Biomechanical model of the xylem vessels in vascular plants.

BACKGROUND AND AIMS: The xylem, or water transport system, in vascular plants adopts different morphologies that appear sequentially during growth phases. This paper proposes an explanation of these morphologies based on engineering design principles. METHODS: Using microscopic observations of the different growth stages, an engineering analysis of the xylem vessels as a closed cylinder under internal pressure is carried out adopting pressure vessel design concepts. KEY RESULTS: The analysis suggests that the xylem vessel structural morphology follows the 'constant strength' design principle, i.e. all of the material within the wall of the xylem is loaded equally to its maximum allowable stress capacity, and the amount of material used is therefore systematically minimized. The analysis shows that the different structural designs of the xylem vessel walls (annular, helical, reticulate and pitted) all quantitatively follow the constant strength design principle. CONCLUSIONS: The results are discussed with respect to growth and differentiation. It is concluded that the morphology of the xylem vessel through the different phases of growth seems to follow optimal engineering design principles.

Transfusion associated graft versus host disease in immunocompetent patients. Report of two cases.

Transfusion graft versus host disease (TGVHD) in immunocompetent patients is a recently recognized disease, reported mainly after open heart surgery, and almost always fatal. We report two cases of TGVHD in immunocompetent patients after open-heart surgery. The disease is characterized by the occurrence of fever, skin rash, liver failure and pancytopenia. Preventive measures include exclusion of first- and second-degree relatives as blood donors, and/or irradiation of blood to be transfused.