Molecular confirmation of Potamogeton × bottnicus (P. pectinatus × P. vaginatus, Potamogetonaceae) in Britain

Restriction fragment length polymorphisms in the internal transcribed spacer region of ribosomal DNA and in the chloroplast genome combine to confirm the existence of Potamogeton×bottnicus ( P. pectinatus × P. vaginatus ) in Britain. One of the parents, P. vaginatus , is not currently a member of the British flora, but did occur here in the past (the latest fossil fruits date from 30000 BP) and the hybrid may therefore owe its origin to ancient cross-pollination involving indigenous material or to more recent long-distance dispersal.     

Context-dependent memory: colour versus odour

An olfactory stimulus and a visual stimulus were employed in a context- dependent memory study using a prose passage as the to-be-remembered item. Ninety-five university students (aged 17-35 years) learned the passage of prose in the presence of one of the stimuli and were then asked to recall the passage with the original context either reinstated or not reinstated. The results revealed a significant context-dependent memory effect for the olfactory cue but not for the visual cue. They demonstrate support for the effectiveness of odours as context cues and it is suggested that context-dependent memory processes may underlie the formation and retrieval of odour-evoked autobiographical memories.      

Evidence for a Structural Role of Protein in Algal Cell Walls

Load-extension measurements were made on three filamentous algaebefore and after either digestion with proteolytic enzymes ortreatment with dithiothreitol. Large differences in tensileproperties of the walls were observed, particularly after pronasedigestion, in two algae, Cladophora and Chaetomorpha, whichcontain hydroxyproline in the wall. Pronase had little or noeffect on a third alga, Nitella, lacking hydroxyproline. A smallerdifference was found on treatment with dithiothreitol, a specificreducing agent for disulphide bonds. These results suggest thata hydroxyproline containing protein is a structural componentof these algal walls, and that hydroxyproline itself is involvedin the carbohydratepeptide linkage.     

Two new genera, Crassoseta and Brachysternopsis, of the family Diplogyniidae (Acari, Mesostigmata, Trigynaspida)

The genus Crassoseta is erected for three new species of diplogyniid mites taken from passalid beetles collected in Mexico, Brazil and Costa Rica; Brachysternum cornutum Hyatt, 1964 is transferred to this genus and a key to species is given. Brachysternopsk gen.n. is established for B. flechtmanni gen. et sp.n. taken from an ant, Atrica capiguara. in Brazil.     

Chromosomal localization of a tandemly repeated DNA sequence in Trifolium repens L

INTRoDUCTIoNlYho1iumrePensL,whiteclover,isaneconomicallyimportantplantspeciesintemperatepastures.Asbrieflyreportedby[1],ithas16pairsofchromosomes(2n=32).Asyet,nodetailedcytologicalexaminationofthisspecies,suchasC-banding,hasbeenrep0rted.Inthelastdecade,thetechnique0fC-bandinghasbeenusedt0examinehighlyrepeatedsequencesinplantchrom0s0mesandhasprovidedausefultoolf0rtheanalysis0fcyt0geneticstructureincr0pplants[2-71.Inplants,thechr0m0s0mall0calizationofhighlyrepeatedDNAsequencesbyinsituhybr…     

Electron Microscopic Observations of the Structure of Sieve-connexions in the Phloem of Angiosperms and Gymnosperms

The sieve elements of Pinus silvestris L., Sorbus aucupariaL., Vitis vinifera L., and Cucurbita pepo L. have been examinedelectron microscopically in ultra-thin section, and the structuresof the corresponding sieve areas or sieve plates have been describedand compared. In Pinus the sieve areas contain groups of connectingstrands which enter the wall from the lumen side as individualsbut coalesce within it in the median cavity. This cavity hasdeveloped by wall breakdown in the middle lamella and primarywall region and corresponds to the median nodule visible undera light microscope. Neither in this nor in the other speciesobserved is there any visible closing membrane. Structural differences between the four species are shown tosuggest that the major evolutionary trend in the evolution ofspecialized conducting strands has been the enlargement of theconnecting strands from groups of small separate strands toa smaller number of larger strands as the median cavity becomesenlarged to form a canal through the wall. The connecting strands appear invariably to be dense, highlyosmiophilic and continuous with the cytoplasmic surface of thecell. No signs of micropores or of other tubular structure inthe strands have been found. The structures thus revealed aremore nearly compatible with active transport of materials acrossthe sieve plate than they are with any purely physical mechanism.It is suggested that they are incompatible with any mass flowhypothesis.