The stele is called monarch, diarch,.
Or many protoxylems. When the protoxylem strands are situated at the periphery of the stele, abutting on the pericycle, as in all roots, and many of the more primitive Pteridophyte stems, the stele is said to be exarch.
When there is a single protoxylem strand in the centre of the stele, or when, as is more commonly the case, there are several protoxylem strands situated at the internal limit of the xylem,, the centre of the stem being occupied by parenchyma, the stele is endarch.
When the protoxylems have an intermediate position the stele is inesarch (many Pteridophytes and some of the more primitive Phanerogams).
As the primitive stele of a Pteridophyte is traced upwards from the primary rout into the stem, the phloem becomes continuous round the xylem.
In the majority of ferns, at a higher level, after the stele has increased greatly in diameter, a large-celled true pith or medulla, resembling the cortex in its characters, and quite distinct from conjunctive, from which it is separated by an internal endodernlis, appears in the centre.
Ftc. 3.Dlarch stele of root of a F stem of young Fern.
Such a break is known as a leaf-gap. A little above the departure of the leaf-bundle the stele again closes up only to be again broken by the departure of the next leaf-bundle.
In such cases the vascular system is said to be polycyclic in contrast with the ordinary monocyclic condition, These internal strands or cylinders are to be regarded as peculiar types of elaboration of the stele, and probably act as reservoirs for water-storage which can be drawn upon when the water supply from the root is deficient.
In other species, however, a peculiar type of polystely is met with, in which the original diarch stele gives rise to se-called dorsal and ventral stelar cords which at first lie on the surface of the primary stele, but eventually at a higher level separate from it and form distinct secondary steles resembling the primary one.
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 stele of Equisetum is of a very peculiar type whose relations are not completely clear.
The whole stele may be surrounded by a common external endodermis; sometimes there is an internal endodermis in addition, separating the bundles from the pith; while in other cases each bundle possesses a separate endodermis surrounding it.
It is probable that this type of stele is a modification of a primitive protostele, in which the main mass of stelar xylem has become much reduced and incidentally separated from the leaftraces.
The Stele In But, unlike the ferns, there is in the seed-plants no in- s d I ~ ternal phloem (except as a special development in ee pan $~$~ certain families) and no internal endodermis.
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.
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.
The bundles sometimes keep their arrangement s v in a ring corresponding with the stele, though the continuous cylin 0 der no longer exists (species of - Ranunculus).
Annuiar vessel, superficially obvious order through & I,~iterceiiuIar canal, the conjunctive tissue of the stele, 1.
The conjunctive of a root-stele possessing a pith is often sclerized between the pith and the pericycle.
Sometimes all the parenchyma within the stele undergoes this change.
In other cases, however, continuous primitive stele is developed, extending from the primar stem to the primary root, the leaf-traces arising later.
In cases where the development of the embryo is advanced at the resting period, traces run from the cotyledons and determine the symmetry of the stele of the primitive axis, the upperpart of which often shows stem-structure, in some respects at least, and is called the hypocoty- ledonary stem or hypocotyl, while the lower part is the primary root .~-,
In other cases the root structure of the stele continues up to the cotyledonary node, though the hypocotyl is still to be distinguished from the primary root by the character of its epidermis.
In these the stele becomes obvious in transverse section at about the same level as that at which the first leaf-traces are developed.
The differentiation of metaxylem follows according to the type of root-stele, and, finally, any stereom there may be is developed.
The connections of its stele witl that of the parent axis are made across the pericycle of the latter Its cortex is never in connection with the cortex of the parent, but with its pericycle.
The formation of additional cambial cylinders or bands occurs in the most various families of Dicotyledons and in some Gymnosperms. They may arise in the pericycle or endocycle of the stele, in the cortex of the stem, or in the parenchyma of the secondary xylem or phloem.
This was not only in itself an important contribution to plant anatomy, but served as the starting-point of a series of researches by Van Tieghem and his pupils, which has considerably advanced our knowledge of the details of histology, and also culminated in the foundation of the doctrine of the stele (Van Tieghem and Douliot, Sur la polystlie, Ann.
The great turgidity which is thus caused exerts a considerable hydrostatic pressure on the stele of the root, the vessels of the wood of which are sometimes filled with water, but at other times contain air, and this often under a pressure less than the ordinary atmospheric pressure.
This pressure of the turgid cortex on the central stele is known as root pressure, and is of very considerable amount.
It is a basalt stele inscribed in hieroglyphic, demotic and Greek with a decree of the priests assembled at Memphis in favour of Ptolemy V.
- Naram-Sin on the Stele of Victory.
The so-called " Stele of the Vultures," now in the Louvre, was erected as a monument of the victory.
Stele Of Victory Of Naram-Sin, King Of Agade.
Sculpture From The Stele Engraved With Kiiammurabi'S Code Of Laws.
With the growth of scientific geography they came to be located somewhat less vaguely, and indeed their name was employed as the equivalent of the Assyrian and Hebrew Cush, the Kesh or Ekosh of the Hieroglyphics (first found in Stele of Senwosri I.), i.e.
Merawi) at the foot of Jebel Barkal, "the sacred mountain," which in time became formidable, and in the middle of the 8th century conquered Egypt; an Egyptian campaign is recorded in the famous stele of King Pankhi.
The pericycle and mesocycle together form the conjunctive tissue of the stele in these simplest types.
When the diameter of the stele is greater, parenchymatous conjunctive tissue often occupies its centre and is frequently called the pith.
Such a vascular cylinder is called a haplostele, and the axis containing it is said to be haplostelic. In the stele of the root the strands of tracheids along the lines where the xylem touches the pericycle are spiral or annular, and are the xylem elements first formed when the cylinder is developing.
As the stele is traced farther upwards it becomes bulkier, as do the successive leaf-bundles which leave it.
The centre of the S~hooo- stele is however often occupied by a large-celled pith resembling the cortex in structure, the cortex and pith ~ together being classed as ground tissue.
In this use the term loses, of course, its morphoI logical value, and it is better to call such a segment of a broken-up I stele a meristele, the whole solenostele with overlapping leaf-gaps being called a dictyostele.
In the stems of many water-plants various stages of reduction of the vascular system, especially of the xylem, are met with, and very often this reduction leads to the formation of a compact stele in which the individuality of the separate Reduced bundles may be suppressed, so that a closed cylinder lmpbost~h1c of xylem surrounds a pith.
The structure of the stele of the primary Fin.
Developed at its base, usually becoming continuous with the cylinde of the root; the strand of the second leaf is formed in a similar wa~ and runs down to join that of the first, so that the stem stele is forme.
It is possible to suppose that this condition is derived from the astelic condition already referred to, but the evidence on the whole leads to the conclusion that it has ansen byan increase in the number of the bundles within the stele, the individuality of the bundle asserting itself after its escape from the original bundle-ring of the primitive cylinder.
I 7.Transverse section of the stele of the stem of a water-plant (Naias);