A new microcasting method is being developed for the study of plant vascular organization. It consists in injecting a low viscosity silicon elastomer in xylem vessel lumina, and in extracting the reticulate cast by partial or complete destruction of cellular walls.
This method allows easy visualization of vessels over a wide range (up to 500 mm) with a surface detail resolution better than 0,1mm. Examination of long cell files casts in their original mutual arrangement makes the following observations and measurements easy to carry out on a statistical basis:
Ordering and diameter of vessel elements, particularly at vessel ends.
Localization of vessel element series whose perforations are abnormal, such as lateral perforations.
Vessel ends: large springwood vessels are not isodiamedric over their entire length: at both ends, several tens of cells have diameters increasing from the minimal diameter of the extreme vessel members (1020 mm) to the mean vessel diameter (100200mm or more). Such an observation suggests two contradictory hypothesis concerning vessel formation as finite functional units:
both extreme cells (only one perforation) are early determined as intended vessel ends.
both extreme cells become vessel ends as limit-points of a dynamic process (turgescence) acropetally and basipetally extending in a cell file.
Lateral perforations: at the level of the nodes of a stem, numerous cauline secondary vessels, bordering leaf and branch gaps, frequently have extensive deformations, compared with their internodal segments: zigzag course of the vessels at the upper part of the gaps, laterally perforated sharp-pointed vessel elements, joined together in very irregular arrangements. Are such deformations the exact image of former cambial cells and the trace of mechanical constraints, due to the growing of petioles and buds?
Key words: xylem, vessel element, casting.
Correspondence: Jean-Pierre André, INRA-Agronomie, 45 Blvd du Cap - BP 2078, 06606 Antibes, France
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