摘要:

Anatomical, histochemical, and mechanical studies indicated the presence of a highly modified and weakened stem base inKochia scopariaL. Schrader. This base, the abscission zone, is the site for stem abscission. In autumn progressive desiccation of the plant is accompanied by the gradual loss of stem flexibility and concomitant increase in rigidity. The tissues of the stem remain relatively tough, but abscission zone tissues become very brash or brittle. When conditioned plants are stressed by winds, the stem acts as a moment arm, and large stresses are generated in the abscission zone. Rupture then occurs across the stem base, often abruptly. Strength tests indicated that breakage occurred with 40% less stress if a soil‐inhabiting fungus (Rhizoctoniasp.) had degraded the nonlignified cell wall components of the abscission zone. Abscission, therefore, is caused by the wind, an external driving variable, but tissue desiccation, changes in anatomy, and decay are internal, preparatory variables.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1978.tb06082.x

出版商:Wiley    

年代:1978

数据来源: WILEY

摘要:

Pollen of 48 species ofSchefflerawas studied by light and scanning electron microscopy. Eight pollen types are distinguished on the basis of sexine sculpturing and structure, apertural shape, and shape and size of the grains. These pollen types correlate well with other data such as the number of carpels, the condition of corolla, infloresence types, and geographical distribution of each species. To determine the evolutionary status of sexine structure, an association between sexine characters and the number of carpels was sought. Assuming polymery of floral parts to be a primitive condition in Araliaceae, the undifferentiated sexine, which occurs in the multicarpellate species, also appears to be primitive. Other unspecialized features include short grains, and non‐sculptured, imperforate tectum. Several species in New Guinea and the Solomon Islands belong to this putative ancestral pollen type, from which other types may have been derived.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1978.tb06083.x

出版商:Wiley    

年代:1978

数据来源: WILEY

摘要:

Development of the anther wall was studied with special reference to the tapetum inPyrostegia ignea. The archesporium in each microsporangium is horseshoe‐shaped. The inner tapetum develops earlier from the vegetative cells of the connective region while the outer differentiates a little later from the parietal layers. Thus, the tapetum has a distinct dual origin. The two tapetal layers exhibit a pronounced structural dimorphism. Sometimes, sterile septae, partitioning the sporogenous tissue, develop in microsporangia. A prominent membrane with Ubisch granules (orbicules) is organised on the inner tangential surface of the tapetal protoplasts facing the uninucleate microspores.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1978.tb06084.x

出版商:Wiley    

年代:1978

数据来源: WILEY

摘要:

A developmental study of anther tapetum inTecoma stanshas shown that the hypodermal archesporial layer differentiates in each microsporangium by cutting off a primary parietal layer to the outside (epidermal) and a primary sporogenous layer to the inside (connective). The primary parietal layer divides periclinally, producing the outer secondary parietal layer, which by further divisions, forms the future endothecium and the middle layer. On epidermal side, the inner secondary parietal layer gives rise to tapetum. The remainder of the tapetum on the inside (connective) is contributed by the parenchymatous connective cells lying just outside the sporogenous cells. The tapetum thus follows the dicotyledonous type of ontogeny. It also shows a distinct dual origin and is structurally dimorphic.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1978.tb06085.x

出版商:Wiley    

年代:1978

数据来源: WILEY

摘要:

Aquilegia elegantulaGreene andA. caeruleaJames occur in montane and subalpine habitats in the southern Rocky Mountains of western North America. The red and yellow flowers ofA. elegantulaare nodding, odorless, protogynous, and secrete a concentrated (44%) sucrose nectar in the floral spurs. Seed set in flowers under pollinator exclosures was 12% while seed set in open‐pollinated flowers was 65%. The flowers ofA. elegantulaare pollinated primarily by the Broad‐tailed Hummingbird (Selasphorus platycercus[Swainson]) and by at least three species of pollen‐foraging bumblebees, of whichBombus occidentalisGreene is the most common. The blue and white flowers ofA. caeruleaare erect, mildly fragrant, protandrous, and secrete a 26% sucrose nectar. Seed set in caged flowers in the field averaged 39%. in uncaged flowers 54%. The most important pollinators ofA. caeruleaare the crepuscular hawkmoth,Hyles(=Celerio)lineata(Fabricius) and ten species of pollen‐foragingBombus. The most abundant bumblebee species,B. occidentalis, is also a frequent nectar thief. Differences in pollination systems alone probably do not constitute an effective anti‐hydridization mechanism betweenA. elegantulaandA. caerulea, but do serve to reinforce differences in habitat and flowering time that distinguish the two species.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1978.tb06086.x

出版商:Wiley    

年代:1978

数据来源: WILEY

摘要:

Although the laticifers of several species ofMammillariacan 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.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1978.tb06087.x

出版商:Wiley    

年代:1978

数据来源: WILEY

摘要:

Three clones ofSpirodela polyrhizaL. (Schleid.) formed dormant bodies called turions. A clone from Puerto Rico did not form turions under all conditions tried. In those clones producing turions, formation was stimulated by the addition of sucrose (10–50 mM) to the nutrient solution. Increased levels of Ca(NO3)2plus sucrose stimulated turion production. In the absence of NO3–, Ca++was more effective than K+in stimulating turion formation. Turion buoyancy was not light dependent, nor was it promoted by sucrose. Normal turions required light for germination, whereas sucrose‐induced turions germinated in the dark. Dark germination was not promoted by either Ca++or K+. Sucrose stimulation of turion formation and subsequent promotion of dark germination was attributed to metabolic rather than osmotic effects. One hundred mM sucrose concentrations inhibited turion buoyancy and germination. Turions formed one primary abscission layer which separated them from the stolon and the mother frond. Subepidermal idioblasts appeared to seal the stolon stump after separation.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1978.tb06088.x

出版商:Wiley    

年代:1978

数据来源: WILEY

摘要:

Laboratory experiments were performed to evaluate observed seasonal changes in leaf morphology of the desert perennial shrub,Encelia farinosaGray. Plants were grown under low or high conditions of photosynthetically active irradiation, soil water potential (Ψsoil), and leaf temperature (8 different experimental regimes). The relative growth rate, leaf water vapor conductance, leaf water potential, and leaf length were all greater for the high Ψsoilregimes, the largest leaves occurring at low irradiation. High irradiation during growth led to thicker leaves with a higher internal to external leaf area ratio (Ames/A); low Ψsoiltended to increase Ames/A somewhat. High irradiation also led to decreased absorptance to solar irradiation caused by increased pubescence. High leaf temperature during development resulted in slightly smaller, thicker leaves with higher Ames/A. Thus, irradiation appeared to have its major influence on leaf thickness, Ames/A, and absorptance, with a secondary effect on leaf length; Ψsoilaffected primarily leaf length, growth rate, and water status, and secondarily Ames/A. Results are discussed with regard to recent ecophysiological studies on the observed seasonal changes in leaf morphology ofE. farinosa.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1978.tb06089.x

出版商:Wiley    

年代:1978

数据来源: WILEY

摘要:

The mature spermatozoid ofLycopodium cernuumis a blunt ended, fusiform cell, 8–10 μm long by 4–5 μm wide. A multilayered structure (MLS) and a subtending anterior mitochondrion are located at the anterior of the cell. The MLS is coiled through 1–1.5 gyres in a shallow sinistral helix around the periphery of the cell. The MLS would be triangular in outline if unwound and laid flat, about 1.4 μm wide, 7.5–8 μm long, and 80 nm thick. The MLS comprises four layers, S1–S4. The S1forms the spline, a supportive sheet of microtubules; the S2, lamellate in younger stages, is an homogeneous, darkly staining layer in the mature sperm; the S3and S4retain their lamellate appearance and are delimited by lateral connections. Approximately 200 S1microtubules extend posteriorly from the MLS at about 45° to the MLS long axis and form a partial sheath around the nucleus. The two basal bodies are located on opposite sides of the cell external to the MLS. Each is tangential to the curve of the MLS and surrounded by a globular matrix. At their attachment, the axonemes are oriented laterally and are antiparallel to each other. Distally, the flagella, each about 38 μm long, trail behind the cell as it swims. The nucleus is roughly ovoid, about 4 μm diam, and centrally or sometimes laterally located. The greater volume of the nucleus is occupied by condensed, amorphic chromatin. Cavities within the chromatin are often seen to contain spheroidal inclusions that have two differently staining regions. The inclusions are also located at the periphery of the chromatin. The posterior of the cell is occupied by several small mitochondria and an amyloplast, about 2 μm diam containing numerous starch grains.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1978.tb06090.x

出版商:Wiley    

年代:1978

数据来源: WILEY

摘要:

The pistillate flowers ofHorsfieldiaare morphologically similar to those ofMyristicaandKnema, and are composed of a single whorl of thick, fleshy tepals, and an unsealed, monocarpellate pistil bearing a single ovule. The carpel is vascularized by two ventral bundles, a pair of dorsal bundles, and several supernumerary bundles. The ovule vascularization is derived from the supernumerary bundles. Paired dorsal vascular bundles are an uncommon feature of uncertain significance. Carpelsof MyristicaandKnemalack any clearly defined dorsal vasculature, and the ovule vascular supply is derived from both the ventral and supernumerary bundles. The organization of the staminate flowers ofHorsfieldiaagrees with the myristicaceous pattern observed inMyristicaandKnema. Each androecium consists of a single whorl of anthers fused or partially fused to a massive connective column. Each anther consists of a pair of bisporangiate lobes and a single vascular bundle. The androecial forms observed are interpreted as forming a series of intermediates between the monadelphous type of androecia of two South American genera,CompsoneuraandDialyanthera, and one African genus,Brochneura, and the solid, columnar androecia which are predominate in the family. Accumulating evidence supports a proposed South American or west Gondwanaland origin of the Myristicaceae.

ISSN:0002-9122    

DOI:10.1002/j.1537-2197.1978.tb06091.x

出版商:Wiley    

年代:1978

数据来源: WILEY