Micrometre-scale refrigerators

A superconductor with a gap in the density of states or a quantum dot with discrete energy levels is a central building block in realizing an electronic on-chip cooler. They can work as energy filters, allowing only hot quasiparticles to tunnel out from the electrode to be cooled. This principle has been employed experimentally since the early 1990s in investigations and demonstrations of micrometre-scale coolers at sub-kelvin temperatures. In this paper, we review the basic experimental conditions in realizing the coolers and the main practical issues that are known to limit their performance. We give an update of experiments performed on cryogenic micrometre-scale coolers in the past five years

Observations of reaction fibers in leaves of dicotyledons

Journal ArticleHALLE, OLDEMAN, AND TOMLINSON (1978) have discussed the transference of function between compound leaves and plagiotropic branches in the context of tree architecture. In trees conforming to Cook's model (see Halle, Oldeman, & Tomlinson, 1978), plagiotropic branches resemble compound leaves in their determinate growth and their periodic abscission. Conversely, certain trees (primarily in the Meliaceae) have compound leaves that behave like plagiotropic branches

Winter xylem embolism and spring recovery in Betula cordifolia, Fagus grandifolia, Abies balsamea, and Picea rubens

Journal ArticleXylem embolism was monitored from mid-winter to mid-summer in four co-occurring species: Betula cordifolia (Reg.) Fern., Fagus grandifolia Ehrh., Abies balsamea (L.) Mill., Picea rubens Sarg. The study site was a west-facing slope in the northern Green Mountains of Vermont, U.S.A.; Betula and conifers were sampled at 914 m; Fagus was collected at 827 m near its local altitudinal limit. Embolism was quantified by the percent the hydraulic conductivity of branch segments was below the maximum obtained following removal of air embolism in xylem conduits. Between early February and early May, the deciduous species averaged 60 to 84% embolism compared to 15 to 60% for the conifers. From April 24 to May 25, embolism in Betula dropped from 81 to 8%; this recovery was associated with root pressures up to 86 kPa as measured with manometers at the lower trunk. Betula trees in which root pressure was eliminated by overlapping saw cuts still showed 75% embolism in June; only 4% was present in control trees cut in a similar fashion after leaf flush. Root pressure was weak (3 kPa) and uncommon in Fagus, and trees remained 80% embolized through June showing considerable dieback

Evolution of water transport and xylem structure

Journal ArticleLand plants need water to replace the evaporation that occurs while atmospheric CO2 is diffusing into photosynthetic tissue. The water-for-carbon exchange rate is poor, and evolutionary history indicates a progression of innovations for cheap water transport--beginning in order with capillary suction at cell walls, stomatal regulation, hydroids, tracheids, secondary xylem, endodermis, and vessels. The radiation of plants in the Silurian and Devonian occurred when the need for water was at an all-time low because of high CO2 concentration. Transport improvements appeared as water demand increased and CO2 dropped to current values in the Carboniferous and Permian. Stomatal regulation and high-conductivity conduits permitted larger plants and a transition from poikilohydric to homoiohydric water relations. The evolution of conduits from hydroids through tracheids to vessels reflects the need to balance resistance to implosion and cavitation versus maximum hydraulic conductance and minimum conduit investment. Localization of rigidifying lignin away from the lumen surface and porous wall regions during tracheid evolution, and the origin of pits, acted to maintain wall strength and permeability while minimizing cavitation. Vessels mark the pinnacle of efficiency, making vines and dense, stiff woods possible without sacrificing conductivity or cavitation resistance. However, vessels make cavitation-resistant wood more expensive and may compromise refilling efficiency versus tracheids. Vascular networks maximize hydraulic conductivity and protection from cavitation at minimum investment by following Murray's law and localizing resistances to the periphery. A future challenge is to quantify the significance of xylem structure in terms of the carbon cost of transpiration and the net carbon profit via gas exchange

Observations on the structure and function of hydathodes in Blechnum lehmanii

Journal ArticleThe fronds of many ferns in the Polypodiaceae (Ogura, 1972) and Cyatheaceae (Weiler, cited in Lippmann, 1925) possess swollen vein endings associated with specialized adaxial epidermal cells. Their structure is similar in all ferns (Gardiner, 1883; Potonie, 1892; Poirault, 1893; Goebel, 1930; Guttenburg, 1934), including Blechnum lehmannii Hieron. (Figs. 1-3)

Inter-tracheid pitting and the hydraulic efficiency of conifer wood: the role of tracheid allometry and cavitation protection

Journal ArticlePlant xylem must balance efficient delivery of water to the canopy against protection from air entry into the conduits via air-seeding. We investigated the relationship between tracheid allometry, end wall pitting, safety from air-seeding, and the hydraulic efficiency of conifer wood in order to better understand the trade-offs between effective transport and protection against air entry

Xylem hydraulics and the soil-plant-atmosphere continuum: opportunities and unresolved issues

Journal ArticleReceived for publication July 23, 2002. Soil and xylem are similar hydraulically. An unsaturated conductivity curve for soil is called a vulnerability curve for xylem?but the underlying physical basis is the same. Thus, any transport model that treats unsaturated soil conductivity would benefit by also incorporating the analogous xylem vulnerability curves. The full-text PDF of this article is freely available at http://agron.scijournals.org/cgi/reprint/95/6/136

Embolized conduits of rice (Oryza sativa, poaceae) refill despite negative xylem pressure

Journal ArticleEmbolism reversal in rice plants was studied by testing the plant's ability to refill embolized conduits while xylem pressures were substantially negative. Intact, potted plants were water-stressed to a xylem pressure of -1.88 + 0.1 MPa and a 66.3 + 3.8% loss of xylem conductivity (PLC) by cavitation. Stressed plants were carefully rewatered, allowing xylem pressure to rise, but not above the theoretical threshold of c. -0.15 MPa for embolism collapse

Cavitation fatigue - the weakening of cavitation resistance of xylem and its reversibility

Journal ArticleXylem function is essential for the growth and survival of higher land plants. Xylem must not only be efficient under favorable conditions to facilitate high rates of stomatal conductance and carbon uptake, but it should also remain functional under drought conditions, when water potential (Ψ) drops to low values. Since water in the xylem is lifted up to the leaves by negative pressure (Steudle, 2001; Zimmermann, 1983), it is intrinsically vulnerable to cavitation