North Temperate Regions Research Articles

EMBRYOLOGY OF CALYPSO BULBOSA. I. OVULE DEVELOPMENT

Calypso bulbosa is a terrestrial orchid that grows in north temperate regions. Like many orchids, the Calypso has ovules that are not fully developed at anthesis. After pollination, the ovule primordia divide several times to produce a nucellar filament which consists of five to six cells. The subterminal cell of the nucellar filament enlarges to become the archesporial cell. Through further enlargement and elongation, the archesporial cell becomes the megasporocyte. An unequal dyad results from the first meiotic division. A triad of one active chalazal megaspore and two inactive micropylar megaspores are the end products of meiotic division. Callose is present in the cell wall of the megaspore destined to degenerate. In the mature embryo sac the number of nuclei is reduced to six when the chalazal nuclei fail to divide after the first mitotic division. The chalazal nuclei join the polar nucleus and the male nucleus near the center of the embryo sac subsequent to fertilization.

The rust flora of Sino-Himalayan forests

From 1975–1980, an investigation of rusts of the Sino-Himalayan flora was undertaken by an interdisciplinary research team of the Academia Sinica. The Sino-Himalayan plateau (Xizang) is a varied and unique physicogeographical region with combined effects of high altitude, vast expanse, and latitudinal position. A special category of montane (1000–6000 m) and its forest rust flora is proposed. This preliminary study of the hosts and their rusts includes 110 species and 26 genera in five families of rusts. According to biogeographical and ecological analysis, the Sino-Himalayan rust flora can be characterized as north temperate. This flora is classified into three zones: (i) the plateau zone; (ii) the temperate zone; and (iii) the subtropical zone. A distinct group of rust genera and species is found in each zone. The main genera and species recognized as Sino-Himalayan are also cosmopolitan in north temperate plateau regions. Sino-Himalaya rust flora mainly is a branch of East Asia flora and has close relationships with adjacent flora as well as with North American flora. This flora constitutes a potential source of rust diseases for forests in this and other regions.

Limnological imbalances: an antipodean viewpoint

SUMMARY. 1. It is argued that modern limnology is excessively concerned with the study of fresh waters in the northern temperate region. Limnology has been and is unbalanced in its interests and emphases and, as a result, many widely held limnological concepts need revision.2. The genesis of the argument is outlined. It involved the initial assumption that Australia was limnologically distinctive. Later, it was realized that since most of the world is arid, semi‐arid, tropical or semi‐tropical, Australia was much less distinctive than originally thought.3. Other limnological assumptions considered in the paper concern saline lakes and salinization, the importance and nature of temporary bodies of standing fresh water, biological diversity in tropical fresh waters, the evolutionary importance of permanent freshwater lakes, and the global applicability of the River Continuum Concept. Consideration suggests that some widely‐held views on these matters are erroneous.4. Thus, saline lakes are more important (scientifically, commercially), widespread and numerous environments than is generally thought. Salinization is a significant environmental hazard for many rivers in semiarid regions. Temporary bodies of standing fresh water are geographically widespread and abundant aquatic environments with high species diversity, and in which many species have restricted distributions and low dispersal abilities. Biological diversity appears to be low in tropical fresh waters. Permanent freshwater lakes are perhaps less important loci for the evolution of the freshwater biota than generally thought, temporary fresh waters more so. Finally, the River Continuum does not seem to be applicable to many running waters outside the north temperate region.5. A number of suggestions are made of how a better balance in limnology may be arrived at: how a more globally comprehensive intellectual framework may be built. Suggestions include: an increased awareness of the frequently different natures of waters within and beyond the northern temperate region; recognition of the potential benefits to be gained from the study of waters outside the northern temperate region;

Helminths of stocked rainbow trout (Salmo gairdneri) with special reference to Clinostomum complanatum.

Rainbow trout (Salmo gairdneri) stocked into a small prairie dug-out were examined for helminths at harvest time. Five species of helminths were found (Diplostomum sp., Diplostomulum scheuringi, Clinostomum complanatum, Crepidostomum farionis and Pomphorhynchus bulbocolli); C. farionis and P. bulbocolli mature in fish while the remaining species utilize fish-eating birds as definitive hosts. Examination of stomach contents indicated that all invertebrate hosts required for the life-cycles of these helminths were present in the dug-out. The most conspicuous and prevalent helminth was C. complanatum as fish were "wormy" and unmarketable due to the presence of high numbers of metacercariae. Metacercariae of C. complanatum were recovered from most organs of rainbow trout. The low survival of stocked fish suggests that C. complanatum may cause some host mortality, but the condition of infected and uninfected fish was similar. It appears that a community of ichthyoparasites can become established in a population of stocked fish in a single growing season in north temperate regions.

The epidemiology of bovine ostertagiasis in the north temperate regions of north America

Ostertagia ostertagi is widely distributed and is the most pathogenic of the parasitic nematodes affecting cattle in this region. Clinical ostertagiasis is seen mainly in calves and yearlings but outbreaks tend to be sporadic; the subclinical disease is of greater importance. Studies on the population dynamics of the free-living stages have shown that infective larvae can survive on pasture over the relatively severe winter conditions encountered in this region but that such pasture contamination declines during the succeeding grazing season and is lost by midsummer. In the host there is gradual acquisition of worms from pasture over the summer period, with relatively high burdens accumulating in the autumn. A dramatic shift in the proportion of adult to immature worms occurs during the autumn (October). By the time calves are housed, the proportion of larvae (mainly L 4) is > 80%. This relative distribution of adults to larvae continues through the winter months until early spring when there appears to be a shift to a higher proportion of adults, presumably due to resumption of development of the L 4. These persistent L 4 stages are considered to be undergoing hypobiosis. Spring infections of calves appear to be mainly acquired from larvae that have successfully overwintered on pasture, and availability of these larvae is drastecally reduced by midsummer. There appears to be negligible development of the eggs that result from the spring infections until late summer and early autumn. At this time because of favorable climatic conditions there is rapid larval development which can result in heavy infections in susceptible calves in the autumn. By late autumn a high proportion of the larvae that are ingested show hypobiosis and remain in this state during the winter months. In spring there is gradual resumption of development but rarely do we see the dramatic Type II ostertagiasis that has been reported in Europe. In this region it appears that there are probably not more than two generations of O. ostertagi during the grazing period: one acquired from overwintered larvae or larvae from associated carrier calves and a subsequent one from the offspring of worms that develop from these larvae.

Adaptive Radiation of Salamanders in Middle American Cloud Forests

Adaptive Radiation of Salamanders in Middle American Cloud Forests

Germplasm enhancement with potato haploids

Haploid-species hybrids were produced from crosses between 40 Solanum tuberosum Group Tuberosum haploids (2n= 2x = 24) as females and wild 2n = 2x = 24 Solanum species S. berthaultii, S. bollviense, S. canasense, S. infundibuliforme, S. microdontum, S. raphanifolium, S. sanctee-rosae, and S. tarijense as males. Several important results were obtained: variation for tuberization in the hybrids was striking and discrete, with plants that did not tuberize adjacent to full sibs with large, smooth tubers; tuber yield in some hybrids significantly exceeded that of the parents—1.5 kg/hill compared to 0.5 kg/hill in the haploid parents and no tubers produced by the species parents; haploids derived from the same tetraploid parent differed greatly in their ability to produce hybrid families with good tuberization; the species parents also influenced tuberization in hybrid progeny—the particular genotypes of S. berthaultii and S. tarijense were the best species parents. Wild potato species contain valuable traits and the allelic diversity necessary to increase heterozygosity, but they do not tuberize in the North Temperate region. Therefore, hybridization with haploids is a valuable method of putting this germplasm into a form that tuberizes, for ease of evaluation and maintenance. Haptoid-species hybrids that produce 2n gametes provide the basis for an effective method of transferring germplasm from the 2x level to tetraploid cultivars. They also provide the essential materials for breeding schemes involving the production of 4x progeny from 4x × 2x or 2x × 2x crosses.

Effects of temperature on the development, growth, and survival of larvae and pupae of a north-temperate chrysomelid beetle.

Developoment, growth, and survival of larvae and pupae of the red turnip beetle, Entomoscelis americana Brown, were studied in 10 constant and four alternating temperature regimes (10 to 32.5° C), in field-cages, and in natural populations in Manitoba. This beetle has a northtemperate distribution in North America. Larval and pupal development occurs in spring and normally is completed before the end of June. Growth and development occurred at all constant temperatures tested, but survival was low at the extreme temperatures. Therefore, the threshold and upper limit were near 10 and 32.5° C. The developmental times of the sexes did not differ and decreased with temperature, except possibly at 32.5° C. The average weight of adult females increased with temperature up to 32.5° C and those of males up to 25° C. Considering developmental rate, survival, adult weight, and incidence of malformed adults, the optimum temperature was estimated to be near 27.5° C.Development was accelerated significantly (6 to 9%) in alternating regimes with temperatures differing by 10° C, but not in regimes differing by 5 and 15° C. All alternating regimes increased adult weight, 5 to 17% for females and 2 to 10% for males. Field cage studies confirmed the increase in adult weight, but not the acceleration in development.A three-parameter normal function described accurately the relationship between developmental rate and constant temperature. A computer simulation model based on this equation estimated developmental times in field cages to within one to five days. For natural populations the model overestimated the developmental times by five to 16 days. The discrepancies between model estimates and observed developmental times in natural populations apparently were due to the elevation of larval and pupal body temperatures above air temperatures by behavioral thermoregulation. The elevation of body temperature was estimated to be equivalent to the addition of 5 to 6° C to the maximum daily air temperature. The adaptations and responses of this beetle to the cool spring temperatures of the north-temperate region are discussed.

Biological consequences of ice rafting in a New England salt marsh community

Ice rafting of salt marsh peat is a recurrent phenomenon in north temperate regions. This process was simulated in a northern New England salt marsh to test several hypotheses concerning the effects of peat transport from high to low intertidal heights on the growth and mortality of key sessile organisms: the ribbed mussel Geukensia demissa (Dillwyn), the fucoid alga Fucus vesiculosus L. var. spiralis (Farlow) and the cordgrass Spartina alterniflora (Loisel.). Growth rates increased when Geukensia and Fucus were transported to the lower intertidal; however, Spartina died when similarly transported. Predation pressure (primarily from Carcinus maenus L.) on Geukensia was greater when it was rafted to the lower intertidal zone than in the upper intertidal habitat and was size specific; mussels >3.5cm reached a size-escape from crab predation. A winter survey of dislodged mussels revealed that 72% of the mussels collected were dead and 86% had been overgrown by large Fucus plants, >2.5 × the natural frequency of Fucus overgrowth (32%). In marsh habitats where hard substratum is rare, 91% of the Fucus were growing on Geukensia. A dislodgement experiment showed that a significantly greater percentage of Geukensia was dislodged after ice-out when Fucus was attached to the shell than those mussels without Fucus overgrowth. In the spring, a population survey conducted in the salt marsh examined densities, biomass and population structure of Geukensia, as well as densities, percent cover and biomass of Fucus. Values obtained in the foremarsh were compared to those from the peat islands recently rafted to the tidal flats. Both biomass and densities of Geukensia were similar in the two areas; however, the size-frequency distributions of the mussels were different. Since fewer large mussels, Fucus and Fucus-overgrown mussels were found on the newly transported peat islands, this pattern appears to reflect dislodgement of larger Geukensia by attached algae during ice transport. Two ice-related sources of mortality were identified for Geukensia: (1) Fucus overgrowth acted as a vector for mussel dislodgement and was an indirect source of mortality; and (2) ice crushing was a direct source of mortality for non-overgrown mussels.

Structure in temperate predaceous diving beetle communities (Coleoptera: Dytiscidae)

Collection records of adult dytiscid beetles (Coleoptera, Dytiscidae) from 312 sites within the province of Alberta, Canada, were analyzed for patterns of similarity in species distribution. Sites compared on the basis of species occurrence (Jaccard coefficient of similarity and group average cluster analysis) separated into 12 principal clusters, interpreted as communities. However, cluster inclusion occurred at very low levels of similarity and 30% of sites were unclustered outliers. Ecological charade us tics of sites that were recognized as varying between dusters were salinity, productivity, stability, water temperature, substrate type, flow and vegetation. An index relating these qualitatively defined variables to species numbers is presented.An analysis of co‐occurrence patterns of species was conducted as an alternative approach to definition of multispecies associations. Each possible combination of co‐occurrence between pairs of species occurring in more than 5 collections each was tested for significance (p = 0.005) by means of Yates corrected χ2 tests. Results of tests were expressed in binary terms, blank – no significance, x ‐ significant. Species were then clustered on the basis of patterns of significant co‐occurrences as above. This analysis was used as the basis for ordination of species in a symmetrical matrix with species ordered so as to maximize density of points of significant co‐occurrence (x) around the principal diagonal. Areas in the matrix showing high densities of co‐occurrence were interpreted as representing communities.The cluster analysis of species produced a better cluster pattern than obtained for sites but while generally similar, the species clustering pattern was less fine. The principal species groups consisted of those of saline water, alpine/subalpine lotic sites, other lotic habitats, and lentic habitats with two subgroups, species in forest areas and grassland species. The ordination, while reflecting this grouping of species, demonstrated that each group graded into others through loss of certain species and acquisition of others and thus showed the continuum nature of these communities. Restricting ordination to congeneric species indicated that these do not usually show identical patterns of co‐occurrence but neither are patterns coincident with mutual exclusion. Within genera, considerable overlap in occurrence between species is the norm but species tend to occupy slightly different positions relative to one another along qualitatively defined ecological gradients. Relative position of species in these ordinations shows good correlation with patterns of geographical distribution lending support to the interpretation of ordination patterns reflecting relative ecological segregation. The dytiscid beetle fauna of north temperate regions is especially rich. Also, the large number of significant co‐occurrences observed between species shows that species packing in many habitats of this region is dense. It is speculated that this species richness is possible because of the seasonality of habitats at these latitudes which reduces competition and predation from other groups and produces seasonal pulses of high productivity permitting a species rich fauna to develop because of abundant resources. It is likely that factors other than interspecific competition for resources are important in shaping dytiscid communities in habitats with strong nutrient pulses.

Special Report: The Present Evolutionary and Taxonomic Status of the Fern Genus Polystichum: The 1984 Botanical Society of America Pteridophyte Section Symposium

The fern genus Polystichum has presented major problems in definition and circumscription of species since its description by Roth in 1799. Part of the problem is the vast diversity within the genus: the number of species is reported as more than 175 by Copeland (1947). In addition, hybridization is extremely common (Knobloch, 1976), and agamospory has been reported among experimental plants from Europe (Vida & Reichstein, 1975). There is also substantial evidence of phenotypic and ontogenetic variability within species of the genus. Good progress has been made in solving evolutionary and taxonomic problems in the north-temperate and boreal regions (Manton, 1950; Manton & Reichstein, 1961; Kurata, 1964; Sleep & Reichstein, 1967; Daigobo, 1972; W. Wagner, 1973; D. Wagner, 1979); however, little taxonomic and virtually no evolutionary work has been done on the genus in tropical or austral regions. Hence this large genus of dryopteroid ferns is in need of substantial attention from systematic and evolutionary biologists, especially in tropical regions. Christopher Haufler at the University of Kansas organized a symposium on Polystichum for the American Institute of Biological Sciences meetings in Fort Collins, Colorado, during August of 1984. As Haufler noted in his opening remarks, the purpose of the symposium was to draw attention to a complex and poorly understood genus of ferns, rather than to solve problems by consensus of the speakers. Participants in the symposium focused on the evolutionary problems encountered in New World Polystichum, primarily from a taxonomic and floristic viewpoint. This paper summarizes the Fort Collins symposium.

Rhytisma Acerinum . [Descriptions of Fungi and Bacteria

Abstract A description is provided for Rhytisma Acerinum . Information is included on the disease caused by the organism, its transmission, geographical distribution, and hosts. HOSTS: Many Acer spp., particularly A. pseudoplatanus in Europe and A. rubrum in North America. DISEASE: Causes tar spot of Acer spp. Stromata appear on living leaves during the summer, and the ascomata mature the following spring on the fragile decomposed fallen leaves. It apparently does not cause serious damage to the tree, but the black stromata are unsightly on ornamental trees. Many species of Acer are of horticultural importance. The disease can be controlled using copper and dithiocarbamate preparations (60, 2789), though burning of fallen leaves in autumn is simple and effective. In Italy, incidence of the disease is lower after a cold dry winter than a warm wet one (58, 5032). GEOGRAPHICAL DISTRIBUTION: Very widely distributed in north temperate regions. TRANSMISSION: Unknown: probably through ascospore dispersal into the air and direct infection of the leaves in humid weather.

Late glacial and late Holocene moraines in the Cerros Cuchpanga, central Peru

Small ice fields on the western cordillera northeast of Lima were expanded to three times their present size in the recent past, and the regional snow line was probably about 100 m lower than it is today. Outwash from the expanded glaciers formed deltas of silt in valley-bottom lakes. When the ice lobes retreated, the reduced outwash was trapped behind recessional moraines, and the clear meltwater infiltrated into the limestone bedrock and emerged at the heads of the deltas in spring pools. The delta surfaces then became covered with peat, and radiocarbon dates for the base of the peat (1100 ± 70 and 430 ± 70 yr B.P. for two different deltas) indicate that the maximum ice advance was older than those dates and, thus, older than the Little Ice Age of many north-temperate regions. Much older moraines date from expansion of the same local summit glaciers to even lower levels in the main valleys, which had previously been inundated by the cordilleran ice field. The cordilleran deglaciation and this expansion of local glaciers probably occurred between 12,000 and 10,000 yr ago, on the basis of slightly contradictory radiocarbon dates.

The Ecology and Sociobiology of Bumble Bees

The bumble bees (genus Bombus Latreille) and their parasites (genus Psithyrus Lepeletier) comprise a group of several hundred species found primarily in north temperate regions. During the past 25 years, no less than 5 books on the general biology of bumble bees have appeared (2,34,57,81,101). Various aspects of bumble bee biology have been covered as part of more general reviews: bees as pollinators (80), the energetics of pollination (54, 61), forag­ ing and mating behavior (3 1), the sociobiology (121,226,227), biogeography (104), and social behavior (105) of bees, and bees as crop pollinators (33, 102). In addition, the bionomic characteristics of the group have been discussed by Sakagami (186). Morse ( l16a) recently reviewed the ecology and behavior of bumble bees and Milliron (108) gave a relatively complete bibliography of bumble bee literature prior to 1962. Our purpose in the present review has been to provide a guide to, and some critical assessment of, part of the literature on the genus Bombus that has appeared during the past decade. We have concentrated primarily on two areas in which research activity has been particularly high: (a) social behavior and (b) evolutionary ecology, within which we have treated foraging behavior exten­ sively, both because of the volume of recent work and because of current interest in bumble bees as components in the ecology of pollination. Limita­ tions of space precluded the incorporation of several important aspects of bumble bee biology, such as sensory physiology, mating behavior, parasites and commensals, biogeography, and biosystematics.

Physio-Ecological Factors Relating to Reproduction and Phenology in Fontinalis dalecarlica

Fontinalis dalecarlica BSG from North Carolina was studied to determine the effects of photoperiod and exposure to air on archegonial production, growth, branching and rhizoids in order to develop a preliminary model for phenology. Mosses were cultured for 16 weeks in four photoperiods at 8C and 1560 lux in artificial streams. The North Carolina population of Fontinalis dalecarlica is a quantitative short day plant and produced archegonia first in the 6 hr light/18 hr dark photopheriod, but in longer photoperiods had produced an equal number by the end of 16 weeks. Long photoperiods favored growth, branching and rhizoids. Reduced growth and branching in the longest photoperiod (18 hr light/6 hr dark) suggests that light inhibition may occur on longer days. Aerated mosses produce more archegonia than submersed mosses. These observations suggest that the moss is adapted to fluctuating water levels by dispersing its gametes while the water level is low. These experiments provide a model for the timing and mechanisms of sperm dispersal that can now be tested in nature. The genus Fontinalis Hedw. has been used fre- quently in physiological studies as a representative aquatic plant (Blackman & Smith 1910; Steemann Nielsen 1947); it has been represented in many pol- lution studies (Frahm 1974; Sodergren 1976; Bur- ton & Peterson 1979); and it is widely distributed in the north temperate region. Its large size and relative abundance further support its choice for detailed biological study. The present study pro- vides a suggested phenological model to serve as a

The family Thelypteridaceae in Europe

Thelypteridaceae are mainly tropical ferns, the total number of species is about one thousand. The five European species represent five different groups, each of which has recently been accorded generic status. Thelypteris palustris Schott (including three varieties) extends throughout north temperate regions, with a closely allied species south of the equator; chromosome numter n=35. Phaegopteris connectilis (Michx) Watt (triploid with base number 30) and Oreopteris limbosperma (All.) Holub (n=34) are members of small genera confined to north temperate regions. Stegnogramma pozoi (Lag.) K. Iwa-cs. (n=36) represents a genus of 12-15 species, mainly in Africa and Asia, with one in Mexico. Christella dentata (Forsk.) Brownsey & Jermy is a wide-ranging and variable tetra- ploid species of a pantropic genus (c. 60 app) the centre of distribution of wich appears to be Burma-Assam (base number 36).
 The genus Cyclosurus (s. str.) comprises a small pantropic group of species with grow (like Thelypteris) in open permanently swampy ground; I belive Cycicsorus and Thelypteris to be closely related genera. Stegno- gramma is related to Sphaerostephanos which (in arrangement of Holttum) is the most diversified genus in the Old World with c. 140 spp. in Malesia . Christella is probably also related to Sphaerostephanos though less nearly than Stegnogramma.

The biology and zoogeography of Helicopsyche borealis (Trichoptera: Helicopsychidae): a Nearctic representative of a tropical genus

Members of the caddisfly family Helicopsychidae were present in the Upper Jurassic, and the present-day distribution of the genus Helicopsyche shows typical southern continent dispersion, suggesting that it was in existence before the breakup of Pangea. Most of the approximately 90 species of Helicopsyche are endemic to the tropics, indicating a tropical origin for the genus. At certain locations, e.g. Mexico, Sri Lanka, and New Caledonia, pronounced speciation has occurred. Only two species have successfully colonized North Temperate regions, H. borealis in North America (the most widespread species) and H. sperata in Europe. In a stream in southern Ontario, H. borealis is probably univoltine although difficulty in separating instars complicates interpretation. Dyar's Rule was applied to assist in this interpretation. Microdistribution of larvae in the stream was examined and microcurrent regime around single rocks was judged to be an important controlling factor. Microdistribution extended into the hyporheic zone of the stream bed. The ability to live interstitially is proposed to explain the unique helical shape of the larval case as the case is very resistant to crushing. Case composition was very similar in 5th instar individuals. Larvae were capable of limited case repair but could not build an entire new case. Larval diet, even in the interstices, consisted chiefly of detritus and diatoms, although the proportions of each changed with seasonal availability. Larvae of H. borealis have the ability to regulate their oxygen consumption in a low oxygen environment and this also fits them for an interstitial life. Comparative biological data for other helicopsychids are presented.

Further Evidence for Osmoregulation in Epidermal Leaf Cells of Seagrasses

A comparison of the epidermal leaf cell ultrastructure of three seagrasses, Thalassia testudinum (tropical, high salinity), Zostera marina (North temperate, moderate salinity), and Ruppia maritima (North temperate, brackish) provides confirmation for the theory that an invaginated plasmalemma-mitochondrial transport system is developed at least in part as a response to salt concentration. Cytochemical localization of presumed Cl− ion provides further evidence for the presence of a salt secretion or exclusion mechanism. Immature epidermal leaf cells communicate with each other and with mesophyll cells through numerous plasmodesmata, but during cell maturation these cytoplasmic connections are lost and the apoplastic transport system develops to replace the symplastic one. The two North temperate region seagrasses contain cytoplasmic lipids which are absent in the tropical species. Thalassia and Zostera have chloroplasts which lack starch, but stain densely for polysaccharides with thiocarbohydrazide. The polysaccharide staining is essentially negative in the chloroplasts of Ruppia, but mesophyll chloroplasts of this brackish water species contain starch. These and other cytological findings are compared with other seagrasses.

FURTHER EVIDENCE FOR OSMOREGULATION IN EPIDERMAL LEAF CELLS OF SEAGRASSES

A comparison of the epidermal leaf cell ultrastructure of three seagrasses, Thalassia testudinum (tropical, high salinity), Zostera marina (North temperate, moderate salinity), and Ruppia maritima (North temperate, brackish) provides confirmation for the theory that an invaginated plasmalemma‐mitochondrial transport system is developed at least in part as a response to salt concentration. Cytochemical localization of presumed Cl− ion provides further evidence for the presence of a salt secretion or exclusion mechanism.Immature epidermal leaf cells communicate with each other and with mesophyll cells through numerous plasmodesmata, but during cell maturation these cytoplasmic connections are lost and the apoplastic transport system develops to replace the symplastic one. The two North temperate region seagrasses contain cytoplasmic lipids which are absent in the tropical species. Thalassia and Zostera have chloroplasts which lack starch, but stain densely for polysaccharides with thiocarbohydrazide. The polysaccharide staining is essentially negative in the chloroplasts of Ruppia, but mesophyll chloroplasts of this brackish water species contain starch. These and other cytological findings are compared with other seagrasses.

Spatially resolved methane band photometry of Saturn: II. Cloud structure models at four latitudes

Spatially resolved measurements of Saturn's reflectivity in the 6190-, 7250-, and 8996-Å methane bands are analyzed to determine cloud vertical structures in the Equatorial Zone, South Equatorial Belt, and North and South Temperate Regions near latitudes ±30°. Radiative transfer models are computed for a simple two-parameter structure. The parameters are A 0, the methane column abundance in an aerosol-free layer at the top of the atmosphere, and A 1, the specific abundance of methane in a semi-infinite homogeneous gas and cloud mixture deep in the atmosphere. For the Equatorial Zone, a model with A 0 = 37 ± 3 m-am and A 1 = 26 ± 2 m-am fits all three bands. For the North Temperate Region, a model with A 0 = 39 m-am and A 1 = 47 m-am comes close to fitting all three bands. For the South Equatorial Belt and South Temperate Region, a single A 0 and A 1 do not fit all three bands. The structure for the South Equatorial Belt resembles that for the North Temperate Region. The level where unit cloud optical depth occurs in the South Temperate Region is deeper than the corresponding level at other latitudes. Some suggestions are proposed to explain differences between model parameters derived using different absorption bands.