. L'origine d'une flore terrestre, une théorie fondée sur les faits de l'alternance. La morphologie des plantes. 346 LYCOPODIALES à sa base : il est clair que la relation de l'apex à l'intersection de l'premiers murs a une (Fig. 183 a). L'axe produit bientôt pour former des feuilles successives disposés en spirales. Le cotylédon et les feuilles ont les caractères ordinaires des feuilles de l'espèce. Le tissu ci-dessous devient rapidement comme l'hypocotyle allongé dont la durée est déterminée par le niveau auquel le prothallus réside dans le sol : où c'est à ou près de la surface le
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. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. 346 LYCOPODIALES to its base: it is clear that the relation of the apex to the intersection of the first walls has been a close one (Fig. 183 a). The axis soon proceeds to form successive leaves spirally arranged. The cotyledon and subsequent leaves have the ordinary characters of the foliage leaves of the species. The tissue below soon becomes elongated as the hypocotyl, the length of which is determined by the level at which the prothallus lies in the soil: where it is at or near to the surface the hypocotyl may be quite short: where deeply seated it lengthens, so that the first leaves are exposed above ground (Fig. 184). It is traversed by a vascular strand, which is monarch below, but near to the first leaf, or later, it becomes diarch, and shows two lateral tracheidal strands. The first root originates exogenously from the upper tier, just above the foot, and is succeeded by other roots of endogenous origin at higher points (Fig. 183. b). Here, then, is an embryogeny characterised by its great direct- ness and simplicity. The only complication is the varying elon- gation of the hypocotyl according to the level of the prothallus in the soil; and there is good reason to think that this is an immediate adaptation to meet the varying levels of development of the game- tophyte in the soil, in an embryo which is pertinaciously subaerial. The nursing of the embryo by the prothallus is not long continued, nor is it structurally provided for, there being no development of an elaborate " calyptra, '' as in some other species: the embryo soon escapes from the prothallus, and fends for itself. The whole condition of the embryo is such as bespeaks a simple and primitive state. Probably this view would never have been in doubt had it not been for the existence of different arrangements seen in other species of the genus, which happened to have been described some years earlier.
