Leaf-trace Sentence Examples
In a few cases the hydrom strand is continued into the cortex of the stem as a leaf-trace bundle (the anatomically demonstrable trace of the leaf in the stem).
This bundle is continued down into the cortex of the stem as a leaf-trace, and passing very slowly through the sclernchymatous external cortex and the parenchymatous, starchy internal cortex to join the central cylinder.
In other cases the leaf-gaps are very broad and long, the meristeles separating them being reduced to comparatively slender strands, while there is present in each gap a network of fine vascular threads, some of which run out to the leaf, while others form cross-connections between these leaf-trace strands and also with the main cauline meristeles.
The vascular supply of the leaf (leaf-trace) consists of a single strand only in the haplostelic and some of the more primitive siphonostelic forms. In the microphyllous groups Leaf.trace of Pteridophytes (Lycopodiales and Equisetales) in and Petlolar which the leaves are small relatively to the stem, the Strands, single bundle destined for each leaf is a small strand whose departure causes no disturbance in the cauline stele.
In the haplostelic ferns the leaf-trace appears as a single strand with a tendency to assume the shape of a horseshoe on cross-section, and this type is also found in the more primitive solenostelic types.Advertisement
In the more highly developed lorms, as already indicated, the leaf-trace is split up into a number of strands which leave the base and sides of the leaf-gap independently.
The leaf trace of any given leaf rarely consists of a single bundle only (unifascicular); the number of bundles of any given trace is always odd; they may either be situated all together before they leave the stele or they may be distributed at intervals round 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).
Each double leaf-trace passes through four internodes before becoming a part of the stele; the double nature of the trace is a characteristic feature.
The vascular bundle entering the stern from a leaf with a single vein passes by a more or less direct course into the central cylinder of the stem, and does not assume the girdle-like form characteristic of the cycadean leaf-trace.Advertisement
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.
That these three sterile segments, with their sporangiophores, are together comparable to one of the bracts of Sphenophyllum, with its sporangiophores, is shown by the vascular supply in each case being derived from a single leaf-trace, So far as is at present known, the Sphenophyllales were homosporous.
Two leaf-trace bundles started from each angle of the stele, and forked, in passing through the cortex, to supply the veins of the leaf, or its subdivisions.
A single leaf-trace, usually collateral in structure, passed out into each leaf.
Coal Measures, is now the best-known of all Palaeozoic plants, the central wood has disappeared altogether and is replaced by pith; the primary wood is only represented in the leaf-trace strands, which form a ring of distinct collateral bundles around the pith; (From a model after Oliver.) FIG.Advertisement