Allan Witztum

On the strength, stiffness and stability of tubular plant stems and leaves

thin walled tubes of circular cross-section are efficient structural elements and thus, as would be expected, are not uncommon in plants, especially those which belong to the Monocotyledonae. Traditional analyses of the strength of these tubular members utilize formulae which were developed for isotropic materials. The present paper deals with the great influence of the high elastic anisotropy of the plant tissue on the mechanical behavior of such tubular stems and leaves in bending. It will be seen that under these circumstances the propensity of the tube to fail due to non-linear effects (deformation of the cross-section) is greatly increased.

Fibre cables in the lacunae of Typha leaves contribute to a tensegrity structure

BACKGROUND AND AIMS Cables composed of long, non-lignified fibre cells enclosed in a cover of much shorter thin-walled, crystal-containing cells traverse the air chambers (lacunae) in leaves of the taller species of Typha. The non-lignified fibre cables are anchored in diaphragms composed of stellate cells of aerenchyma tissue that segment the long air chambers into smaller compartments. Although the fibre cables are easily observed and can be pulled free from the porous-to-air diaphragms, their structure and function have been ignored or misinterpreted. METHODS Leaves of various species of Typha were dissected and fibre cables were pulled free and observed with a microscope using bright-field and polarizing optics. Maximal tensile strength of freshly removed cables was measured by hanging weights from fibre cables, and Instron analysis was used to produce curves of load versus extension until cables broke. KEY RESULTS AND CONCLUSIONS Polarized light microscopy revealed that the cellulose microfibrils that make up the walls of the cable fibres are oriented parallel to the long axis of the fibres. This orientation ensures that the fibre cables are mechanically stiff and strong under tension. Accordingly, the measured stiffness and tensile strength of the fibre cables were in the gigapascal range. In combination with the dorsal and ventral leaf surfaces and partitions that contain lignified fibre bundles and vascular strands that are strong in compression, the very fine fibre cables that are strong under tension form a tensegrity structure. The tensegrity structure creates multiple load paths through which stresses are redistributed throughout the 1-3 m tall upright leaves of Typha angustifolia, T. latifolia, T. × glauca, T. domingensis and T. shuttleworthii. The length of the fibre cables relative to the length of the leaf blades is reduced in the last-formed leaves of flowering individuals. Fibre cables are absent in the shorter leaves of Typha minima and, if present, only extend for a few centimetres from the sheath into the leaf blade of Typha laxmannii. The advantage of the structure of the Typha leaf blade, which enables stiffness to give way to flexibility under windy conditions, is discussed for both vegetative and flowering plants.

SEED DISPERSAL BALLISTICS IN BLEPHARIS CILIARIS

During drying of the bilocular capsular fruit of Blepharis ciliaris (L.) B.L. Burtt (Acanthaceae, subfamily Acanthoideae), elastic potential energy is stored in the septum that separates the two locules. When the tip of the capsule is wetted, a failure develops in the tissue seams joining the two valves of the capsule, which propagates rapidly from the apex of the capsule downwards. This converts the elastic potential energy to kinetic energy, part of which is used to launch the two seeds from the peculiar hook-like jaculators on which they sit. The ballistics of seed ejection in Blepharis ciliaris is described and related to the morphology of the plant.

Button botany: plasmodesmata in vegetable ivory

The hard endosperm of species of the palm genus Phytelephas (elephant plant), known as vegetable ivory, was used in the manufacture of buttons in the nineteenth century, the early twentieth century, and again in more recent times. Here, we show that the pathways for intercellular communication, including the cytoplasm in opposite pits and the plasmodesmata that traverse the cell wall, can be visualized in century-old inexpensive buttons that are readily available in antique shops.

Lignified and nonlignified fiber cables in the lacunae of Typha angustifolia

The leaves of Typha are noteworthy in terms of their mechanical properties. We determined the mechanical properties of the fiber cables within the leaf. We found that in vegetative plants, the lignified fiber cables isolated from the leaf sheath and nonlignified fiber cables isolated from the leaf blade of Typha angustifolia differ in their diameter, swelling capacity, Young’s modulus, tensile strength, and break load. These differing properties are related to their contributions to stability in the two regions of the leaf.

Fiber cables in leaf blades of Typha domingensis and their absence in Typha elephantina : a diagnostic character for phylogenetic affinity

Vertical fiber cables anchored in horizontal diaphragms traverse the air-filled lacunae of the tall, upright, spiraling leaf blades of Typha domingensis, T. angustifolia, T. latifolia and T. × glauca. The fiber cables may make a mechanical contribution to leaf blade stiffness while allowing flexibility under windy conditions. We examined the very tall, upright, spiraling leaf blades of T. elephantina, which can be over 4 m long, for fiber cables. In the tall species of Typha, there are two alternative architectures for upright leaf blades. T. domingensis utilizes fiber cables to enhance stiffness in the tall, upright concavo-convex leaf blades, whereas T. elephantina may maintain their tall stature in the absence of fiber cables by having a different cross-sectional geometry. These alternative architectures can be used as a diagnostic character along with other morphological characters to assess phylogenetic affinity in Typha. The very tall T. elephantina which lacks fiber cables may be more closely relat...