317 

 ON FISSIDENS: 



WITH A NEW VARIETY OP F. PUSILLUS. 



By J. A. Wheldon, F.L.S. 



The genus Fissidens has been always a source of intorestj 

 and also of trouble, to the botanist. The species are closely 

 related, and their specific characters elusive and not always well 

 defined. As the family Fissidentaceae contains both cladocarpous 

 and acrocarpous plants, its position varies considerably in the 

 different systems of classification. Some insert it between the 

 Acrocarpi and the Pleurocarpi, others place it in a distinct class 

 by itself. In Lindberg's arrangement, greater importance being 

 attached to the structure of the peristome than to the position 

 of the seta, it is placed next the DicranacecB. The older bryo- 

 logists, ignoring the very specialised leaf-structure, included all 

 the species of Fissidens in the genus Dicramcm. 



The morphology of this anomalous leaf has been variously 

 explained. The earliest bryologists considered that the whole 

 leaf-structure corresponded to the ordinary moss leaf, and that 

 the sheathing portion resulted from part of the tissue on one side 

 being split into two layers. This explanation was proposed by 

 B. de la Pylaie (in Journ. de Bot. 1814, 135) and was so improb- 

 able that it was soon discarded. In 1819 Eobert Brown (in Trans. 

 Linn. Soc. xii, 575) advanced the suggestion that the vaginant 

 portion represented the true leaf, all the rest being an outgrowth 

 from it. This view was supported by Bruch and Schimper 

 (Bryol. Europ. i, 2), but rejected by Lindberg -'= and Braithwaite 

 (Brit. Moss Flora, i, 166, 1881). Lindberg regarded the whole 

 expansion, with the exception of one of the sheathing laminae, 

 as the true leaf, the excepted portion being regarded as a stipular 

 appendage which had become adnate. In 1899 Mr. E. S. Salmon 

 (in Ann. Bot. xiii, 103) fully reviewed these various theories, and 

 produced convincing evidence in support of Brown's suggestion. 



Mr. Salmon omitted one point which is well exemplified in 

 the new variety to which this paper is devoted. In this moss the 

 inferior lamina never reaches the base of the nerve, and frequently 

 only half way to it. If the superior and inferior laminae are 

 regarded as representing the true leaf, then we must admit the 

 possibility of the lower part of the nerve functioning as a petiole, 

 a structure unknown in the rest of the mosses. Nor is any moss 

 known to me in which the lamina fails to be directly connected 

 with the stem from which it springs, from which one may fairly 

 deduce the adventitious origin of these outgrowths. 



Several groups of critical species occur within the range of 

 even the British section of Fissidens, and very diverse opinions 

 prevail as to the status of many of the named forms. The plants 

 in the form-circle of Fissidens viridulus constitute one of these 



* Lindberg: Utkast till en natur. Grupp. Europ. bladm. med topp. frukt,, 

 16, 1878. 



