Gas Exchange Requirements Research Articles

Structure and function of skin in the pelagic sea snake, Hydrophis platurus.

We describe and interpret the functional morphology of skin of the Yellow-bellied sea snake, Hydrophis platurus. This is the only pelagic sea snake, and its integument differs from what is known for other species of snakes. In gross appearance, the scales of H. platurus consist of non-overlapping, polygonal knobs with flattened outer surfaces bearing presumptive filamentous sensillae. The deep recesses between scales ('hinge') entrap and wick water over the body surface, with mean retention of 5.1 g/cm of skin surface, similar to that determined previously for the roughened, spiny skin of marine file snakes, Acrochordus granulatus. This feature possibly serves to maintain the skin wet when the dorsal body protrudes above water while floating on calm oceanic slicks where they forage. In contrast with other snakes, including three species of amphibious, semi-marine sea kraits (Laticauda spp.), the outer corneous β-protein layer consists of a syncytium that is thinner than seen in most other species. The subjacent α-layer is also thin, and lipid droplets and lamellar bodies are seen among the immature, cornifying α-cells. A characteristic mesos layer, comprising the water permeability barrier, is either absent or very thin. These features are possibly related to (1) permeability requirements for cutaneous gas exchange, (2) reduced gradient for water efflux compared with terrestrial environments, (3) less need for physical protection in water compared with terrestrial ground environments, and (4) increased frequency of ecdysis thought to be an anti-fouling mechanism. The lipogenic features of the α-layer possibly compensate for the reduced or absent mesos layer, or produce layers of cells that comprise what functionally might be termed a mesos layer, but where the organization of barrier lipids nonetheless appears less robust than what is characteristically seen in squamates.

Escalation of extracorporeal life support as a bridge to lung transplantation in end-stage lung disease

Extracorporeal membrane oxygenation (ECMO) as a bridge to lung transplant (LTx) is not uncommon, but it is not commonplace yet. We present a case of a 45-year-old man with cystic fibrosis with recent deterioration in lung function who was initially supported with veno-venous (VV) ECMO. However, he subsequently required conversion to veno-veno-arterial (VVA) ECMO. After 21 days of support, he underwent double lung transplantation, with an uneventful postoperative course. This case shows that, in patients with end-stage respiratory failure awaiting lung transplantation, extracorporeal life support may require escalation to improve gas exchange and address circulatory requirements.

Gas Transfer of an Implatable Artificial Lung

The water flow characteristics in the implantable artificial lung (IAL) were evaluated by in vitro experiments. The 50-fiber modules were of different lengths but contained well-spaced fibers; and the 300-fiber module had a volume fraction of 0.9. A lower number of tired hollow fibers produce a higher water-side film coefficient of oxygen transfer at a constant velocity of water-side flow. The area of the water flow decreases with increasing hollow fiber packing density, thus if water is induced to flow uniformly in the IAL module, the oxygen transfer rate should monotonously increase with the packing density. The water flow in the IAL is not uniform and depends on the hollow fiber packing density. The stagnation of the water flow occurs in the bundle of the hollow fibers at the water inlet. Intravascular oxygenation represents an attractive, alternative support modality for patients with ARDS. The concept of intravascular oxygenation as an alternative ARDS therapy originated with Mortensen (7), who developed an intravenous oxygenator (IVOX) consisting of a bundle of crimped hollow fiber positioned in the vena cava. In phase I clinical trials, the IVOX provided an average of 28% of basal gas exchange requirements for patients with severe ARDS (8-14). The clinical study, however, concluded that more gas exchange was needed for intravascular oxygenation to be clinically effective in ARDS treatment. We are developing an intravenous membrane oxygenator (IMO) with a design goal of 50% of basal oxygen and carbon dioxide exchange requirements for end-stage ARDS patients. Like the IVOX, the IMO consists of a bundle of manifolded hollow fiber, and is intended for intravenous placement within the superior and inferior vena cava. The target level of gas exchange in the IVOX, and consequently, the IMO, incorporates a polyurethane balloon concentric with the fiber bundle, which rhythmically inflates and deflates to provide active blood mixing, and thus enhances gas exchange. Our current efforts focus on device improvements intended to provide the target levels of gas exchange, given the constraints imposed by intravenous placement on fiber bundle size and hence fiber area for gas exchange. Although critical care techniques have been improved, the high mortality of severe ARDS has not significantly changed (10-15). In an implantable artificial lung (IAL), the greater part of the oxygen transfer resistance is located in the blood-side laminar film (16), and various methods have been attempted to make the laminar film thin and improve the oxygen transfer rate (17, 18). In the present study, the water flow characteristics in the IAL were evaluated by in vitro experiments. The effect of hollow fiber packing fraction on water flow condition was evaluated to produce effective constant of water with the hollow fibers. Oxygen transfer rates were evaluated, and the optimum hollow fiber packing fraction was determined at an outside diameter of hollow fibers of 380 μm. II. THEORYT The Reynolds number (NRe =Lv/υ) characterizes the flow regime and is the ratio of inertial force to viscous force. The Schmidt number (NSc=υ/D), analogous to the Prandtl number in heat transfer, characterizes the fluid properties and is the ratio of momentum transport to diffusive transport. The Peclet number (NPe=Lv/D), which is the product of NRe, characterizes the relative importance of convective and diffusive processes and is the ratio of bulk mass transport to diffusive mass transport. The Sherwood number (NSh=KL/D), also known as the mass transfer Nusselt number, likewise characterizes the relative importance of convective and diffusive transport; it is the ratio of total transport to diffusive transport. The mass transfer Stanton number (NSt=K/v) is proportional to the ratio of the actual mass flux to the mass flux capacity of the flow, i.e. the amount of mass potentially transferable per unit time and cross-sectional area.

Phase Contrast Imaging Reveals Low Lung Volumes and Surface Areas in the Developing Marsupial

Marsupials are born with immature lungs when compared to eutherian mammals and rely, to various extents, on cutaneous gas exchange in order to meet metabolic requirements. Indeed, the fat-tailed dunnart is born with lungs in the canalicular stage of development and relies almost entirely on the skin for gas exchange at birth; consequently undergoing the majority of lung development in air. Plane radiographs and computed tomography data sets were acquired using phase contrast imaging with a synchrotron radiation source for two marsupial species, the fat-tailed dunnart and the larger tammar wallaby, during the first weeks of postnatal life. Phase contrast imaging revealed that only two lung sacs contain air after the first hour of life in the fat-tailed dunnart. While the lung of the tammar wallaby was comparatively more developed, both species demonstrated massive increases in air sac number and architectural complexity during the postnatal period. In addition, both the tammar wallaby and fat-tailed dunnart had lower lung volumes and parenchymal surface areas than were expected from morphometrically determined allometric equations relating these variables to body mass during the neonatal period. However, lung volume is predicted to scale with mass as expected after the neonatal marsupial reaches a body mass of ∼1 g and no longer relies on the skin for gas exchange. Decreased lung volume in the marsupial neonate further supports the maxim that cutaneous gas exchange occurs in the marsupial neonate because the respiratory apparatus is not yet capable of meeting the gas exchange requirements of the newborn.

Inexpensive non-toxic flocculation of microalgae contradicts theories; overcoming a major hurdle to bulk algal production

There are two major energy and cost constraints to bulk production of single cell microalgae for biofuels or feed: expensive culture systems with high capital costs and high energy requirements for mixing and gas exchange; and the cost of harvesting using high-speed continuous centrifugation for dewatering. This report deals with the latter; harvesting by flocculation where theory states that alkaline flocculants neutralize the repelling surface charge of algal cells, allowing them to coalesce into a floc. It had been assumed that with such electrostatic flocculation, the more cells to be flocculated, the more flocculant needed, in a linear stoichiometric fashion, rendering flocculation overly expensive. Counter to theory of electrostatic flocculation, we find that the amount of alkaline flocculant needed is a function of the logarithm of cell density, with dense cultures requiring an order of magnitude less base than dilute suspensions, with flocculation occurring at a lower pH. Various other theories abound that flocculation can be due to multi-valent cross-linking, or co-precipitation with phosphate or with magnesium and calcium, but are clearly not relevant with the flocculants we used. Monovalent bases that cannot cross-link or precipitate phosphate work with the same log-linear stoichiometry as the divalent bases, obviating those theories, leaving electrostatic flocculation as the only tenable theory of flocculation with the materials used. The cost of flocculation of dense cultures with this procedure should be below $1.00/T algae for mixed calcium:magnesium hydroxides.

Mathematical and Computer Simulation of the Biological Life Support System Module 1/2. Description of the model

Received 7.12.2009, received in revised form 14.12.2009, accepted 21.12.2009 The mathematical model based on kinetic coefficients and dependencies obtained during the experiments was constructed to estimate the character of functioning of the experimental module of biological life support system (BLSS) and the possibilities of its controlling. The mathematical model consists of two compartments - the 'phytotron' model (with wheat and radish) and the 'mycotron' model (for mushrooms). The following components are included into the model: edible mushrooms (mushroom fruit bodies and mycelium); wheat; radish; straw (processed by mycelium); dead organic matter in the phytotron (separately for the wheat unit and for the radish unit); worms; wormscoprolites; vermicompost used as a soil-like substrate (SLS); bacterial microflora; mineral nitrogen, phosphorus and iron; products of the system intended for humans (wheat grains, radish roots and mushroom fruit bodies); oxygen and carbon dioxide. At continuous gas exchange, the mass exchange between the compartments occurs at the harvesting time. The conveyor character of the closed ecosystem functioning has been taken into account - the number of culture age groups can be controlled (in experiments and in the model - 4 and 8 age groups). The conveyor cycle duration can be regulated as well. The module was designed for the food and gas exchange requirements of 1\30 of a virtually present human. The model estimates the values of all dynamic components of the system under various conditions and modes of functioning, especially those, which are difficult to be realized in the experiment. The model allows dynamic calculation of biotic turnover closedness coefficient for main considered elements. The coefficient of matter biotic cycle closure for systems based on matter supplies has been formalized.

Experimental safety and efficacy evaluation of an extracorporeal pumpless artificial lung in providing respiratory support through the axillary vessels

We sought to investigate the safety and feasibility of implanting the pumpless interventional lung assist device (Novalung; Novalung GmbH, Hechingen, Germany) to the axillary vessels either by means of direct cannulation or end-to-side graft interposition and the capability of either type of vascular access to provide respiratory support during apneic ventilation in adult pigs. Ten pigs were ventilated for 4 hours (respiratory rate, 20-25 breaths/min; tidal volume, 10-12 mL/kg; fraction of inspired oxygen, 1.0; positive end-expiratory pressure, 5 cm H2O). Thereafter, the interventional lung assist device was surgically connected to the right axillary artery and vein by using direct cannulation (n = 5) or end-to-side ringed polytetrafluoroethylene graft interposition (n = 5), and ventilatory settings were reduced to achieve near apneic ventilation (respiratory rate, 4 breaths/min; tidal volume, 1-2 mL/kg; fraction of inspired oxygen, 1.0; positive end-expiratory pressure, 20 cm H2O). Hemodynamic and intrathoracic volumes and lung cytokine levels were measured. Blood flow through the interventional lung assist device was 1.7 +/- 0.4 L/min or 30% +/- 14% of the cardiac output, and the mean pressure gradient across the interventional lung assist device was 10 +/- 2 mm Hg. The interventional lung assist device allowed an O2 transfer of 225.7 +/- 70 mL/min and a CO2 removal of 261.7 +/- 28.5 mL/min. Although the amount of blood flow perfusing the interventional lung assist device was significantly higher (P < .01) with direct cannulation (2.1 +/- 0.3 L/min) compared with that seen in graft interposition (1.3 +/- 0.3 L/min), the latter allowed similar respiratory support with reduced hemodynamic instability. The axillary vessels are a safe and attractive cannulation site for pumpless partial respiratory support. Compared with direct cannulation, graft interposition was equally able to support the interventional lung assist device-driven gas exchange requirements during apneic ventilation with better hemodynamic stability.

Lesions in the cerebellar fastigial nucleus have a small effect on the hyperpnea needed to meet the gas exchange requirements of submaximal exercise

The purpose of this study was to test the hypothesis that an intact cerebellar fastigial nucleus (CFN) is necessary for the hyperpnea to meet the gas exchange needs of submaximal exercise. Bilateral stainless steel microtubules were implanted in the cerebellum inside (n = 12) or outside (n = 2) the CFN for injection (0.5 to 10 microl) of the neurotoxin ibotenic acid. All goats had difficulty maintaining normal posture and walking for up to 1 mo after the implantation of the microtubules and again for hours or days after the neurotoxin was injected. Postmortem histology indicated there were 55% fewer living neurons (P < 0.001, n = 9, 3,720 +/- 553 vs. 1,670 +/- 192) in the CFN of the experimental goats compared with a control group of goats. As is typical for goats before implantation of the microtubules, the decrease in arterial Pco(2) from rest during mild and moderate treadmill exercise was 2.0 +/- 0.39 and 3.5 +/- 0.45 Torr, respectively. Implantation of the microtubules did not significantly change this exercise hyperventilation. However, neurotoxic lesioning with 10 mul ibotenic acid significantly (P < 0.05) attenuated the decrease in arterial Pco(2) by 1.3 and 2.8 Torr at the first and second workload, respectively. The modest attenuation of the exercise hypocapnia at both workloads in CFN-lesioned goats suggests that the CFN is part of the control system that enables the ventilatory response to meet the gas exchange requirements of submaximal exercise.

Mesenchymal expression of vascular endothelial growth factors D and A defines vascular patterning in developing lung

The lung has specific vascular patterning requirements for effective gas exchange at birth, including alignment of airways and blood vessels and lymphatic vessels. Vascular endothelial growth factors (VEGF) are potent effectors of vascular development. We examined the temporal and spatial expression of VEGF-D and specific VEGF-A isoforms at each stage of lung development. VEGF-D, expressed only by cadherin-11-positive cells of the mesenchyme, is first detected at embryonic day (E) 13.5, a period of active vasculogenesis. VEGFR-3, its cognate receptor, is detected earlier on days E11.5 to E14.5, in both blood vessels and lymphatic vessels and later, on day E17.5, in only lymphatic vessels. VEGF-A is expressed in the mesenchyme throughout lung development and also by the epithelium midway through organogenesis. Before E14, the predominant forms of VEGF-A are the soluble isoforms, VEGF-A120 and 164. Not until E14.5 do epithelial cells at the tips of expanding airways express VEGF-A, including VEGF-A188, an isoform with high affinity for extracellular matrix. Our results demonstrate unique temporal and spatial expression of VEGF-D and specific VEGF-A isoforms during lung development and suggest these related factors have distinct functions in vascular and lymphatic patterning of the lung.

Gas exchange potential in reptilian lungs: implications for the dinosaur–avian connection

The theory that birds evolved from a group of small terrestrial theropod dinosaurs has created much controversy. One argument proposed against this theory is that the lungs of early theropods were incapable of sustaining endothermic gas exchange requirements and could not have given rise to the lungs of birds. A reexamination of the comparative physiological and morphological literature combined with a theoretical analysis of gas exchange potential indicates that non-avian lungs would not constrain the gas exchange requirements of early endotherms. Furthermore, our analysis suggests that factors besides diffusive gas exchange were important in the evolution of the distinct morphology of the highly effective avian and mammalian lungs.

Competition between gas exchange and speech production in ventilated subjects

Competition between airflow requirements for speaking and gas exchange occurs in ventilator-dependent tracheotomized subjects who can `steal' air from alveolar ventilation during the ventilator's inflation phase to produce sound. We wondered whether these subjects adopted strategies to minimize hypoventilation when speaking, particularly when ventilatory drive and respiratory discomfort are increased by hypercapnia. We recorded speech and ventilatory and speaking volumes in five ventilated subjects during reading and extemporaneous speech. All subjects spoke during the ventilator's inflation (and expiratory) phase, losing approximately 15% of their inspired tidal volume. During induced hypercapnia (15 mmHg increase in Pet CO 2 ) which caused shortness of breath, all subjects could still speak adequately. Two subjects `adapted' to hypercapnia by reducing the air used for speaking during inflation. In contrast, one subject reacted, as normal subjects do, by increasing the airflow per syllable (a mal-adaptive strategy in ventilated subjects). These changes were modest despite the strong hypercapnic stimulus.

Clinical strategies in intravascular gas exchange.

Specific therapies in the management of acute pulmonary failure remain elusive, with attention being focused instead on novel supportive measures. The benefits of extracorporeal gas exchange support remain uncertain, and the perceived simplicity of intravascular gas exchange has, therefore, attracted much interest. Initial clinical experience with the intravascular oxygenator (IVOX) device confirms its safety and simplicity, but estimated mean gas-transfer values represent only 25% of basal gas-exchange requirements. The inherent limitations of IVOX as an oxygenator are discussed, providing a rationale for considering IVOX as primarily a CO2 removal device. Reappraisal of the clinical place of intravascular gas exchange and the identification of specific applications most likely to yield benefit to patients are suggested. Design modifications enhancing efficacy are anticipated, further strengthening the potential of intravascular gas-exchange devices in selected patients with pulmonary failure.

Peculiar Constraints on Life Histories Imposed by Protective or Nutritive Devices for Embryos

SYNOPSIS. A general constraint for sexually produced offspring of multicellular organisms is that they must begin as a single cell (lest they be chimeric) and that the embryos that produce multicellular structures perform many functions less capably than more developed individuals. Beyond this limitation, predictive models have examined optimized tradeoffs between number of offspring and size of offspring. Broadly applicable predictions have been sought, and two general expectations have resulted. (1) Optimal parental investment per offspring should not be affected by total investment per clutch when offspring size is much smaller than clutch size. (2) Selection should usually favor an optimal size of offspring rather than varied sizes within a clutch. However, constraints intrinsic to mechanisms of parental protection or provisioning may change these predicted outcomes of selection on life history traits. (1) Provisioning and packaging of protected embryos may introduce dependence of offspring size on adult size or clutch size, (a) Clutches of embryos are a large mass of metabolizing tissue without a circulatory system but with requirements for gas exchange; when dissolved gases are transported by interstitial flow between embryos, requirements for supply of oxygen may require larger offspring in thicker and hence more massive clutches, (b) Offspring size can become dependent on adult size through capsule size or size of the birth canal, and offspring size is thereby linked to clutch mass when clutch mass increases with adult size. (2) Random placement of protected embryos may increase variation around an optimum size or quality of offspring. Possible examples are (a) determination and packaging of nurse eggs that will be consumed by embryos and (b) hormonal interactions among offspring in a uterus. Constraints that violate general expectations and arise from varied and specific mechanisms of parental protection or provisioning may be common.

In Vivo Gas Transfer Performance of the Intravascular Oxygenator in Acute Respiratory Failure

The intravascular oxygenator (IVOX) has undergone both animal and clinical trials. Data from the animal studies have demonstrated that the device is capable of transferring up to approximately 100 ml/min of oxygen and carbon dioxide. Initial data from the human trials suggest that gas transfer, although approaching these levels, varied widely in patients with respiratory failure. We studied the factors affecting gas exchange in 26 patients with severe acute respiratory failure who underwent intravenacaval support of gas exchange with IVOX. The patients underwent monitoring of IVOX gas transfer rates, hemodynamics, blood gases, and ventilation parameters at scheduled intervals following device insertion. All devices functioned following implantation. The mean value for O2 transfer was 64 +/- 21 SD ml/min (range 15-114 ml/min) and for CO2 transfer was 48 +/- 17 ml.min-1 (range 14-112 ml/min). CO2 transfer correlated positively with device surface area, cardiac output, and mixed venous Pco2 and negatively with duration of implantation. O2 transfer did not correlate with any patient factors probably due to error inherent in the measurement of this variable. Independent measurements of IVOX gas transfer by respiratory gas exchange in a subset of patients with normal values of mixed venous Pco2 were in good agreement with the routine measurements and indicated that the device provided up to 26% of gas exchange requirements in this subset. We conclude that IVOX transfers clinically useful amounts of oxygen and carbon dioxide in vivo. Factors that influence gas transfer include device surface area, PvCO2, cardiac output, and duration of implantation. Optimization of these factors (such as with permissive hypercapnea) could result in enhanced performance in vivo.

The development of an implantable artificial lung.

This report describes the development of an implantable gas exchange device. The device is composed of hollow fiber elements wound around a central open core enclosed in a compliant outer casing, offering very low resistance to blood while providing adequate gas exchange. The purpose of this study was to determine if this device design can completely support the gas exchange requirements of a large animal when the device is placed in series with the main pulmonary artery (PA). Six 40-80 kg adult sheep were used. The device was placed with vascular grafts anastomosed end to side on the proximal and distal main PA. The study began with the entire right ventricular blood flow being diverted through the device by occlusion of a snare around the PA between the vascular grafts. Total gas exchange then was provided by the device and the endotracheal tube was clamped. Results showed that this pumpless potentially implantable device is capable of completely supporting the gas exchange requirements of the experimental animals for up to 8 hours in the acute setting without significant change in cardiac index (CI) and oxygen consumption (VO2) compared with baseline. CI = 55.0 +/- 17.0 cc/min/kg versus 45.0 +/- 17.3 cc/min/kg. VO2 = 1.90 +/- 0.96 cc O2/min/kg versus 2.08 +/- 0.54 cc O2/min/kg.

Ventilation in Cataglyphis bicolor: Regulation of carbon dioxide release from the thoracic and abdominal spiracles

Using several different experimental approaches, we show that the discontinuous ventilation cycle (DVC) in the arid-adapted ant Cataglyphis bicolor is of the passive suction ventilation (PSV) type. Gas flow is tidal, without significant active ventilation. Direct observations of spiracular activity and recordings of endotracheal pressure with simultaneous gas exchange measurements are fully consistent with this ventilation regime. Separate simultaneous recordings of the thoracic and abdominal spiracles show that they have distinct functions and capabilities. The thoracic spiracles act as high-capacity gateways to the tracheal system and are responsible for some 80–90% of overall gas exchange when the ant is active. At rest, a single open mesothoracic spiracle can meet an ant's oxygen uptake and carbon dioxide release requirements. The abdominal spiracles combined have less than half the diffusive capacity of a single thoracic spiracle (although they can meet the gas exchange requirements of an ant at rest) and their contribution to overall gas exchange is minor. However, they are important during the Flutter (F) phase of the DVC, especially in its early stages, before the thoracic spiracles take over (the thoracic and abdominal spiracles exhibit inverse modulation during the F phase).

Development and current status of a new intracorporeal membrane oxygenator (IVOX).

The IVOX (intravascular oxygenator) is an intracorporeal, hollow-fibre membrane oxygenator and carbon dioxide (CO 2) removal device. The IVOX is surgically placed into the vena cava via a femoral or jugular venotomy. Oxygen (O 2) is pulled through the hollow fibres by a vacuum pump controlled by a flow meter. There is no extracorporeal circulation of blood. Gas exchange occurs as the patient's blood flows over several hundred hollow fibres. Inlet and outlet gas conduits exit a small skin incision for inflow of 0 2 and outflow of CO 2. Studies in sheep show that the IVOX can support approximately 30% of gas exchange requirements. The position of the IVOX in the vena cava does not affect haemodynamics or cause thromboembolic complications. It can remain in place for up to 22 days without affecting haematologic or blood chemistry parameters. The IVOX is currently undergoing clinical trials at selected medical centres in patients with acute respiratory failure.

Implantable Lung-Assist Device Being Tested on Patients With Acute Respiratory Failure

THE UNIVERSITY of Texas Medical Branch (UTMB), Galveston, is one of seven sites testing an intravascular membrane oxygenator on patients with acute, potentially reversible respiratory failure. Although the experimental device (<i>Int J Artif Organs</i>. 1989;12:384-389) provides less than half the gas exchange needs of a patient, it may be enough to prevent or significantly reduce lung injury that often results from high pressures and increased oxygen tensions produced by mechanical ventilation, says Joseph B. Zwischenberger, MD, associate professor of cardiothoracic surgery, and director of the university's Extracorporeal Membrane Oxygenation Program. Says Zwischenberger, the intravascular oxygenator or IVOX (Cardiopulmonics Inc, Salt Lake City, Utah) is a "booster lung" rather than an artificial lung for replacing diseased or injured lungs. Studies with sheep suggest that it may be able to provide 30% of a patient's gas exchange requirements. While not enough to replace mechanical ventilation for patients suffering acute respiratory failure, it

Comparative structure-function studies within a strongly dimorphic plant, Melocactus intortus (Cactaceae)

Melocactus intortus is a species of strongly dimorphic plants whose different parts show strong correlations between structure and function. Each individual has one unbranched monopodial shoot; during the first several years of life, this grows as a photosynthetic juvenile with a transparent epidermis, numerous stomata and a system of cortical bundles that permeate the chlorophyllous outer cortex. Cortical bundles undergo secondary growth. The xylem of the stele contains wide vessel elements, vascular tracheids and fibres, resulting in a wood that is probably relatively strong and efficient at water conduction. After several years, the plant undergoes a transition to being adult and all further shoot elongation growth results in a region of body with different anatomy and which is capable of flowering, the cephalium. The epidermis is thin, lacks stomata and is soon converted to a cork cambium. The cortex is nonphotosynthetic and cortical bundles do not have secondary growth. The wood of the adult portion contains only narrow vessels and parenchyma. The juvenile and adult portions of each shoot have distinct functions as well as distinct requirements for gas exchange; water and nutrient transport; and mechanical strength. When interpreting the selective value of the various anatomical features, it is critically important to consider the specific role and requirements of the particular portion of the plant in which they occur.

The Taxonomic Status of the Small Ground-Finch, Geospiza (Aves: Emberizidae) of Genovesa Island, Galápagos, and Its Relevance to Interspecific Competition

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