Mesarch Sentence Examples
The main events in this transition appear to have been (I) disappearance of the central xylem of the protostele and replacement by pith, leading to the survival of a number of (mesarch) collateral bundles (see below) at the periphery of the stele; (2) passage from mesarchy to endarchy of these bundles correlated with a great increase in secondary thickening of the stele.
A leaf-trace, as it passes through the cortex, has a collateral structure, the protoxylem being situated at the inner edge of the xylem; when it reaches the leaf-base the position of the spiral tracheids is gradually altered, and the endarch arrangement (protoxylem internal) gives place to a mesarch structure (protoxylem more or less central and not on the edge of the xylem strand).
Similarly in the sporophylls of some cycads the bundles are endarch near the base and mesarch near the distal end of the stamen or carpel.
Palaeozoic genera, has not entirely disappeared from the stems of modern cycads; but the mesarch bundle is now confined to the leaves and peduncles.
A point of anatomical interest is the occurrence in the vascular bundles of the cotyledons, scale-leaves, and elsewhere of a few centripetally developed tracheids, which give to the xylem-strands a mesarch structure such as characterizes the foliar bundles of cycads.Advertisement
The leaf-traces, where they traverse the cortex, have the structure of the foliar bundles in Cycads, for they are of the collateral type, and their xylem is mesarch, the spiral elements lying in the interior of the ligneous strand.
The leaves of the more primitive members of this series were entirely fern-like and possessed a fern-like vascular strand; while in the later members, including the modern Cycads, the leaf bundles, remaining unaffected by secondary thickening, are mesarch, while those of the stem-stele have become endarch.
The vascular bundles, in particular, show precisely the characteristic collateral mesarch or exarch structure which is so constant in the recent family (see Anatomy Of Plants).
The mesarch structure of the leaf-bundles is met with in a less pronounced form in the flower peduncles of some cycads.