ORGANIZATION AND ONTOGENY OF THE VASCULAR SYSTEM IN THE PETIOLE OF EASTERN COTTONWOOD

The topologic arrangement of petiolar bundles varies within the length of the cottonwood petiole. Each petiolar bundle is formed by the subdivision and aggregation of acropetally differentiating subsidiary bundles in a predictable pattern. The subsidiary bundles provide vascular continuity between the stem and specific portions of the leaf lamina. Spot-labeling of individual veins with ¹⁴CO₂, freeze substitution, and microautoradiography were used to establish the relation between the secondary veins of the lamina and the vasculature of the petiole. Within the petiole vasculature each subsidiary bundle was continuous with a specific portion of the lamina and seemed to have a separate function. Subsidiary bundles continuous with the central leaf trace were closely related functionally to the tip region of the lamina, while the subsidiary bundles continuous with the lateral leaf traces were functionally related to the middle and basal portions of the lamina.     

ONTOGENY OF THE PSEUDOHERMAPHRODITIC REPRODUCTIVE SYSTEM IN NASSARIUS VIBEX (GASTROPODA: BUCCINIDAE: NASSARIINAE)

Pseudohermaphroditism or imposex is an abnormality of the gastropodreproductive system caused by the effects of an environmentalpollutant, tributyltin, in which parts of the male reproductivetract develop in females. In this study, the post-larval developmentof the reproductive system in pseudohermaphroditic male andfemale Nassarius vibex was investigated relative to normal developmentto determine how and when the abnormalities associated withpseudohermaphroditism develop. Two populations of N. vibex were compared:one exhibiting normal development, and one in which the animalswere effected by pseudohermaphroditism. In the affected N. vibexpopulation, a penis and anterior spermiduct developed very earlyin ontogeny in both males and females. Normal reproductive systemdevelopment was affected in some females, evidenced by fusion ofthe induced spermiduct and anterior oviduct, or by abnormaloviduct development. Several males displayed precocious maturationof the gonad in addition to early development of the anteriorspermiduct and penis. (Received 25 April 2000; accepted 1 July 2000)     

ANALYSIS OF REPRODUCTIVE SYSTEM ONTOGENY AND HOMOLOGY IN NASSARIUS VIBEX (GASTROPODA: BUCCINIDAE: NASSARIINAE)

The post-larval development of the reproductive system in Nassariusvibex was investigated using a histological approach, to establishthe sequence of organogenesis and the contribution of differentorganogenetic components to the adult system. This researchis part of a broader investigation with two primary objectives.The first is to provide a detailed analysis of reproductivesystem development that can be used to re-evaluate existingcharacter sets and generate new characters for reconstructing gastropodphylogenies. The reproductive tract has always been a promisingbut problematic system to work with in phylogenetic research,because of the uncertain homology of many parts of the system.Most of the reproductive system itself develops during post-larvalontogeny, a phase of gastropod development that has been investigatedin very few taxa; thus this line of research has the potentialto provide a wealth of new information. The second objective isto establish comparable organogenetic series for representative gastropodtaxa that can be used to investigate the significance of heterochronicprocesses in the evolution of gastropod organ systems. The reproductive system in Nassarius vibex develops very latein ontogeny from two ontogenetic components in the male andthree in the female. Development and differentiation of theorgan system is not completed until near maturity. In both sexes,ductal components develop first and fuse to form the rudimentaryreproductive tract. Gonads and accessory reproductive structuresdevelop after the ductal components are fused, and glandulartissues do not appear until near maturity. The penis does notdevelop until near maturity. The capsule and albumen glandsin this species are broadly conjoined, unlike the situationin other members of Nassarius. The ingesting gland developsbetween them as a dorsal outgrowth of the glandular lumen. (Received 25 April 2000; accepted 1 July 2000)     

CELL WALL CHEMISTRY AND FINE STRUCTURE IN LEPTOIDS OF DENDROLIGOTRICHUM (BRYOPHYTA): THE END WALL

Leptoids (sieve elements) of Dendroligotrichum are characterized by a highly oblique end wall which is composed of cellulose (birefringent; IKI-H₂SO₄-positive), polyuronides (toluidine blue-positive), pectins (hydroxylamine-positive) and natural aldehydes (silver hexamine and silver proteinate-positive). Cytochemically the end wall appears identical to the unevenly thickened lateral wall. Electron cytochemical localization of aldehydes with silver proteinate reveals two distinct wall layers in comparison to the 3-layered lateral wall. Plasmodesmata are present in the end wall with a frequency of 15-20 per μm². A characteristic feature of end wall plasmodesmata is an expanded median cavity which is 0.12-0.15 μm in diameter. Frequently an electron-dense substance, whose chemical nature and origin are unknown, occludes the plasmodesmata.     

FLOATING STOMATES (ADETOSTOMY) IN FERNS: DISTRIBUTION AND ONTOGENY

A unique guard cell apparatus occurs in certain species of Anemia (Schizaeaceae) and in certain species of the unrelated family Polypodiaceae. A guard cell pair is completely surrounded by one epidermal cell, with no attachment to lateral (anticlinal) walls of adjacent epidermal cells. This condition of “floating stomates” is called adetostomy. In rare cases the surrounding epidermal cell is in turn entirely enclosed by another single epidermal cell. The ontogeny of floating stomates has been variously interpreted. This study supports the early view of Rauter that a funnel-shaped guard mother cell is initiated in place entirely within a protodermal cell.     

MICROTUBULE ARMS AND CYTOPLASMIC STREAMING AND MICROTUBULE BENDING AND STRETCHING OF INTERTUBULE LINKS IN THE FEEDING TENTACLE OF THE SUCTORIAN CILIATE TOKOPHRYA

Microtubules attached to the pellicle at the tips of tentacles pivot through about 140° on these attachments, splay apart, and bend along their longitudinal axes when feeding occurs. The tubules could be bending in response to pellicular contractions; active bending, sliding, or contraction of the tubules may not be involved. Intertubule links apparently prevent tubules from splaying apart at certain levels. These links are probably under tension during feeding. They stretch; they sometimes become half as thick and eight times as long as they are before feeding. Often, tubules joined together by these links also change in shape; they become slightly flattened and elliptical in cross section. Cytoplasm from the ciliate Tetrahymena is drawn down a feeding tentacle inside an invagination of the Tokophrya cell membrane from the tentacle tip. The positions of arm-bearing microtubules around such invaginations indicate that arms are involved in moving invaginations along. The edges of the perforated Tetrahymena cell membrane are "sealed" to the cell membrane of Tokophrya around each feeding tentacle tip.     

THE THICKENED LEPTOID (SIEVE ELEMENT) WALL OF DENDROLIGOTRICHUM (BRYOPHYTA): CYTOCHEMISTRY AND FINE STRUCTURE

Leptoids (sieve elements) of Dendroligotrichum exhibit unevenly thickened lateral walls. The thickened wall areas are predominantly confined to the radial walls. With the light microscope the thickened wall cannot be resolved into distinct layers, but rather is optically homogeneous. Standard histochemical tests reveal that these walls are rich in cellulose (birefringent; IKI-H₂SO₄-positive) with small amounts of polyuronides (toluidine blue-positive) and pectins (hydroxylamine-positive) and are non-lignified. They also contain abundant natural aldehydes as revealed by the Schiff, silver hexamine, and silver proteinate reagents. Aldehyde blockades (sodium borohydrate, sodium chlorite) confirmed the presence of aldehyde groups in the cell wall. At the ultrastructural level, the lateral walls of sieve elements react strongly with uranyl and lead salts and yield little fine structural information. Electron cytochemical localization of aldehydes with silver proteinate revealed three distinct wall regions: outer, middle and inner. The outer and middle regions appeared polylamellate while the inner region contained no reaction product. The nacreous sieve elements of vascular plants are compared to the thickened sieve elements in bryophytes.     

THE ONTOGENY OF THE PLEUREMBOLIC PROBOSCIS IN NUCELLA LAPILLUS (GASTROPODA: MURICIDAE)

The ontogeny of the proboscis in Nucclla lapillus was investigatedusing light and scanning electron microscopy. The proboscis develops by elongation of the body wall surroundingthe mouth, whilst the rhynchocoel is formed by imagination ofthe body wall surrounding the proboscis. Elongation of the snoutduring development of the proboscis results in the anteriormovement of the anterior oesophagus and part of the mid-oesophagus(the valve of Leiblein) which is drawn through the circum-oesophagealnerve ring. The acinous salivary glands and the radular sacalso come to lie anterior to the nerve ring. The mid-oesophagealgland of Leiblein and the glandular dorsal folds are not drawnthrough the nerve ring, and develop behind it. The anterioroesophagus elongates at a later stage of development to producethe oesophageal length required for extension of the adult proboscis.Modifications to this sequence of events, or changes in therate of growth of the various parts of the foregut, might accountfor the differences between the neogastropod and neotaenioglossanpleurembolic proboscis. The intraembolic proboscis found inthe Conoidea and the Pseudolivoidea may have been derived viaa modification of the developmental sequence which producesthe muricoidean pleurembolic proboscis. (Received 10 May 1996; accepted 15 August 1996)     

SPINDLES, SPINDLE PLAQUES, AND MEIOSIS IN THE YEAST SACCHAROMYCES CEREVISIAE (HANSEN)

The intranuclear spindle of yeast has an electron-opaque body at each pole. These spindle plaques lie on the nuclear envelope. During mitosis the spindle elongates while the nuclear membranes remain intact. After equatorial constriction there are two daughted nuclei, each with one spindle plaque. The spindle plaque then duplicates so that two side-by-side plaques are produced. These move rapidly apart and rotate so that they bracket a stable 0.8 µm spindle. Later, during mitosis, this spindle elongates, etc. Yeast cells placed on sporulation medium soon enter meiosis. After 4 hr the spindle plaques of the more mature cells duplicate, producing a stable side-by-side arrangement. Subsequently the plaques move apart to bracket a 0.8 µm spindle which immediately starts to elongate. When this meiosis I spindle reaches its maximum length of 3–5 µm, each of the plaques at the poles of the spindle duplicates and the resulting side-by-side plaques increase in size. The nucleus does not divide. The large side-by-side plaques separate and bracket a short spindle of about 1 µm which elongates gradually to 2 or 3 µm. Thus there are two spindles within one nucleus at meiosis II. To the side of each of the four plaques a bulge forms on the nucleus. The four bulges enlarge while the original nucleus shrinks. These four developing ascospore nuclei are partially surrounded by cytoplasm and by a prospore wall which originates from the cytoplasmic side of the spindle plaque. Eventually the spore nuclei pinch off and the spore wall closes. In some of the larger yeast cells this development is completed after 8 hr on sporulation medium.     

EVOLUTION AND MORPHOGENETIC RULES: THE SHAPE OF THE VERTEBRATE LIMB IN ONTOGENY AND PHYLOGENY

The notion of a “developmental constraint” has become a catchphrase for a collection of poorly defined notions about how ontogeny affects phylogeny. In this paper, we shall attempt to define this idea more precisely by examining the vertebrate limb from three viewpoints. First, theoretical models of morphogenesis suggest several generalizations about how limb geometry is laid down during development. Comparative studies and experimental manipulations of developing limbs independently confirm these generalizations, which amount to a set of “construction rules” for determining how the major features of limb architecture are established in ontogeny. Armed with these rules, we can inquire how limb morphology can be varied during evolution and suggest a more precise operational definition of “developmental constraints” on morphological evolution.     

INHERITANCE AND MEIOTIC ELIMINATION OF A VIRUS GENOME IN THE HOST ECTOCARPUS SZLZCULOSUS (PHAEOPHYCEAE)1

The marine brown alga Ectocarpus siliculosus (Dillwyn) Lyngbye is frequently infected by a latent DNA virus that multiplies in modified sporangia and gametangia of the host. We describe a polymerase chain reaction (PCR) procedure for the amplification and detection of viral DNA in Ectocarpus. PCR analysis of parents and progeny plants confirmed that virus DNA passes through meiosis like a Mendelian trait. An infected sporophyte produced equal numbers of gametophytes with and without the viral genome. Thus, meiosis in sexual populations of the host acts as a mechanism for the creation of virus-free progeny.     

ONTOGENY AND STRUCTURE OF THE SECONDARY PHLOEM IN EPHEDRA

The secondary phloem in Ephedra is atypical of the gymnosperms in general and exhibits several angiosperm-like characteristics. The ray system of the conducting phloem consists of parenchymatous, multiseriate rays. The axial system contains parenchyma cells, sieve cells, and unusual albuminous cells reminiscent of the specialized parenchyma cells found in some angiosperms. These cell types may intergrade with each other. P-protein in the developing sieve element appears early in the form of a single, ovoid slime body. Later, smaller slime bodies appear and quickly disperse. In the mature sieve element the single, ovoid slime body is lost, and P-protein is then evident in the form of a parietal cylinder, thread-like strands, amorphose globules, or a slime plug. Necrotic-appearing nuclei are commonly found in mature sieve cells.