. L'origine d'une flore terrestre, une théorie fondée sur les faits de l'alternance. La morphologie des plantes. Les membres SPORIFÈRE 377 au milieu de l'entre-nœud (Fig. 202), et dans Stachannularia Cingularia ou en haut de l'entre-noeud (Fig. 204). De tels faits, ces ici que brièvement esquissé, doit être pris en compte dans l'examen de la morphologie du strobile du Equisetales, et décider de la vrai chararacter des sporangiophores. Mais avant qu'il est renseigné sur leur structure détaillée et le développement doivent être examinés. Spore-Producing Membres. Naturellement, le développement de la spore-p

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. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. SPORE-PRODUCING MEMBERS 377 in the middle of the internode (Fig. 202), and in Stachannularia or Cingularia at the top of the internode (Fig. 204). Such facts as these, here only briefly sketched, must be taken into account in discussing the morphology of the strobilus of the Equisetales, and in deciding the true chararacter of the sporangiophores. But before this is entered upon their detailed structure and development must be examined. Spore-Producing Members. Naturally the development of the spore-producing members can only be followed in the living genus, though from the similarity of their mature features to those seen in the fossils it is probable that there was substantial similarity in these also. In Equisetum the axis, which is about to produce a strobilus, ceases active growth in length, retaining a conical form: the sporangiophores arise upon it in acro- petal order, as convex swellings (Fig. 205). The details show some varia- tion in different species : they are here described for Equisetum arvense and limosum.1 In the first stages the spor- angiophores are not unlike the sterile leaf-sheaths, involving, as seen in longitudinal section, some six cells, which grow out with a fan-like tracery and repeated anticlinal walls (Fig. 206 a). This similarity has been used as an argument favouring the view that the sporangiophore and the bract-leaf are results of " metamorphosis" of essentially the same part, a point which will be taken up later. Single superficial cells near the margins of the convex outgrowths are early recognisable as the parent cells which give rise to all the essential parts of the sporangia, though adjoining cells also grow out together with these to form the sporangial body: the origin of the sporangium is thus of the eusporangiate type (Fig. 206 a, b). At an early stage there is active growth in the middle region of the sporangiophore, wh

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