. The classification of flowering plants. Plants. 52 FLOWERING PLANTS [CH. of the foliar organs and of the peduncles is therefore a primitive character and an indication of the antiquity of the group. Associated with the bundles both in the cotyledons and subsequent leaves, transfusion tissue is found spreading from the centripetally formed wood round the sides of the bundle to the phloem. WorsdelP has pointed out that this may be regarded as an extension of the centripetal wood and that its occurrence in Conifers indicates the derivation of the collateral bundle in those leaves from an origin
. The classification of flowering plants. Plants. 52 FLOWERING PLANTS [CH. of the foliar organs and of the peduncles is therefore a primitive character and an indication of the antiquity of the group. Associated with the bundles both in the cotyledons and subsequent leaves, transfusion tissue is found spreading from the centripetally formed wood round the sides of the bundle to the phloem. WorsdelP has pointed out that this may be regarded as an extension of the centripetal wood and that its occurrence in Conifers indicates the derivation of the collateral bundle in those leaves from an originally mesarch arrangement. From a physiological point of view the transfusion tissue takes the place of the finer branchings of the reticulate venation in the Angiosperms, ensuring an efficient water-conducting system in the leaves. Fig. 5. A—D. Stangeria paradoxa. (After Lang.) A, longitudinal section of young microsporangium shewing two of the four cells (indicated by x) forming the archesporial plate. B, a more ad- vanced stage, each cell of the plate has divided into an outer wall-cell and an inner cell which has again divided to form two sporogenous cells (shaded). C, mature microsporangium containing microspores, and shewing the remains of the tapetum {t) and the crushed wall-cells (tv). A, B x 133 ; C X 33. D, longitudinal section of ovule, X 5; p, pollen-chamber ; e, embryo-sac. E, mesarch bundle of leaf of Cycas ; p (in figure) protoxylem; a, centripetal xylem; c indicates a tracheid of the centrifugal xylem; h, phloem; e, crushed elements of the protophloem. The Cycads are dioecious. The sporophylls are arranged spirally, generally in large numbers, and on cones which terminate the growth of the axis which bears them. The stamens, which are closely crowded, are flat as in Cycas (fig. 4, D) or peltate as in Zainia (fig. 3, A), the latter resembling in shape the sporophylls of Equisetum. They bear on the under surface numerous, rarely few, pollen-sacs (microsporangia),
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectplants, bookyear1904
