and a chromatid core has been achieved using a simplified silver staining method in squash preparations from meiotic chromosomes of grasshoppers. This technique permits the exact localization of kinetochores on the chromosomes whether metacentric, acrocentric or ‘telocentric’. The sister kinetochores can be observed from mid-diplotene stages but they are not differentiated during first meiotic metaphase. However they can be observed again at the onset of anaphase 1. The existence of a positively stained chromatid core in meiotic divisions is also reported. This core appears well defined inside each chromatid from diplotene to the end of the second meiotic division. The visualization of these cores in first meiotic metaphase clearly shows the points at which the chiasmata took place.

In Allium cepa mierospores, the nucleolar organizing region appears as an area of low density situated between two nucleolar masses. It consists of a series of zones with a density similar to that of the chromatin surrounded by areas of lower density. The dense zones are sometimes arranged in an orderly pattern of 2–4 rows. The organizing region consists of filaments, about 100 Å in diameter, which are seen to be concentrated in the dense areas, and more scattered in the rest of the region. — The alcoholic PTA staining technique reveals the presence of an appreciable quantity of arginine-Iysine rich histones in the organizing region, as well as a similarity between the dense areas of this region and the rest of the chromatin: a similarity which is also brought out by the thallium alcoholate technique, used for DNA staining. By means of uranyl-BDTA-lead RNP material can be shown to be predominantly located in the low-density areas of the organizing region in the form of fibres of about 80 Å in diameter, presumably representing the newly synthetized r-RNA (45 S RNA). A pattern is suggested for the organizing region, in which areas of functional chromatin (euchromatin) would appear alternating with areas of non-functional chromatin (heterochromatin).

The Nuoleolar Organizer Region (NOR) was studied in Allium cepa. The cells analyzed were: meristematic root tip cells, cells from the layers of the anther (exothecium, intermediate layer cells, endothecium and tapetum) and meiocytes from preleptotene to diptotene. Conventional electron microscope techniques and serial section were used in the study. Ultracytochemical tests, using preferential techniques for nucleic acids were also applied. The results show that: a) Although the NOR may have a very variable form in the different cells analyzed, in each case its fine structure is similar, being constituted of chromatin differentiated in high and low density zones. b) In each case, its cytochemical characteristics are similar; highly positively to EDTA (preferential stain for RNP) being observed in the low density zones. As scattered chromatin these would constitute the active regions of the NOR. c) In certain cases a clear relationship is seen between NOR volume and the size of the nucleolus. d) The chromatin associated to the NOR always appears as very dense zones. e) The NOR-Nucleolus relationships are established exclusively with the latter's fibrillar pars. f) The nucleolar segregation phenomenon takes place sometimes as a result of a NOR contraction and in other cases depends on the position of the NOR in relation to the nucleolus. g) In preleptotene, the NOR shows its characteristic morphology and is positive to EDTA, while from zygotene to diplotene the structure of the NOR presents no differences from the rest of the chromatin and only the lateral elements of the synaptonemal complex or its remnants are positive to EDTA. h) The synaptonemal complex runs through the NOR with no differentiation and on occasions its lateral elements show associations with the nucleolus.

Nucleolar cycles were related to mitotic and amitotic phases in meristematic cells of Allium cepa L. roots growing in control conditions and in others where amitosis was induced by a 6-h treatment with ethidium bromide.The induced amitosis in these cells was followed by an unbalanced distribution of chromosomal material between the two independent daughter cells resulting from it.The first discrepancy between mitosis and amitosis was found at the level of prophase nucleolar disorganization, which was greatly modified. The nucleolar cycle was permanently stopped in amitosis even when recovery from treatment was allowed. The kinetics of amitosis appearance in the meristem suggest a decision point located round about mid-prophase. This decision point seems to involve nucleolar function in prophase. These observations confirm the fact that prophase is one of the decisive stages in the differentiation of a cell.

Induction of polynucleate cells in onion root meristems by inhibition of two sequential cytokineses is used to study controls operating in cell cycle progression. Triggering of both replication and metaphase occurs synchronously in nuclei sharing a common cytoplasm, independent of their ploidy or intracellular position. The replication rate appears to be activated by the simultaneous intracytoplasm presence of other replicating nuclei. On the other hand, central position of a nucleus in the cell as well as increase in ploidy leads to slowing down of replication rate. The relative advance and lag of S period in the different nuclei in a common cytoplasm is partially counter balanced by differential times of G2. Moreover prophase lengthening in the fast interphase nuclei points to a negative control exerted by the lagging nuclei mediated by cytoplasm. Finally, it could be worth emphasizing similarities in the control mechanism operating in cycle progression both in animal and plant cells.

Suelos. Características petroquímicas y micromorfológicas de Haploxeralfs, por P. Arévalo. J. Gallardo y J. Benayas.-- Determinación de manganeso total en suelos, por J. Moreno Caselles, M. Guillén y M. Romero.-- Rendolles de la Sierra de María (Almería), por L J. Alias y J. Martínez.-- Estudio de tres perfiles de suelos de La Alpujarra, por Barahona. E., García Chicano. J. L. Guardiola. J. L. lriarte. A, Pérez-Pujalte, A. y Quirantes, J.-- Detección de concordancias fisonómico-edáficas por J. M. Gómez. R. Rodríguez González. A. García Miranda y C. de Hoyos Alonso.-- Suelos y sedimentos de zonas semiáridas, por A. García Rodríguez, J. Forteza Bonnin y L F. Lorenzo Martín.-- Suelos sobre jumillita de la Sierra de las Cabras (Albacete), por Joaquín Hernández Bastida, María Teresa Fernández Tapia y Francisco Alcaraz Ariza.-- Estudio microscópico de aridisoles de las Islas Canarias, por A. Rodríguez Rodríguez, M. C. Pérez Angles. M. C. Díez de la Lastra Bosch y E. Fernández Caldas.-- El material original: propiedades de los suelos de Galicia, por F. Macias. R. M. Calvo, C. García. E. García-Rodeja y B. Silva.-- Las sierras de Queixa e invernadeiro y sus estribaciones, por Grupo de Edafólogos de Santiago.-- Suelos forestales de la cordillera costero-catalana, por A. Escuredo. V. R. Vallejo y J. Bech.-- Materia orgánica de suelos forestales, por A. Escuredo. V. Vallejo y J. Bech.--Dificultades en el empleo del sistema "USDA-Soil Taxonomy", por Carlos Roquero de Laburu.-- Nematodos fitoparásitos de la superfamilia "criconematoidea", por A. Bello y Ma. Paz Lara.—Silicatos. Retención del diazinon por montmorillonita, sepiolita y caolinita, por G. Dios Cancela, S. González García y M. Martín Aguilar.—Geoquímica. Alteración de minerales de estaño, por Guijarro, J. Casas, J. y Hoyos, M.A.—Nutrición y Fisiología Vegetal.-- Capacidad fotosintética de maíz, por U. Velázquez, M. Lasaña y J. Cardus.-- El alerce en Chile. II. Su nutrición mineral, por M. Lachica, C. González O. y M.

The nucleolus is not a permanent organelle in the life of a cell; at the initiation of a new cycle, the nucleolar formation seems to depend on RNA synthesis directed by nucleolar RNA polymerase1. We have compared the rate of nucleologenesis in meristem cells of Allium cepa L. and in those of four Vicia species with different numbers of ribosomal cistrons and different DNA contents3,4 as well as the rate of nucleologenesis in four different cell populations lying side by side in the root of Zea mays. The rate of nucleologenesis per ribosomal cistron seems to be constant in the different species growing in similar conditions, but this mean rate was modified in metabolically different subpopulations in the same root.

En núcleos de microsporas deAllium cepa se describen estructuras de 1 µm de diámetro, de baja densidad y estrechamente relacionadas con varias masas de cromatina, las que se equiparan con los denominados «micropuff» recientemente descritos.

This paper reports the effect of abscisic acid (ABA) on the root growth in Allium cepa and the effects on the growth components, namely, cell flow and final cell size, and number of functional cells in each meristematic rib. With a continuous treatment by ABA steady state kinetics of growth is attained for each concentration, characterized by a constant growth rate. This growth is inhibited only by ABA 10-5M, 10-6M and 10-7M. The 10-7M concentration seems to act only on the cell size. At ABA 10-6M a slight decrease in the cell flow is also observed and at 10-5M the results suggest a decrease in the number of functional meristematic cells, in addition to the other two previously recorded effects. All these results are compared with those obtained by the same ABA concentrations in the short term experiment.

In microspores of Allium cepa, the nucleolus generally takes the form of two hemispherical masses showing the same degree of development, consisting of the fibrillar and the granular components intermingled. The nucleolar organizing region (NOR) can be observed between the two nucleolar masses and is often in close connection with the masses of chromatin. It consists of a series of zones similar in density to the chromatin itself, arranged in a relatively orderly pattern in a region of low density. These dense zones consist of an accumulation of filaments measuring 100 Å, while the less dense area consists of similar but less concentrated filaments connecting the dense zones with one another. Apparently it is this latter area which is in connection with the nucleolar masses. It may be supposed that in order to carry out its function the NOR assumes a pufflike structure.

An ultrastructural and cytochemical analysis of the attachment sites of the synaptonemal complex to the nuclear envelope in rat spermatocytes is presented. The association is formed by the ends of the lateral elements in the form of a plate associated with the inner membrane, as well as by formations of fibrillar character found in the perinuclear space and in the cytoplasm adjacent to the outer membrane. By means of uranyl-EDTA-lead staining, both the area of association between the lateral elements and the fibrillar formations can be strongly stained, while the same areas appear unstained when the tissue has been previously treated with RNase. Staining with alcoholic PTA also produces a strong staining effect at the level of the attachment plates of the lateral elements, and very weak staining of the perinuclear space and of the formations in the adjacent cytoplasm. On the basis of this analysis of both the associations of autosomes and of the XY pair, we suggest that this associating structure consists partly of RNA and partly proteins, presumably histones.

La rápida elongación de las células durante la diferenciación se debe, entre otros motivos, a un incremento de la membrana envolvente; este incremento es originado por el aporte de vesículas del aparato de Golgi cuyas membranas coalecen con el plasmalema y su contenido se deposita externamente junto al material amorfo de la membrana.

By examining the cells of the endothecium and exothecium of Allium cepa anthers we have been able to study the formation and development of the vacuoles. These appear to arise from local expansions of the ER which, in the course of cell differentiation, increase in size until they form one single vacuole occupying most of the cell volume. The three phases in the development of the vacuoles, local expansions of the ER provacuoles, young or star-shaped vacuoles, and fully developed, or spherical vacuoles, take “pari passu” with the differentiation and elongation of the parenchyma of the anther.

This paper reports the results of a study of the first beginnings of sporopollenin granules in the cells of the tapetum. The endoplasmic reticulum was observed to be the system responsible for their production, first by the formation of electron-dense bodies (pro-Ubisch bodies). These constitute the nuclei of the sporopollenin granules upon which laminae with the characteristics observed in the exine of pollen-grains are deposited in formation of the young Ubisch bodies. The latter ultimately assume a spherical shape (adult Ubisch bodies), whereupon they seem to be carried by channels across the cytoplasm of the tapetum into the locule of the pollen-sac. The origin, development and possible function of sporopollenin granules are discussed in the light of the theories and observations of other authors and of the writers themselves.

This is a study, by electron microscopy, of the structure and possible function of a “dense wall” in the pollen mother-cells (PMC) after the last mitosis, carried out on Allium cepa anthers. On the formation of the “special callose wall”, the paper also describes the contribution of the membranes from vesicular elements to the formation of a new plasmalemma closely related to the “special callose wall”, and the formation of this layer from the inner contents of vesides which are of nuclear origin and other vesicular elements.

ARR et al. 1-3 were pioneers in the study which led to the discovery of a definite localization of the chromatin in relation to the nuclear membrane when they found that in the somatic cells of the cat the sex chromatiu is located at the nuclear membrane. BURGOS* demonstrated the accumulation of Feulgen reactive material on the inner aspect of the nuclear membrane in unfertilized eggs of Arbacia punetulata, and GA'x "5 studied the relations between chromatin and nuclear membrane outpocketings. However, the re]ation between chromatin network and the membrane in the interphase nucleus remains practically unexplored. In this paper we study a certain relation often observed between chromatin and nuclear membrane in the interphase nucleus of meristematic cells.

This paper describes the changes undergone by the layer dividing the vegetative from the generative cell. The abundant contents of the space inside the membrane, consisting of opaque bodies, vary considerably as the pollen grain matures, and decrease in volumepari passu with the formation of the dense bodies in the plasm of the vegetative cells. The dense bodies constitute a layer of droplets surrounding the enveloping membrane of the generative cell within the cytoplasm of the vegetative cell. Connexions have been observed between the dense bodies and the partition layer which seem to indicate that the latter is apparently responsible for their organization, and is the area in which the synthesis of their contents takes place as well. The deposition of the dense bodies on the plasmalemma of the pollen grain points to their participation in the formation of one or other of the layers in which the grain is enveloped.

The cytopla,smic continuity b~tween raer{st,eraatic cells was a raatter of controversy till S'trugger (1957) and Buvat (1957) studied the cytoplasmic bridges which traverse the walls with the electronmicroscope. Later Whatey et al. (t960) .sugaested ~he continuity of the tubules .of the endopla,sraic reticulura acros.s the cell wall, and Porter and Machado (1960) po,inted out that the configuration .of the plasmodesm is related to the formation of the cell plate a~t the telophase. A't this moraent the cell plate is cr.o,ssed by tracts ,of the endoplasmic reticulum which determine the location o[ the pla.smo, desms. Recently, Frey- Wyss 1 in get al. (1964), studying the cell plate formation, macle the same observa~tions as Porter and Machado. Nevertheless, .a,s Buvat (1953) writes, "the cont, inuous passage of the filaments of the endopla.smic reticnlum from a cell to its neiahbours is still debatable," and only a few details of the ultrastructure .of the plasmodesms are known. In thi,s comraunicati.on we present some new evidence on tMs question, and sugg,e,st a model of the plasraodesra as well.

In Angiosperms, after meiosis and the formation of microspores, these develop a series of processes leading to maturation. It may be said that one of the first important steps towards maturation is taken when the haploid nucleus undergoes post-meiotic mitosis to give rise to two cells, the vegetative and the generative, which are formed with marked spatial polarity, dividing the cytoplasm very unequally (Vaznart 1963), so that the generative cell possesses little cytoplasm while the vegetative occupies most of the pollen grain. The former undergoes a mitotic division, and produces two spermatic cells which later effect the double fecundation characteristic of Angiosperms, while the latter forms the pollen tube which is to carry the spermatic cells towards fecundation. The subjects of the study carried out by us were the various changes observed mainly in the vegetative nucleus and the evolution of the membrane systems from cytoplasmic organules in the course of development and maturation of the pollen grain.

1. The prophases of meioses in a particular pollen-sac of the monocotyledons Scilla non-scripta and Allium cepa run a synchronized course and their protoplasts are joined together by connexions. 2. The connexions are of different dimensions and they may be of either cytoplasmic or nuclear character, or they may arise from bridges set up by cell organules. 3. The connexions have been observed throughout the prophase of meiosis only. Their possible origin and the consequences are discussed. © 1969, Japan Mendel Society, International Society of Cytology. All rights reserved.

[EN]: A rib meristem is characterized by a complexus of longitudinal rows or ribs of cells which divide at right angles to the growth axis. This growth pattern is clearly represented in the cortex of the root. The longitudinal walls of the rib are primary longitudinal walls, the transverse walls being formed, basically, by middle lamella. At each division cycle, 2 daughter cells are formed, and the celt plate forms the new middle lamella between them. The fine structure of the cell plate formation has been studied by several authors TM. Moreover, FREY-WYSsLING et al. 2 showed that the middle Iamella formed by the coalescence of small Golgi vesicles, grew thicker through the supply of new Golgi vesicles. The aim of this paper is to study the development of the middle lamelta in relation to the cell division cycles., [ES]: La lámina media que separa transversalmente a las células del meristemo radical en columna se desarrolla, en virtud del aporte de pequeñas vesículas del aparato de Golgi, incrementando su espesor desde 0,1–0,2 hasta 0,4–0,5 µ. Las características morfológicas de estas paredes transversales permiten distinguir fácilmente las células hermanas y hasta las 4 células que se originan de 1 célula madre en 2 ciclos de división.