. L'origine d'une flore terrestre, une théorie fondée sur les faits de l'alternance. La morphologie des plantes. Chapitre XII. Les SPORANGIOPHORES ET sporophylles. La position théorique abordée dans le dernier chapitre est caractérisé non seulement par sa simplicité mais aussi par son élasticité. Il porte avec lui aucune obligation d'assumer que tous les organes de l'appendiculaire devrait faire preuve de précision ou de constance dans leur disposition sur l'axe qui porte tout, ni même qu'ils étaient tous pareils dans [leur caractère initial ou d'une fonction. Maintenant, comme une question de fait, un examen des cônes de telles plantes comme le Psilo- taceae

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. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. CHAPTER XII. SPORANGIOPHORES AND SPOROPHYLLS. The theoretical position taken up in the last chapter is characterised not only by its simplicity but also by its elasticity. It carries with it no obligation to assume that all the appendicular organs should show definiteness or constancy in their disposition upon the axis which bears them, nor even that they were all alike in [their initial character or function. Now, as a matter of fact, an examination of the strobili of such plants as the Psilo- taceae, Horsetails, and Calamarians shows that irregularities of arrangement of the parts are common: it is impossible to reduce the arrangement of the appendages in the cone of Tmesipteris or of Equisetum maximum to any regular scheme (Fig. 73): the appendages of both vary in radial angle and in level of insertion. There is also great variability in the disposition of the leaves in the genus Lycopodium, being sometimes whorled, some- times irregularly spiral. It is true that cases do exist among the strobiloid Pteridophytes which show regularity in the disposition of their parts, but in their shoots at large a regular disposition of the appendages cannot be held to be a general feature. Such irregularities, so far as they are of primary origin, are difficult to explain on the basis of any sympodial construction of the strobilus: to a theory of enation, as expounded in the previous chapter, they offer no obstacle; for if the appendages originated from the surface of the pre-existent axis, as suggested, they might equally well appear in regular positions, or be disposed with greater or less irregularity—as, indeed, is seen to be the case. The comparative classification of those appendages of the strobilus which are seen in the different types of Pteridophytes has always presented. Fig. 73. Transverse section through a spor- angiferous bud of Tmesipteris. ax— axis. y=foliage leaves

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