How to use Leaf-traces in a sentence
The gaps in the outer tubular stele, however, are formed by the departure of aerial branch-traces, instead of leaf-traces as in the ferns.
The remaining bundles (compensation bundles) which go to make up the cylinder are such as have branched off from the leaf-traces, and will, after joining with others similarly given off, themselves form the traces of leaves situated at a higher level on the stem.
The constitution of the stele of a flowering plant entirely from endarch collateral bundles, which are either themselves leaf-traces or will form leaf-traces after junction with other similar bundles, is the great characteristic of the stem-stele of flowering plants.
In other cases, however, continuous primitive stele is developed, extending from the primar stem to the primary root, the leaf-traces arising later.
In these the stele becomes obvious in transverse section at about the same level as that at which the first leaf-traces are developed.Advertisement
The complicated girdle-like course is characteristic of the leaf-traces of most recent cycads, but in some cases, e.g.
This is of interest from the point of view of the comparison of recent cycads with extinct species (Bennettites), in which the leaf-traces follow a much more direct course than in modern cycads.
Close to the apex of a shoot the vascular bundles of a leaf make their appearance as double strands, and the leaf-traces in the upper part of a shoot have the form of distinct bundles, which in the older part of the shoot form a continuous ring.
Araucaria, the leaf-traces persist for a considerable time, perhaps indefinitely, and may be seen in tangential sections of the wood of old stems. The leaf-trace in the Coniferales is simple in its course through the stem, differing in this respect from the double leaf-trace of Ginkgo.
U1 - U', leaf-traces, numbered according to the phyllotaxis, It 5 belonging to the lowest leaf of the five; ph, a group of primary phloem; pd, periderm, formed from pericycle.Advertisement
Its outer margin is crenulated, the leaf-traces being given off from the middle of each bay.
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 leaf-traces can be distinguished as distinct strands at the periphery of the stele, as shown in fig.
Heterangium shows, on the whole, a decided preponderance of Filicinean vegetative characters, though in the leaf-traces and the secondary tissues the Cycads are approached.
In most cases the leaf-traces passed out from the stem in pairs, as in the recent Ginkgo; dividing up further as they entered the leaf-base.Advertisement
Among existing types the genus Macrozamia appears to show the nearest approach to this simpler structure of the leaf-traces.
In a Floridan species of Zamia the leaf-traces are described as characterized by a more direct course from the stele of the stem to the leaves than in most modern genera, thus agreeing more closely with the extinct Bennettites.
The latter is produced (except in cases of complete astely where a cylinder is never formed) after a number of leaf-traces have appeared on different sides of the stem so as to form a circle as seen in transverse section, the spaces intervening between adjacent bundles becoming bridged by small-celled tissue closing the cylinder.