Descripteur : fibrous dorsal cap centrifugal differentiation
Type du descripteur : categorical
This character is unique to palms and is quite delicate to interpret. Indeed, it could be observed in successive TS of the same Mauritia-Type stem that there is a centrifugal differentiation of the fibrous dorsal cap. Thus in the zone of transition, it appears that the few proximal fibres (the initial fibres near the phloem) are fully developed and strong lignified whereas the marginal cells of the fibrous dorsal cap are abruptly demarked from the proximal, without lignification and with a TS appearance approaching that of parenchymatous cells. In subcortical zone, most of the cells of the fibrous dorsal cap are lignified and present only a single or a bi-layered wall with a TS appearance approaching that of fibrous cells but the marginal cells are always elongate and aligned in a radial plane passing through the fvb centre. We interpret this as follow: during the growth, a change in cell differentiation occurs in the marginal (peripheral) zone of the fibrous part depending on the radial position of the fvb. When fvb are formed in the apex, the cells synthesized in the marginal zone of the fibrous part are parenchymatous. But later in the stem growth, at the transitional and subcortical zone, these cells synthesized in this marginal zone undergo a different cell differentiation: more the fvb are near the cortex, more these marginal cells are lignified until they become elongated fibres-like, an intermediate state between sclereids and fibres. This explains the previously described phytoliths distribution. Indeed, at the periphery of a fvb, cell differentiation is no constant throughout the longitudinal stem axis depending on its radial position. Therefore these marginal fibres-like are not well developed in the subcortical and transitional zones, thus stegmata are almost always absent and phytoliths are not developed. Sometimes, in the transitional zone, phytoliths are very few and only found between the proximal fibres and the sclerenchymatous cells, the future marginal fibres-like cells. It is for the same reason that no para-fibro vascular bundle parenchyma is observed in the Mauritia-Type stems: this parenchyma is always removed by fibres-like cells.
centrifugal differentiation of the dorsal cap
- present
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[Legend of the figure]
Centrifugal differentiation of the dorsal cap / stretching of fibrous cells.
The 3 regions of the dorsal cap in the zone of transition in Iriartella setigera; a: the proximal showing well differentiated fibres; b: the medium showing parenchymatous cells with slight lignification; c: the distal showing parenchymatous cells associated to the dorsal cap and “becoming” above as in b then in a, thus enlarging the fibrous part, unstained TS with polarized light.
present
present
present
present
- absent
-
absent
absent
