FASCIATION AND DICHOTOMOUS BRANCHING IN ECHINOCEREUS (CACTACEAE)

In Echinocereus reichenbachii dichotomous branching and fasciation (cresting) are rare events. Both were found together in only a few of many populations investigated and are interpreted as variants of a single phenomenon. They may occur at any stage of shoot development, but crest meristems arise most commonly on young branches among clusters of normal shoots. Sometimes they appear on unbranched young plants or seedlings, very rarely on older shoots. Dichotomy results from the division of an apical meristem into equal parts each of which functions independently, producing a forked shoot. Fasciation involves the extension of a single meristem into an apical ridge. The product is a flabellate shoot that becomes undulate if growth along the summit continues. In longisection linear meristems appear similar to radial sections of normal shoots; in median sagittal section they have a much extended central mother cell zone within which the cell pattern resembles a rib meristem. Although crest meristems become sluggish or even inactive with age, localized renewed growth may occur spontaneously or be induced by injury. In this species the random production of normal shoots from crest meristems (defasciation) was not observed, but if much or all of such a meristem is removed, branches may arise from lateral areoles, and these are always normal. It seems, therefore, that whatever induces fasciation in E. reichenbachii originates in and is restricted to the apical meristem and its immediate vicinity.     

THE ULTRASTRUCTURE OF DEVELOPING LATEX DUCTS IN MAMMILLARIA HEYDERI (CACTACEAE)

Electron microscopy was used to investigate early development of latex ducts in Mammillaria heyderi (Cactaceae). Numerous vesicles (secondary vacuoles) form from invaginations of the plasmalemma near sites of wall thinning, from endoplasmic reticulum (ER), and from vesiculate grana of degenerate plastids. Dictyosomes, though they occur in young duct cells, do not seem to be responsible for the formation of vesicles. Cytoplasmic vesicles may contain fibrillar, globular, or crystalline materials, or may be devoid of any type of particulate matter. They may be responsible for storage of numerous laticiferous components. Lysosomal materials could be stored in some vesicles and contribute to the degradation of the protoplast. Some nuclei contain condensed chromatin and are subject to deformation and collapse. Mitochondria and lipid bodies are common in young duct cells but ER is rare. When ducts form in young tissues, plastids in the lumen do not produce starch grains or extensive membranous networks. The plastids eventually degenerate to become a part of latex. If ducts form in older, established tissues having mature plastids, the plastids undergo extreme modification.     

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.     

PHYSIOLOGICAL RESPONSES TO RAINFALL IN OPUNTIA BASILARIS (CACTACEAE)

An immediate, marked response to small amounts of rainfall occurs in Opuntia basilaris, despite previous drought conditions. The effect of rainfall is upon plant water potential, which is the single most important parameter influencing stomatal opening, CO₂ assimilation, and organic acid synthesis. Nocturnal stomatal opening is initiated following rainfall, and stomata remain open during the daytime. Decreasing stomatal and mesophyll resistances correlate with increasing rates of nocturnal assimilation of ¹⁴CO₂. Photosynthetic rates of ¹⁴CO₂ assimilation are low, despite high plant water potentials and low stomatal diffusion resistances. The decreased mesophyll resistances and increased rates of nocturnal ¹⁴CO₂ assimilation correlate with the increases of nocturnal efficiency of water use and CO₂ assimilation. The diurnal efficiency of water use and CO₂ assimilation is lower than the nocturnal gas exchange efficiency values.     

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.     

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.     

PHENOTYPIC PLASTICITY OF BRANCHING PATTERN IN SUGAR MAPLE (ACER SACCHARUM)

Branching patterns of leader shoots and lower branches were compared in 15 sugar maple trees. The number of first-order branches per second-order branch (R_1:2) differed very significantly between leader shoots and lower branches. Values of R_1:2 averaged 6.47 for leaders and 4.06 for lower branches. Stepwise bifurcation ratios between successive higher orders did not differ significantly. The difference in R_1:2 between leader shoots and lower branches within individual trees is the result of developmental-phenotypic interactions, such that the number of leaf-bearing shoots per support shoot decreases over time. These results indicate that branching patterns within individual trees are not stationary, as has commonly been claimed.     

SEEDLING DEVELOPMENT OF OPUNTIA BRADTIANA (CACTACEAE)

Seedling development in Opuntia bradtiana, a north-central Mexican endemic, is similar to that of other opuntias, except for the absence of glochids and the fact that germination is extremely slow and germination percentage low. Hypocotyl and root elongation and epicotyl development are rapid for two weeks after rupture of the seed coat. However at this point hypocotyl elongation nearly ceases, while stem and root development continue at a reduced rate. An eight-month seedling is usually not more than 25 mm tall but has numerous areoles with spines and occasionally one subtending leaf. At this time tubercles have begun to coalesce into the vertical rows of ribs characteristic of the section Grusonia.     

RELATIONSHIPS AMONG SHOOT APICAL MERISTEM ONTOGENIC FEATURES IN TRICHOCEREUS PACHANOI AND MELOCACTUS MATANZANUS (CACTACEAE)

Various morphometric parameters of shoot apical meristems were measured during the ontogeny of Melocactus matanzanus and Trichocereus pachanoi. While some ontogenic features varied dramatically (i.e., apical surface curvature, dK/dl, varied by 0.15–0.17, while the apical volume increased from 10⁵ μm³ to 10⁷ or 10⁸ μm³), the percent volume of the central-mother-cell zone (CMCZ), pith-rib meristem (PRM), and the peripheral zone (PZ) showed remarkable intraspecific constancy: CMCZ = 9.2%, PRM = 10.4%, PZ = 80% for M. matanzanus, and CMCZ = 10%, PRM = 24.5%, PZ = 65% for T. pachanoi. Seedlings (2 plastochrons old) showed distinctly different percent volumes from older forms: CMCZ = 7.8%, PRM = 5.9%, PZ = 86% for M. matanzanus, and CMCZ = 8.9%, PRM = 13%, PZ = 77% for T. pachanoi. Geometric expressions of cell patterns within zones of the apex remained relatively constant throughout development. On the basis of intra- and interspecific comparisons of the percent volumes among 23 taxa and the chronology in the appearance of zones within the apex, it is concluded that while there are no apparent biological reasons for the actual percent volume values computed from our sample set, the constancy in zones reflects a high degree of homeostasis which is achieved early in ontogeny.     

BRANCHING PATTERN IN ONION ADVENTITIOUS ROOTS

The pattern of topographic distribution of lateral roots (LRs) along the adventitious root of Allium cepa L. was studied in relation to the adventitious root vascular system. In the onion LRs arise in vertical files opposite the protoxylem poles, thus forming protoxylem-based ranks of LRs along the mother root. Two protoxylem-based ranks are shown to support large numbers of LRs, whereas the remaining ranks are considerably less prolific. Within each rank short distances between longitudinal neighbor LRs are uncommon. The results suggest that the developing LR influences the location of subsequent LRs forming in the same protoxylem-based rank. The frequency with which longitudinal neighbor LRs formed in adjacent protoxylem-based ranks was significantly high, whereas their appearance in the same rank or in nonadjacent ranks was more restricted. Similarly, the existence of nonrandom clumps of LRs could be demonstrated at certain levels of the adventitious root. The possible effects of interaction between protoxylem-based ranks of LRs as well as likely agents operating from the parent root vascular system are discussed insofar as they represent factors favoring the clumping of LRs.     

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.     

BRIEF INCUBATION OF GAMETANGIA-BEARING CAPS IN ANTIBIOTICS ELIMINATES BRANCHING IN PROGENY OF ACETABULARIA ACETABULUM (CHLOROPHYTA)1

Branching of the stalk of Acetabularia acetabulum L. (Silva) was investigated by inbreeding and by a brief treatment of gametangia with a variety of antibiotics. The position of the branch along the stalk varied, implying that branching was not restricted to any one time in development (base is oldest and apex is youngest). The branching phenotype was not inherited in Mendelian fashion. Although three microscopic structures (“bubbles,”“pustules,” and “scars”) occurred on the stalks of cells that had branched, these structures were not statistically correlated with branching in the population (n=699 cells). However, brief treatment of gametangia with a new antibiotic mixture did eliminate all macro- and microscopic structures associated with branching of the stalk in the subsequent generation. We could not fulfill Koch's postulates or provide clear evidence for the pathogenic nature of cell branching. Our brief antibiotic treatment of gametangaa of Acetabularia acetabulum was rapid, had no adverse effects, and virtually eliminated branching (and any potential pathogens) from laboratory cultures in the subsequent generations. Our method allows biochemical and molecular analyses to proceed uncomplicated by the possible presence of other organisms and provides a clean baseline for the future selection of mutations that may induce heritable branching.     

THE GENUS PELECYPHORA (CACTACEAE): RESOLUTION OF A CONTROVERSY

The four species considered in this article fall into two natural groups. The evidence presented for this conclusion involves not only external features such as floral, fruit, and seed morphology, but also that obtained from internal structure, including tubercle and areole development, the anatomy of surface layers of mature tubercles, the structure of crystals in the ground tissues, and features of tracheary elements. When all evidence is considered, it becomes apparent that Pelecyphora aselliformis and Encephalocarpus strobiliformis are cogeneric as are P. valdeziana and P. pseudopectinata. Yet, the two cogeneric pairs are not closely related to each other. We do not propose to place them in any phylogenetic scheme as yet because general knowledge of the family Cactaceae is still too fragmentary. The first pair is assigned to Pelecyphora, of which a formal taxonomic treatment is presented, and the second pair is tentatively assigned to Thelocaclus.