SCHIZOGENY AND ULTRASTRUCTURE OF DEVELOPING LATEX DUCTS IN MAMMILLARIA GUERRERONIS (CACTACEAE)

The schizo-lysigenous latex ducts of Mammillaria guerreronis, sect. Subhydrochylus, were examined by optical and electron microscopy. These ducts are complex having a distinct outer epithelium without intercellular spaces and an inner epithelium in which schizogenous spaces arise. Schizogeny begins with formation of bulbous pockets and/or production of dark streaks in certain walls. Spaces formed by these processes ultimately contain a combination of electron dense materials, vesicles, and numerous thin, convoluted wall layers. Schizogeny may be responsible for initial formation of lumen and latex and may also separate some inner epithelial cells from the surrounding layers. Lysigeny of the inner epithelial cells contributes materials to the latex and allows enlargement of the duct. The inner epithelial cells contain mitochondria, dictyosomes, apparently functional nuclei, and numerous vesicles. The outer epithelium has fewer vesicles than the inner epithelium. Schizo-lysigeny in the members of sect. Subhydrochylus is considered to be ancestral to the strict lysigeny of the members of sect. Mammillaria. The inner and outer epithelia of M. guerreronis are thought to be homologous to the lumen and epithelium respectively of M. heyderi.     

DICHOTOMOUS BRANCHING IN MAMMILLARIA (CACTACEAE)

Cruciate dichotomous branching is a regular phenomenon in at least two species of cacti: Mammillaria perbella and M. parkinsonii, and there is exomorphic evidence that it occurs in other species of this genus. The specimens examined showed no obvious changes in the phyllotaxy of the two shanks derived by dichotomy. In the ecologically and morphologically specialized Cactaceae, it is quite unlikely that dichotomous branching represents the retention of a primitive character. It is furthermore improbable that it is of taxonomic value above the species level.     

THE STRUCTURE AND DEVELOPMENT OF AN UNUSUAL TYPE OF ARTICULATED LATICIFER IN MAMMILLARIA (CACTACEAE)

Although the laticifers of several species of Mammillaria can technically be classified as being of the articulated type, they differ significantly from all other reported articulated laticifers. They are derived from cells which differentiate only in older tissues, never in meristematic or young regions. The development involves the complete lysis of masses of cells, not just the perforation or resorption of the end walls in a single file of cells. At maturity, the laticifer lumen is lined with a one-to-several layered epithelium which may be quite thick. The laticifers increase in diameter with age, apparently by the lysis of the inner epithelial cells. Laticifers occur in the pith, cortex and tubercles of the vegetative body but were not observed in the roots, flower parts or in seedlings up to eight months old. Seven species were studied, all of which have “milky sap.” and the laticifers of each were virtually identical to the laticifers of the others.     

FURTHER STUDIES OF THE UNUSUAL TYPE OF LATICIFEROUS CANALS IN MAMMILLARIA (CACTACEAE): STRUCTURE AND DEVELOPMENT OF THE SEMI-MILKY TYPE

The development and structure of the laticifers in several species of the section Subhydrochylus of the genus Mammillaria (Cactaceae) were examined. These laticifers were found to be similar to those of the section Mammillaria in that both types develop from the complete lysis of several rows of parenchyma cells, and both types consist of long, branching, tubular lumens which are lined by epithelia. The laticifers of the section Subhydrochylus differ from those of the section Mammillaria in that those of the former are more irregular in shape, lumen development, and epithelium form. Also, the Subhydrochylus laticifers occur only as a single ring in the outermost cortex and tubercle bases, whereas those of section Mammillaria can be found in pith, medullary rays, cortex and throughout the tubercles. Because the structure of the laticifers in the section Mammillaria is much more regular and orderly, it is postulated that they are the derived type and that the laticifers of the section Subhydrochylus more closely resemble the ancestral condition. Two species, M. elegans and M. tegelbergiana, were found to be intermediate in nature, having characteristics of both types of laticifer systems. Solista pectinata was found to have laticifers similar to those in section Subhydrochylus.     

ULTRASTRUCTURE OF THE SECRETORY DUCTS OF RHUS GLABRA L.

The infrastructure and development of the secretory ducts were studied in the secondary phloem of Rhus glabra L. The ducts were found to develop schizogenously. The electron microscope observations may explain the view of several previous authors of schizo-lysigenous development of the duct lumen in the Anacardiaceae. The secreted material consists of lipophilic and polysaccharide substances. The electron micrographs suggest that the lipophilic substances arise in plastids, ER cisternae, Golgi vesicles, and mitochondria. The polysaccharide constituents apparently originate from the outer wall layers of the epithelial cells. The wall layers facing the lumen of the duct disintegrate and form, together with the secreted osmiophilic droplets, the gum-resin. Numerous microtubules were found along walls of the epithelial cells.     

THE FUNCTION OF EXTRAFLORAL NECTARIES IN OPUNTIA ACANTHOCARPA (CACTACEAE)

Opuntia acanthocarpa (Cactaceae) possesses extrafloral nectaries embedded in the areoles of new reproductive and vegetative growth. The nectar secreted by these glands attracts ants and is a nutritional food source. Members of one attracted ant species, Crematogaster opuntiae (Myrmicinae), are aggressive and efficient defenders of the plants against cactus-feeding insects. The results of our study are consistent with the ant-guard hypothesis for the role of extrafloral nectaries in O. acanthocarpa. Additionally, individuals of O. acanthocarpa are well protected in comparison with those of O. phaeacantha. The latter generally possess ephemeral extrafloral nectaries and consistently maintain fewer ants.     

ULTRASTRUCTURE OF DEVELOPING AND MATURE NONARTICULATED LATICIFERS IN THE MILKWEED ASCLEPIAS SYRIACA L. (ASCLEPIADACEAE)

The ultrastructure of developing and mature nonarticulated laticifers in Asclepias syriaca L. (the common milkweed) was studied by conventional fixation and staining techniques and by osmium impregnation techniques. The mature laticifer protoplast in A. syriaca possesses a large central vacuole with an intact vacuolar membrane. Formation of this vacuole apparently results from dilation and subsequent enlargement of endoplasmic reticulum and possibly in part by fusion of smaller vacuoles and limited cellular-lytic autophagy. Widespread digestion or autophagy of cytoplasm within vacuoles is not evident. Nuclei, mitochondria, dictyosomes, and small vesicles are the most prominent components distributed in the peripheral cytoplasm. Plastids appear to degenerate as the laticifer matures. The specialized cellular component, latex, which is the vacuolar content of the laticifer, is interpreted to be produced in the cytoplasm and subsequently incorporated into the large central vacuole. Rubber globules, the most prominent latex component, are surrounded by a membrane that does not have a trilaminate structure. Globules are associated with an electron-dense fibrillar component in the vacuole.     

ULTRASTRUCTURE OF THE DEVELOPING MEGASPORE MOTHER CELL OF GINKGO BILOBA

The immature megaspore mother cell of Ginkgo biloba is essentially spherical and is surrounded by a thick, complex wall. A large nucleus occupies the central region of the cell, and the organelles appear to be randomly arranged in the cytoplasm. With approaching maturity and the onset of meiosis, the cell elongates in the direction of the ovular axis. An extensive system of ER develops at the micropylar pole of the cell during elongation, and the plastids and mitochondria migrate to the opposite or chalazal pole. The micropylar end of the mature megaspore mother cell is usually devoid of plastids and mitochondria, but these organelles are densely packed in the chalazal end of the cell below the nucleus. The dictyosomes and dense spherosome-like bodies do not show such polarity in their distribution. At meiosis I plastids and mitochondria are, as a rule, restricted to the chalazal dyad cell that is destined to produce the functional megaspore. The wall of the megaspore mother cell consists of a middle lamella which is irregularly thickened, an outer wall layer resembling the walls of the surrounding nutritive cells, and an inner layer resembling the middle lamella in appearance.     

A MORPHOMETRIC STUDY OF THE ULTRASTRUCTURE OF ECHINOCEREUS ENGELMANNII (CACTACEAE). III. SUBAPICAL AND MATURE TISSUES

A stereological, morphometric study of the ultrastructure of subapical cells, xylem parenchyma, and cortical chlorenchyma of Echinocereus engelmannii shows that each of these cell types has a unique organellar composition. The cells of any of these tissues are unique not only in vacuolation (which is visible at the light microscope level), but also in the nucleus/cytoplasm ratio and the concentrations of mitochondria, chloroplasts, and dictyosomes. Furthermore, the differences between each of these cell types were large and statistically significant. It had previously been found that the cells of each zone of the shoot apical meristems of E. engelmannii are different from those of the other zones, but the present study suggests that, considering the large ranges of structure possible in the nonapical cells of this species, the apical meristem variation should be considered as only a minor difference and that the meristem zones are really quite similar to each other.     

A MORPHOMETRIC STUDY OF THE ULTRASTRUCTURE OF ECHINOCEREUS ENGELMANNII (CACTACEAE). VI. THE INDIVIDUALIZED ULTRASTRUCTURES OF DIVERSE TYPES OF MERISTEMS

The structures of spine basal meristem cells were examined by stereological morphometric techniques and found to be different not only from shoot apical meristem cells but also from spine primordium cells in the relative volumes occupied by cellular organelles. Nineteen types of meristematic cells have been studied in E. engelmannii, and all are distinct ultrastructurally. This suggests that whereas they all appear to be primarily involved in mitosis and cytokinesis, there must be other important aspects to their physiology; possibly these cells have already begun differentiation toward their mature states even while they are parts of active meristems.     

A MORPHOMETRIC STUDY OF THE ULTRASTRUCTURE OF ECHINOCEREUS ENGELMANNII (CACTACEAE). IV. LEAF AND SPINE PRIMORDIA

A stereological morphometric study of leaf primordia (P1 and P2) of Echinocereus engelmannii indicated that primordia are significantly different ultrastructurally from the shoot apical meristem tissues (tunica and peripheral zone) that produce the primordia. Leaf initiation involves readjustments of rates of synthesis and growth of cytoplasm, vacuoles, mitochondria, chloroplasts, and dictyosomes, such that leaf initiation must be a complex process in which different cell components are affected individually. Furthermore, leaf primordia are ultrastructurally distinct from spine primordia. Leaf and spine primordia as young as these are not yet irrevocably determined, thus different types of primordia, from the time of their inception and before their determination, have distinctly unique metabolisms; primordia are not merely generalized, uncommitted outgrowths whose developmental fate is set at some time later than inception.     

HORMONAL CONTROL OF ORGANOGENESIS IN OPUNTIA POLYACANTHA (CACTACEAE)

An axillary bud (areole) of Opuntia polyacantha is composed of an extremely short axis which bears highly modified leaves (spines). After producing the spines, the bud apical meristem becomes quiescent. When the axillary bud is excised and cultured with benzylaminopurine (BAP), the apical meristem increases in complexity and produces leaves on an elongated axis. Gibberellic acid (GA) induces spine production with no elongation of the axis. Naphthaleneacetic acid (NAA) causes no structural change in the meristem but induces root formation in adjacent tissue. The initiation of roots, spines, or leaves is visible within three days. Explants on medium lacking hormones show no structural change, but by the tenth day are insensitive to BAP or NAA. These aged explants can be made to respond to BAP or NAA by merely rewounding them at the time of hormone application. With concentrations previously found to be optimal for single hormone responses, NAA in combination with BAP prevents shoot formation, or in combination with GA prevents spine production. When all three hormones are combined, spines are produced, indicating that GA is active morphologically while BAP and NAA counteract each other. If the level of NAA is decreased, BAP counteracts the NAA and also overrides the GA and leafy shoots are produced. By varying the proportions of BAP and GA, in the absence of NAA, four types of lateral appendage can be produced: leaves, mildly altered leaves, leaf-spine transition forms, and spines.     

EFFECT OF GROWTH RATE,MORPHOGENIC ACTIVITY,AND PHYLOGENY ON SHOOT APICAL ULTRASTRUCTURE IN OPUNTIA POLYACANTHA (CACTACEAE)

Dormant short shoot apices of Opuntia polyacantha were cultured under three conditions: cytokinin and high sucrose to stimulate the formation and rapid growth of a leafy long shoot; cytokinin and no sucrose (slow growth of a leafy long shoot); gibberellic acid and high sucrose (rapid growth of a spiny short shoot). These meristems, and also dormant (uncultured) ones, were analyzed by stereological, ultrastructural techniques. By comparing meristems growing with cytokinin but with or without sucrose, correlations between metabolic rate and apical ultrastructure were studied; comparison of leaf-producing and spine-producing meristems permitted examination of correlations with morphogenic role; comparison with published data for four other species permitted study of phylogenetic effects, and comparison with dormant apices revealed information about meristem activation. Ultrastructure varied according to each condition: metabolic rate, morphogenic activity and species can be distinguished by quantitative methods. Apical ultrastructure is most strongly correlated with rate of growth such that apices of differing species resemble each other if growing at similar rates, whereas apices of a single species differ markedly if growing at differing rates or if performing different morphogenic activities. Hyaloplasm is an excellent indicator of metabolic rate; mitochondria, nuclei, and vacuoles are not.     

A MORPHOMETRY STUDY OF THE ULTRASTRUCTURE OF ECHINOCEREUS ENGELMANNII (CACTACEAE). II. THE MATURE,ZONATE SHOOT APICAL MERISTEM

The shoot apical meristems of adult Echinocereus engelmannii plants are zonate and have a tunica, central mother cells, a peripheral zone, and a pith-rib meristem. An ultrastructural, stereological study showed that each zone has its own distinct ultrastructure, but that the differences between the zones are quite small, both on a protoplasmic basis and on a cytoplasmic basis. Furthermore, the ultrastructure present in the adult apices differed only slightly from that which had been found in seedling apices, demonstrating a long-term stability of structure. The standard deviations found in the sample were small, indicating little variability from one plant to the next and suggesting that there are little or no cyclic changes during the plastochron or a 24-hr photoperiod. The ultrastructures found in the shoot apical meristems differed significantly and markedly from mature tissues of the same plants.     

A MORPHOMETRIC STUDY OF THE ULTRASTRUCTURE OF ECHINOCEREUS ENGELMANNII (CACTACEAE). I. SHOOT APICAL MERISTEMS AT GERMINATION

At germination the shoot apical meristems of Echinocereus engelmannii were discs with a volume of 666,000 μm³ and were composed of a unistratose tunica (volume: 260,000 μm³) and a corpus which was two cell-layers thick (volume: 406,000 μm³). Four days after germination the nucleus constituted 28.9% of the volume of the cell, and the vacuole constituted 24.5%. The mitochondria were 13.3% of the volume of the tunica cytoplasm, the chloroplasts 9.4%, and the dictyosomes only 1.2%. The endoplasmic reticulum was too sparse to be accurately measured. The organelles of the corpus were identical in size and shape to those of the tunica, but there were statistically significant differences in their cellular and cytoplasmic densities: the more distal corpus layer (C1) was less vacuolate (16.6% of the cell volume), and both corpus layers contained more chloroplasts, 12.0% of the cytoplasmic volume in C1 and 14.3% in the more proximal corpus layer (C2). During the first four days after germination there was a dramatic increase in the size of the central vacuole (e.g., from 15.4% to 24.5% in the tunica), and the mitochondria increased in density from 10.2% of the cytoplasmic volume to 13.3%. Chloroplast density also increased in all meristem layers, but the dictyosome density decreased, as much as a 30% loss in C2. There was also a highly significant reduction in the number of cisternae per dictyosome, from 5.47 to 4.77. Surprisingly, there was no change in heterochromatin: ca. 27% of the volume of the nuclei of all layers was heterochromatic at all stages studied. Thus, the organellar structure of corpus cells is distinctly different from that of tunica cells, and as the apical meristem becomes active after germination, the changes which occur are not uniform in the meristem but rather are zone-specific.     

A STUDY OF THE PROPOSED GENUS OBREGONIA (CACTACEAE)

The monotypic genus Obregonia Frič was compared with Ariocarpus, Lophophora, Strombocactus, and certain other cactus genera. Obregonia and Ariocarpus are similar in characters of seeds, seedlings, habitat, fruits, and general habit. They differ in time of flowering, point of flower origin, areole structure, presence or absence of druses and a mucilage system, and presence or absence of spines. Obregonia and Lophophora are similar in characters of seeds, habitat, basic areole structure, and fruits. They differ in seedling form, habit of adult plants, presence or absence of spines, and chemical analysis. Strombocactus resembles Obregonia in few ways except in basic areole structure and some aspects of anatomy. The author concludes that Obregonia is an intermediate form between Ariocarpus and Lophophora and deserves generic rank. A formal taxonomic treatment of the genus follows the conclusions.     

东方扁虾精子发生的超微结构

应用电镜技术研究了东方扁虾（Thenus orientalis)精子发生的全过程,精原细胞呈椭圆形,其染色质分布较均匀,线粒体集中于细胞一端形成“线粒体区”。初级精母细胞较大,染色质凝聚成块,次级精母细胞核质间常出现大的囊泡,胞质内囊泡丰富而线粒体数量却明显减少,早期精细胞核发生极化、解聚,部分胞质被抛弃。中期精细胞外观呈金字塔形,分为三区;正在形成的顶体位于塔顶,核位于塔基部,居间的细胞质基质内富含膜复合物,后期精细胞顶体进一步分化。形成顶体帽和内、外顶体物质等三个结构组份。成熟精子核呈盘状或碗状,具有5-6条内部充满微管的辐射臂。