[Illustration: Fig. 400. _Cycas Micholitzii_. Vascular bundles in a forked pinna; _px_, protoxylem; _s_, sheath of thick-walled cells; _cf_, _cp_, centrifugal and centripetal xylem.] In the xylem portion of the bundle from the midrib of a forked pinna of _Cycas Micholitzii_ shown in fig. 400 the centrifugal xylem elements are unusually numerous: the space between the two xylems is occupied by parenchyma and the whole strand is enclosed by a sheath of crystal-containing cells, _s_, with thick inner walls. Fig. 400, 1–4, illustrates the gradual change in the form of the bundle in the region of dichotomy[87]. The ground-tissue of the petiole is abundantly supplied with secretory canals and in the hypodermal region is a cylinder of stereome. In some petioles, _e.g._ _Macrozamia heteromera_[88], the ground-tissue cells are lignified and reticulately pitted, a feature met with in some Mesozoic cycadean leaves[89]. In _Cycas media_ Worsdell noticed a tendency of the leaf-trace bundles towards a concentric arrangement and similar vascular strands are recorded in the peduncle of _Dioon edule_, in various sporophylls[90] and in other cases. It is possible, as Worsdell believes, that the fairly frequent occurrence of concentric bundles in plants characterised by collateral bundles may have a phylogenetic significance. The pinnae are dorsiventral and the veins exarch or pseudomesarch: secretory canals occur between (_Encephalartos_), above, or below the veins. The mesophyll of _Cycas_ is characterised by the presence of isolated xylem-elements passing from the midrib to the edge of the lamina and, as Lignier[91] suggests, these may be regarded as a reduced system of lateral conducting strands. The epidermal cells of the leaflets have straight or slightly curved walls except in _Stangeria_ where they are undulate and fern-like[92]. The stomata, with few exceptions confined to the lower epidermis, are larger than in other gymnosperms (on the average ·075 × ·034 mm.) and are more or less depressed below the surface; the guard-cells are usually surrounded by 4–6 subsidiary cells. The roots exhibit no feature to which attention need be called: the pericycle is several cells broad and as in the stem there may be extrafascicular cylinders of xylem and phloem. CHAPTER XXIX. PTERIDOSPERMEAE. I. LYGINOPTERIDEAE. LYGINOPTERIS.