. Heredity and evolution in plants . relength. The lines indicate the planes of various sections through the cone,published in Wielands American Fossil Cycads. To the right Cycado-cephalus Sewardi Nathorst. Microsporangiate cone, natural size, preservedas an impression on a flat slab. From a fossil-bearing bed of the Trias, atBjuf, Southern Sweden. (Left figure from Wieland, right figure fromNathorst.) closely a plant may resemble both a cycadophyte and a a sense this plant may be called a living fossil. Speci-mens have since come into flower in botanic gardens, andthe typical cycadace
. Heredity and evolution in plants . relength. The lines indicate the planes of various sections through the cone,published in Wielands American Fossil Cycads. To the right Cycado-cephalus Sewardi Nathorst. Microsporangiate cone, natural size, preservedas an impression on a flat slab. From a fossil-bearing bed of the Trias, atBjuf, Southern Sweden. (Left figure from Wieland, right figure fromNathorst.) closely a plant may resemble both a cycadophyte and a a sense this plant may be called a living fossil. Speci-mens have since come into flower in botanic gardens, andthe typical cycadaceous cones (Fig. 91) leave no doubtthat the plant is a true cycadophyte. THE EVOLUTION OP PLANTS 2OQ 142. Derivation of New —Attention should hereagain be called to the fact that the theory of evolution doesnot teach that one given species becomes transformed intoanother, but simply that new species are descended fromolder forms which may or may not continue to exist. Itis not supposed, for example, that ferns developed into. FIG. —Cycadcoidea dacotensis. Semi-diagrammatic sketch of aflower (bisporangiate cone), cut longitudinally; one sporophyll folded, andone (at the right) arbitrarily expanded. At the center is the apical, cone-shaped receptacle, invested by a zone of short-stalked ovules and inter-seminal scales. The pinnules of the sporophylls bear the compoundsporangia (Synangia). Exterior to the flower are several hairy three-fourths natural size. (After Wieland.) cycads, and cycads into higher gymnosperms, but thatthere has been an unbroken line of descent (possibly morethan one) in the plant kingdom, that closely related forms(like ferns and cycads) have descended from a commonancestral type which may or may not now be found. Wemust not, in other words, expect necessarily to find in 210 JIKKKDITY AM) EVOLUTION IN I!. \\ !s fossil forms the direct ancestors of those now living, althougha study of their structure is of the greatest value in ena
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