摘要:

In this unique and comprehensive article, Dr. Cusset presents a historical review of ideas and theories on leaf development, a detailed description and critique of the classical model, a discussion of Jeune's model, and finally his own synthetic model.According to Cusset, the classical model, which has been widely adopted in textbooks, reference books, and research, postulates that (i) leaf morphogenesis results from the successive activity of apical, marginal (or submarginal), and intercalary meristems; (ii) cell files arise from submarginal initials; (iii) the cell is the fundamental unit of morphogenesis. Cusset documents the shortcomings of these assumptions by discussing evidence from publications that deal with modem techniques ranging from the quantitative analysis of mitotic activity and chimeras to marker experiments and clonal analysis. All of these different approaches have led to the same conclusion: for nearly all taxa studied so far, there is no evidence, either in terms of mitotic activity (e.g., mitotic index) or in a functional sense, of apical or marginal meristems. This means that apical or marginal meristems cannot account for the elongation and lateral extension of the leaf blade. Rather, the available data support the following conclusions.(i) The young leaf primordium is fully meristematic, i.e., there is generalized mitotic activity throughout the primordium. Gradually the mitotic activity is restricted to the base and thus the leaf develops through the activity of an intercalary meristem. Both the orientation and the frequency of cell divisions indicate that the leaf margin plays only a minor role in the growth of the lamina. In only a very few cases (e.g., the petals ofNicotiana tabacum), and then in only relatively late developmental stages, has a maximal mitotic index been observed directly at the margin. More commonly, a peak of mitotic activity has been noted in a submarginal region. However, this region is at a considerable distance from the margin, and the increased mitotic activity, which is often not statistically significant, occurs in only some developmental stages. In summary, one may conclude that instead of distinct meristematic entities, the developing leaf shows a patterned continuum of meristematic activity.(ii) The so-called submarginal initials contribute relatively little to the lateral extension of the leaf blade. Hence, it is questionable whether they should be called "initials."(iii) In general, cells do not appear to be the fundamental units of morphogenesis. As pointed out long ago by De Bary: "the plant forms cells, not cells the plant," thus underscoring the need for a more holistic approach.In addition to the preceding conclusions, Jeune's model (developed in Cusset's laboratory) incorporates the notion of growth centres ("centres générateurs"). These are defined as fields with certain physiological properties that are transmitted to other areas of the developing leaf. Each leaf primordium has two growth centres. In compound or lobed leaves, they are responsible for the production of the lateral elements according to the following rules, (i) Each primordium of a leaflet or lobe arises at a fixed distance from the preceding one. (ii) The rhythm of their formation is constant with regard to the plastochron on the axis where the leaf is bom. (iii) As a consequence, the correlation between the number of lateral elements and the logarithm of the length of the blade primordium is linear (which confirms the exponential growth of the primordium). (iv) After the inception of a lateral element, the growth centre that gave rise to it is reconstituted. If, however, the growth centre is removed microsurgically on one side of the leaf primordium, no lateral elements are formed there. The concept of centre générateur closely corresponds to the idea of a growth centresensuWardlaw. Although Jeune's model has been confirmed for a great variety of leaves, both simple and compound, it does not fully apply in all cases, as for example in the leaves ofCastanea sativaandFraxinus excelsior. To cope with these exceptions, as well as other situations, Cusset proposes a synthetic model in which leaf blade development is modulated by the following eight internal effectors: (i) biochemical oscillations, according to the "brusselator," a model directly derived from ideas of Turing and Prigogine; (ii) the calmodulin–Ca2+balance; (iii) a distal phyllopodial organizer, based on microsurgical evidence, but rather hypothetical; (iv) a system orienting the major veins according to the model of Ferré and Le Guyader; (v) interactions between the flux of inductive substances explaining the minor venation according to Mitchison's model; (vi) a microtubule orienting mechanism which might be an internal electromagnetic phenomenon; (vii) an organizing mechanism that eventually explains the particularities of the minor venation of each species; (viii) auxin acting in a complex fashion on several of the preceding effectors. External factors, such as temperature interact with the above internal mechanisms.Although some aspects of the above synthetic model remain rather hypothetical and speculative at the present time, Cusset's proposal may serve as a stimulating working hypothesis. Furthermore, it emphasizes that the study of leaf morphogenesis not only is a technical and morphological problem of limited interest, but also involves aspects of molecular biology and fundamental theoretical and philosopical issues. From this point of view, leaf morphogenesis may serve as a model case for the discussion and elucidation of contemporary morphogenetic and biological problems.

ISSN:0008-4026    

DOI:10.1139/b86-375

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

Mode ofFusarium solanipenetration into rough lemon citrus rootstock seedling fibrous roots was determined and root symptomatology was evaluated using light, scanning, and transmission electron microscopy. Roots contained limited subepidermal infection 15 h after inoculation. Invasion of epidermal cells was principally through broken epidermal and root hair cell walls, but direct penetration also occurred. Cortical infection was primarily intracellular. Severe root rot was present 36–60 h after inoculation and in most cases infection after 60 h resulted in disruption of the phloem and endodermis around the still relatively intact primary xylem. Naturally infected roots from field trees were collected in an advanced stage of rot. In these,F.solanidigestion of middle lamellae of primary xylem was common, and infection and erosion of secondary vessel element walls occurred frequently.

ISSN:0008-4026    

DOI:10.1139/b86-376

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

Xylem always develops within xylem fascicles, and the phloem either comprises phloem fascicles or is dispersed. Xylem and phloem fascicles may be encircled by a sheath of cells different from cells of the interfascicular tissue. The interfascicular tissue is either homogeneous, i.e., consisting of fibers, or heterogeneous, i.e., formed of fiber aggregates and a parenchymatous and (or) sclerenchymatous intrusive network. This network is interpreted as modified pith. A true pith is often present, and four kinds of true pith are recognized.

ISSN:0008-4026    

DOI:10.1139/b86-377

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

Fruit infection byXanthomonas campestrispv.vesicatoria(Doidge 1920) Dye 1978 was monitored by scanning electron microscopy and by bacterial counts. Possible sites for bacterial penetration were through dead flowers and by proliferation of bacteria in the wart (small protuberance) area. Bacterial multiplication was observed in all warts of young, mature, and ripened fruits. Bacterial cells were bound to the fruit surface by fibrillar material. On the fruit surface, bacteria multiplied in small aggregates, submerged in slime. The slime consisted mainly of sucrose units. At later stages of disease development, the slime covered the entire fruit surface in young fruits. Typical scab symptoms appeared only in leaves of inoculated plants, whereas buds, flowers, and fruits of various sizes were symptomless, but later shed; shedding was strongest in young buds. In more mature and ripened fruits, bacterial numbers decreased and there was less shedding.Xanthomonas campestrispv.vesicatoriacould be detected, in enrichment culture, in low numbers at the seed site (ovary) in inoculated fruits that did not shed.

ISSN:0008-4026    

DOI:10.1139/b86-378

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

The seasonal patterns of biomass allocation in flowering and nonflowering specimens ofPinguicula alpinaL.,P.villosaL., andP.vulgarisL. were compared in a subarctic environment.Pinguicula alpinahad a large perennial root system, comprising 50% of the nonreproductive biomass. The other two species had smaller, annual roots, representing 5–10% of their biomass. Nonflowering specimens usually had winter buds about twice as large as flowering specimens,P.vulgarisvaried, however, with site in this respect. FloweringP.alpinaallocated approximately equal proportions of biomass to reproduction (flower stalk, capsule, and seeds) and over-wintering organs (bud and roots), whileP.villosa's investment in reproduction was about five times that invested in the winter bud. ForP.vulgaris, no correlation was found between the size of reproductive organs and the size of the bud. The number of seeds produced per capsule varied from 30 inP.villosato 110–140 in the other two species. Seed weight was lower inP.alpina(15 μg/seed) than inP.vulgaris(24μg) andP.villosa(28μg). Plant biomass and reproductive effort varied between sites forP.vulgaris; however, this variation was apparently not related to differences in nutrient availability between sites.

ISSN:0008-4026    

DOI:10.1139/b86-379

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

Systematic early reduction of two-thirds of the surface of each leaf on the main stem in soya results in (i) a decrease, on the main axis, in the number of leaves formed and the total number of pods, with no change in the rate of phyllogenesis; and (ii) a stimulation of the development of axillary branches which compensate the foliar and reproductive deficit of the main stem through their growth, their leaf surface area, and the number of pods they form. For given growth conditions, it was shown that, to a certain extent, global production (vegetative and reproductive) is maintained at a constant level by compensation processes between the main stem and the branches. The existence of such relations between compartments has regulatory importance for the development of the whole plant. [Translated by the Journal]

ISSN:0008-4026    

DOI:10.1139/b86-380

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

Protein bodies were characterized at an early stage of their maturation in thin green cotyledons of developing radish embryos. They appeared as granules in the cytoplasm of meristematic cells. Their diameter (0.5 to 1 μm) was in the range of that of mitochondria. They were distinguished from vacuoles by their morphology, size, and structure. Some appeared to be associated with cisternae of endoplasmic reticulum or dictyosomes (permanganic fixations). Their evolution was synchronous in the cell and also in the cotyledonary tissue. Compact in appearance, they were constituted of thin packed fibrillar structures, limited by a denser peripheral layer. It is difficult to know whether or not they had a limiting membrane. Some possessed thin dense crystals or globoids (aldehydic fixations). During this early phase, several types of organelles seemed to contribute to the development of protein bodies, including saccules of rough endoplasmic reticulum and dictyosomes. Polyribosomes constituted a network around their surface. They did not exhibit any acid phosphatase activity. In this respect, they differed from the vacuoles, saccules of endoplasmic reticulum, and several neighbouring vesicles.

ISSN:0008-4026    

DOI:10.1139/b86-381

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

A microcomputer coupled to a video digitizer was used to develop methods for measuring areas of tree rings and for delineating and measuring microscopic anatomical features. Digitally imaged areas of individual rings of a loblolly pine cross section were within 2.6% of areas measured with a planimeter and were more accurate than areas calculated from radius widths. Various anatomical features were "mapped" within digitized images of consecutive rings, individual rings, and highly magnified parts of the rings of green ash. Fibers within various ring zones, including those known to have formed when trees were flooded, were readily differentiated on the basis of image brightness. The image-determined ratio of lumen to wall material within different groups of fibers was considered a measure of structural comparability. These techniques thus provide a rapid, accurate method to study relationships between tree rings and factors affecting their development. Methods could be improved by increasing both spatial and brightness resolution.

ISSN:0008-4026    

DOI:10.1139/b86-382

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

As compared with the cell walls of anthracnose-sensitive beans, the cell walls of resistant beans, with the involvement of Cornell gene, adsorb, when cultivatedin vitro, larger quantities of the endopolygalacturonase secreted by the fungus responsible for the disease,Colleotrichum lindemuthianum. This selective adsorption only occurs for the enzyme excreted by the pathogenic fungus. This phenomenon is observed with cell walls from which the constituant glycoproteins inhibitive for the endopolygalacturonase were previously removed. The adsorbing capacity of the two types of cell walls was reduced by a heat treatment (100 °C, 30 min) and the differences between both types of walls for the selective binding with the endopolygalacturonase ofC.lindemuthianumare reduced. This suggests that protein(s) are involved in this specific adsorption. Solubilisation of the pectic substances of the walls by an enzymatic treatment strongly reduced the binding capacity of the walls for the endopolygalacturonase. It appears therefore that ionic bindings with the pectic polymers are also involved. The possible implication of this phenomenon in the resistance to anthracnose is discussed. [Translated by the Journal]

ISSN:0008-4026    

DOI:10.1139/b86-383

出版商:NRC Research Press    

年代:1986

数据来源: NRC

摘要:

The infection of leaf and stem tissues ofPhaseolusbySclerotinia sclerotiorumwas studied using electron microscopy. Direct entry of the intact plant surface was achieved by hyphae from simple or compound appressoria. A closely appressed hyphal tip developed an electron-lucent region at the apex, in which a plasmalemmal invagination formed. This invagination surrounded a region of extracytoplasmic vesicular material which penetrated the cuticle during invasion. Penetration occurred by a narrow infection peg which passed through the cuticle via a narrow pore and caused little change in cuticle integrity. Once the cuticle was breached, a walled subcuticular vesicle developed from which infection hyphae spread laterally. Destruction of the epidermal cell walls was both rapid and extensive. Leaf and stem colonization occurred by intercellular growth of undifferentiated hyphae accompanied by cellular necrosis in advance of the mycelial front.

ISSN:0008-4026    

DOI:10.1139/b86-384

出版商:NRC Research Press    

年代:1986

数据来源: NRC