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 413 OF PLANTS parenchymatous cells, the pcricyclc, and the xylem is separated shaped or semilunar, sometimes isodiametric in cross-section 5). Each segment of the siphonostele frequently resembles from the phloem in the stem by a similar layer, the mcsocycle (Fig. protostele, the segments of inner endodermis, pericycle, phloem, (corresponding with the hydrom sheath in mosses). _ The peri- aand joining with the corresponding outer segments to cycle and mesocycle together form the conjunctive tissue of the formmesocycle a concentric structure. For this reason a stem m vhich the stele. In the root the mesocycle, like the phloem, is interrupted, vascular has this type of structure is spoken of as polyand runs into the pericycle where the xylem touches the latter stelic, thesystem term “stele” being transferred from the primary (Fig. 2). The whole cylinder is enclosed by the peculiarly central cylinder the axis and applied to the vascular strands dilferentiated innermost cell-layer of the cortex, known as the just described. of In this use the term loses, of course, its morphoendodermis. This layer has its cells closely united and sealed to logical value. Where the “ steles” are few and band-shaped on one another, so to speak, by the conversion of the radial and cross-section, the structure is spoken of as “gamostehc ; where transverse walls (which separate each cell from the other cells of they are numerous, and run independently for considerable c isthe layer), or a band running in the centre of these, into corky tances, it is “ dialystelic. ” The splitting up of the vascular tube substance, so that the endodermal cells cannot be split apart to or siphonostele into separate strands not depend wholly upon admit of the formation of intercellular spaces, and an air-tight the occurrence of leaf-gaps. In the does dialystelic forms sheath is formed round the cylinder. Such a vasciilar cylinder is the leaf-gaps are very broad and long,extremely and are filled by a networK called a protostele, and the axis containing it is said to be proto- of slender strands, some of which run out to the leaf, while others monostelic. In the stele of the root the strands of tracheids along cross connexions between these and with the strands the lines where the xylem touches the pericycle are spiral or an- form separating the gaps, which, though stouter the leaf-gap nular, and are the xylem elements first formed when the cylinder steles, are reduced to comparatively narrow than bands. In some is developing. Ea-cli strand of spiral or annular fiist- foinied cases the strands separating the gaps are themselves split up, tracheids is called a protoxylem strand, as distinct from the further increasing the dialystely. Finally, the original dialystelic metaxylem or rest of the xylem, which consists of thick-walled cylinder may be supplemented by additional internal or external tracheids, the pits of which are usually scalariform. _ The stele strands or both, joining by frequent connexions the primary ones. is called monarch, diarch,. . . polyarch according as it contains The leaf-trace consists of a single strand only m some ot the one, two, ... or many protoxylems. When the protoxylem protostelic and more primitive siphonostelic forms, in the latter strands are situated at the periphery of the stele, abutting on the (where it is simply a detached segment ot the siphonostele) ha ing pericycle, as in all roots, and many of the more primitive Pterido- the shape of a crescent or horse-shoe in cross-section with the phyte stems, the stele is said to be exarch. When there is a convexity outwards, and in the petiole downwards.. In the more single protoxylem strand in the centre of the stele, or when, as is forms, as already indicated, the trace is itS(m SP*^JjP more commonly the case, there are several protoxylem strands complex into a number of strands which leave the base and sides ol the situated at the internal limit of the xylem, the centre of the stem independently. In the petiole these may increase m beino- occupied by parenchyma, the stele is endarch. This is the leaf-gap by branching and sometimes form a complicated system casein the stems of most Phanerogams and a few Ptendophytes. number resembling that of the stem, but usually reducible to the primiWhen the protoxylems have an intermediate position the stele is tive bilateral horse-shoe type. The protostelic, siphonostelic, mcsarch (many Pteridophytes and some of the more primitive and “polystelic” conditions are mainly found among the 1 tenPhanerogams). In many cases external protophloem, usually con- dophytes. There is good reason to suppose that the protostelic consistino- of narrow sieve-tubes often with swollen walls, can be dition is primitive in evolution. Essentially the same type was distinguished from metaphloem. As the primitive stele of an found in the primitive conducting system of thegametophyteof tne exarch Pteridophyte is traced upwards from the primary root into mosses. IS o doubt the siphonostele came next, sometimes perhaps the stem, the phloem and mesocycle become continuous round the in relation to the necessity for the formation of a hard peripheral xvlem At the same time the stele becomes more bulky, all its supporting cylinder, but often owing to factors not yet understood, elements increasing in number. Very frequently the number of and by the crowding of the leaf-gaps and other factors the polyprotoxylem strands increases by branching of the original ones stelic ” state was reached. Thus in tracing the stelar system ol a that have come up from the root. Soon a bundle goes off to the “polystelic” fern, for instance, upwards from the primary root, first leaf. This consists of a protoxylem with a few metaxylem the various stages which have led to the evolution ot this type elements, a segment of mesocycle, phloem, pericycle, and usually are passed through. In a few Pteridophytes and the great majority an arc of endodermis, which closes round the bundle as it detaches of the Phanerogams, the protostele of the primary root dilates itself from the stele. Thus such a leaf-bundle contains parts ot either immediately upon entering the stem, or at a higher level, all the tissues of the stele, and is hence called a menstele. It is acquires a true medulla, and the vascular cylinder breaks into a primitively bilaterally symmetrical like the leaf it supphes number of separate collateral bundles (i.c., with xylem to wares though it may acquire a radial symmetry of its own, while the the centre and phloem towards the periphery of the stem) which stele of the stem is primitively radially symmetrical like the stem pass out of the cylinder to supply the leaves after having given itself As the stele is traced farther upwards it becomes bulkier, off branches {continuation bundles) which continue their couise in as do'the successive leaf-bundles which leave it. In many Pten- the cylinder of the stem. The lowest collateral bundles oi t le dophytes the solid protostele is maintained throughout the axis. typical Phanerogamic stem are frequently directly continuous In others a central parenchyma or primitive pith-a, newregion with the (often separate) strands forming the stele of the primary of the primitive stelar conjunctive—appears in the centre oi the root so that no siphonostelic condition of the type alwa,ys found xylem. In many ferns internal phloem appears associated with near the base of the stem of a higher Pteridophyte is passed this primitive pith; and at a higher level, after the stele has through. But the collateral bundles of the stem stele are usually increased greatly in diameter, a large-celled true pith ox medulla united laterally by conjunctive tissue, so that a, hollow cylinder resembling the cortex in its characters and usually quite distinct of stelar tissue is maintained, and the pith is isolated from the from conjunctive, appears in the centre. This is often separated cortex except at the leaf-gaps. This modified siphonostele is from the conjunctive by an internal endodermis. V here internal very characteristic of the Phanerogamic stem. In the great phloem is present this is separated from the internal endodermis majority of cases there is a fairly wide pericycle outside the by an endocycle or “internal pericycle,” as it is sometimes called, phloem, and this is partially or wholly converted into sclerenami from the xylem by an internal mesocycZe-these tv o layers chyma, usually of the prosenchymatous (fibrous) type, in this together with the outer mesocycle and pericycle constitutm the way an efficient peripheral supporting cylinder is secured, and it conjunctive tissue of the now hollow cylindrical stele (The con is no doubt largely owing to the need for this mechanical support that the modified siphonostele is maintained in the Phaneio3inactive frequently forms a connected whole with bands ot starchy ^uTparlchy,i, which, when the ylem “ ^ gamic stem. As it is, the single collateral bundle often asserts appear among the tracheids, the phloem often being its individuality while still within the stele, the conjunctive penetrated by similar bands of phloem - parenchyma.) Io this being obviously arranged round them as centres—the sclerized tvne of stele whether with or without internal phoem, is pericycle, for instance, taking the form of a number of bands, given the name siphonostele to distinguish it h'om thc solid arc-shaped in cross-section, capping the phloems of the bundles, Irotostele characteristic of the root, of !he.,fivf' and the cortex and medulla frequently intruding between two tion of the stem, and in the more primitive Pteridophytes adjacent ones (Fig. 6). In some cases, indeed, the modified of the whole of the axis. The siphonostele is, as it is found siphonostele breaks up altogether, the cortex and medulla among many of the lower ferns, broken by the departure becoming continuous between the bundles, each of which is surof a leaf-bundle, the outer and inner endodermis joining so that rounded by its own investment of tissue corresponding with contL sWe beSils horseshoe - shaped and the cortex continuous junctive, but now called peridesm, the isolated segment of t ie with the pith (Fig. 4). Such a break is known as aleaj-gap. mesocycle being known as mesodesm. This is the condition ot A little abPove the departure of the leaf bundle the stele again astely, entirely parallel with polystely except that the Astely. closes up up, only to be again broken loaTes anduext the separate strands are usually all or mostly leaf-traces. by the are departureded of the The difference between the concentric and the collateral structure PoI “ y phyllotaxy is high, a given leaf-gap is not closed he fore of the strands, depending mainly on the presence or a sence „ stely-’’ tlm next ones appear, and the siphonostele thus internal phloem, and the difference between the rounded s becomes split up into a number of segments, sometimes bandANATOMY