. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. DENNSTAEDTIA-DAVALLIA SERIES 615. Fig. 340. Davallia hymenophyll- oides. Pinnule enlarged. (After Hooker, from Christ's Farrnkraiiter.) uniformity of structure of the sorus on the mixed plan, with flat receptacle, and with no definite rule of orientation of the long-stalked sporangia. The connection of the D,ennstaedtia-Davallia series with such genera as Lindsay a, Pteris, Pellaea, and Adiantum, where the sori are marginal, seems beyond question, and it is strongly supported by the anatomical evidenc
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. DENNSTAEDTIA-DAVALLIA SERIES 615. Fig. 340. Davallia hymenophyll- oides. Pinnule enlarged. (After Hooker, from Christ's Farrnkraiiter.) uniformity of structure of the sorus on the mixed plan, with flat receptacle, and with no definite rule of orientation of the long-stalked sporangia. The connection of the D,ennstaedtia-Davallia series with such genera as Lindsay a, Pteris, Pellaea, and Adiantum, where the sori are marginal, seems beyond question, and it is strongly supported by the anatomical evidence. But, on the other hand, the sori are liable to move from the marginal position: this phyletic change is illustrated by very gradual steps. An inequality of the lips of the indusium is apparent in Microlepia (Fig. 332 a) : it appears often in greater degree in the various forms of Davallia, and is specially marked in § Leucostegia (Fig. 340), where the upper lip appears as the continuation of the leaf- lobe, the lower as a cup-shaped indusium apparently some distance from the margin. Leucostegia has long been recognised as closely related to Cyslopteris, which also has a mixed sorus, without regular orientation of its sporangia, while it is protected by an indusium of similar form to that of Leucostegia (Fig. 341). These examples will serve as illustrating a feature which has probably been widely effective in the descent of the Leptosporangiate Ferns, viz. the retreat of the sorus from the margin to the under surface of the leaf. From Cystopteris the sequence may with probability be traced on to the Aspideae. It would thus appear that a considerable proportion of the Mixtae are referable in origin to forms with a marginal position of the sorus. A further illustration of probable relationship, in this case to Polypo- dium, is seen in Hypolepis. This genus was included by Kiihn and by Prantl1 in the Dennstaedtiinae, though by others it has been placed in the Pterideae. The mar
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