Organography of plants, especially of the archegoniatae and spermaphyta . ates thickened onlyin their middle portion. The chief axis, on the other hand, whose function ispartly a mechanical one, partly that of conveying nourishment \ has almosta cylindric outline on cross-section, although there is a slight flatteningvisible upon the upper and the under sides ^. Whereas in Aneura hymeno- phylloides (Fig. 47) and stronger mechanicalclaims are made upon thechief axis than upon thelateral axes, more of its cellsexhibit thickened walls (), and the difference be-tween the two axes
Organography of plants, especially of the archegoniatae and spermaphyta . ates thickened onlyin their middle portion. The chief axis, on the other hand, whose function ispartly a mechanical one, partly that of conveying nourishment \ has almosta cylindric outline on cross-section, although there is a slight flatteningvisible upon the upper and the under sides ^. Whereas in Aneura hymeno- phylloides (Fig. 47) and stronger mechanicalclaims are made upon thechief axis than upon thelateral axes, more of its cellsexhibit thickened walls (), and the difference be-tween the two axes is there-fore greater. In other words,starting from a thallus withthroughout similar branch-ing (Fig. 12), a progressivedifferentiation into stem andleaf appears, and we areable clearly to follow itsevolution. These species ofAneura possess also are forms which no longer lie with the whole under-surface upon thesubstratum, but which fasten themselves to it by means of special anchoring-organs (Figs. 21, 23). These anchoring organs are distinguished from the. Fig. 22. Aneura fucoides. Upper figure; chief shoot intransverse section. Lower figure; lateral shoot in transverseHighly magnified. section. ^ See Part I, p. 34. The differences between the chief and lateral shoots in the species of Aneura are brought aboutthrough the suppression/rc^w the first of the formation of the wing on the chief axis, and throughthe assimilation-shoots in the middle region of the thallus undergoing only few divisions. There areof course transitions, that is to say, forms in which the difference between the chief axis and thelateral axis is simply one of the greater thickness of the former. Stephanis statement, in Hedwigia,xxii (1893), p. 12, that the thin membranous wing often thickens as it gets older until it becomesa stalk with a cylindric cross-section is, so far as I have observed, an error. The same author says(Colensos New Zealand Hepaticae, in Journal of the Linnean Society, Bota
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