XXXVIl] 



DAVALLIA 



19 



the marginal sori more or less markedly superficial in the adult state, of 

 mixed type, with flat receptacle, and indusial lips often very different from 

 one another, the upper being merged in the general leaf-surface, and with 

 delicate, long-stalked sporangia. The sum of these features mark out Davallia 

 as relatively advanced, though still essentially of the same type as Dicksonia. 

 Comparison of the sorus of Davallia at an early stage of development with 

 those of Thyrsopteris^ Cibotium, Dennstaedtia, and Microlepia (Vol. II, Figs. 

 529, 534-5, 539, 540) demonstrates that in these related Ferns there has been 

 a slide of the sorus, from the marginal position with definite indusial lips, to 

 a superficial position, the upper and more massive lip being incorporated into 

 the flattened expanse of the pinnule ; and the change can even be followed 

 in the individual development. It is, however, significant that the vascular 

 supply terminates as a rule below the receptacle. The series illustrates the 

 change in form of the receptacle. In the earlier terms it is conical with the 

 B 



Fig. 59;. ^ = young sorus of D. griffithiana Hk., in \eitical section, showing the flat receptacle 

 with the first sporangium lying centrally. ^ = an older sorus of the same, showing sporangia of 

 different ages intermixed. j-«/ = upper indusium: //{/"slower. ( x roo.) 



vascular supply extending into it (Vol. II, Fig. 540): but in Davallia it is flat, 

 the vascular tissue spreading out below the surface that bears the sporangia 

 (Fig. 591, B). This has its close relation to the change from a gradate to a 

 mixed sorus, and it has been shown that in this series there is a gradual change 

 from a gradate to a mixed sequence of sporangia. Lastly, the sporangia of 

 the series show a transition from a massive type with thick stalks and oblique 

 continuous annulus to the more delicate structure of Davallia, with vertical 

 interrupted annulus, and a stalk consisting of only a single row of cells. The 

 parallelism that exists in respect of these various structural features confirms 

 the reality of the progression from the Dicksonioid to the Davallioid Ferns, 

 which the habit itself suggests. 



Davallia is the centre of a plexus of Fern-types which have always pre- 

 sented difficulties to the systematist, as is shown by their varied synonymy. 

 A special interest in any phyletic discussion attaches to such related forms, 



2-2 



