414
ANATOMY
of the polystelic Pteridophyte with its tendency to radial organization, and the unipolar wedge-shaped “bundle” of the Dicotyledon usually with its cap or sheath of peridesmic stereom, depending on the different characters of the siphonostele from which the two types have been respectively evolved, though
OF
PLANTS
striking and characteristic enough, are of comparatively minor importance. The characteristic type of arrangement of vascular strands found in monocotyledonous stems depends on a similar extreme assertion of the individuality of the single leaf-trace bundle, combined with an often enormous increase in the number
Fig. 3-
Fig. 2.
7*
Fig. 4-
Fig. 5of bundles present at any given level. The result is a dilation of the stele till it occupies the whole or nearly the whole of the axis, the coitex being reduced to a narrow cylinder or disappearing altogether. The pericycle usually forms a sclerized band at or near the periphery of the stem, and the leaf-trace bundles are scattered in a definite though not superficially obvious order through a general parenchymatous ground tissue, each bundle being commonly capped or surrounded by its own sheath of stereom. In a few cases among the Phanerogams (e.g., Auricula)
Fig. 9. Figs. 2-9.—Diagrams of the Main Types of Arrangement of the Stelar Tissues in Pteridophytes and Phanerogams (as seen in transverse section). The xylem is cross-hatched, the phloem shaded diagonally, protoxylem and protophloem shaded darkly, parenchymatous conjunctive tissue leit white, sclerenchyma dotted. Fig. 2.—Diarch root-stele, with axial metaxylem plate joining the two protoxylems ; the commonest type of primary root-stele. Fig. 3.—Solid exarch protostele (represented as pentarch) : base of most Pteridophyte stems, and throughout stem of more primitive forms. Fig. 4.—Typical siphonostele (represented as mesarch) with internal phloem, broken by departure of a meristelic leaf-tree on the right. Characteristic of base of stem in most ferns, and of whole axis in many. Fig. 5.—Simple dialystelic type; mesarch siphonostele represented as broken by two opposite leaf-gaps (right and left), each containing three meristelic leaf-traces, simulating protosteles and usually known as steles. Characteristic of the stems of most of the higher ferns. Fig. 6.—Modified siphonostele with collateral endarch bundles separated laterally by conjunctive tissue. The regions of the latter outside the phloems are sclerized, as is usually the case. Characteristic of the dicotyledonous stem. Fig. 7.—Separate collateral endarch bundle of anastelic form surrounded by its peridesm which may be partly or wholly sclerized. Xylem and phloem separated by mesodesm. Some dicotyledonous stems. Fig. 8.—Meristelic arc of collateral bundles, united by conjunctive. Common in petioles of Dicotyledons. Fig. 9.—Large polyarch root-siphonostele, showing radial arrangement of primary xylem and phloem in alternate strands, and also additional internal metaxylem, mostly in the form of large vessels. Found in many Monocotyledons. Most roots show some form intermediate between this and that represented in Fig. 2 a polystelic condition originates secondarily in evolution by the association of supplementary internal inversely oriented bundles with detached arcs of the original stele. This is another method of increasing the vascular supply to the leaves when the needs of these exceed the capacity of the original dilated and broken central cylinder. In other cases polystely may originate in a completely astelic type by the association of bundles round common centres (Nymphcea). In the foregoing description of the evolution of the stelar
