78 GYMNOGRAMMOID FERNS [CH. xxxix 



III. Adiantum Linne, 1753 



Adiantum is a widely spread genus containing 184 species of Ferns usually 

 of shady and moist habit, and with a preponderance of species in tropical 

 America. They are characterised by having sori apparently marginal, but 

 really superficial in origin, and covered by a sharply reflexed leaf-margin 

 which has the appearance of an indusium, since it is either membranous or 

 brown-coloured when mature. As a matter of fact there is no true indusium. 

 The sporangia are inserted upon the distal region of the veins that traverse 

 the fertile lobe, and it is the fertile region of the blade itself which is 

 reflexed at the proximal limit of the fertile zone. It is this flexure that gives 

 the essential difference between Adiantum and the central group of the 

 Gymnogrammoid Ferns. Strictly speaking, each such fertile lobe bears a 

 group of sori seated upon parallel veins. The whole structure corresponds 

 to a small but highly specialised leaf-segment of Gymnogranivie reflexed 

 sharply on itself in relation to protection (Fig. 633). 



The genus thus characterised comprises a coherent group of Ferns with 

 upright or creeping scaly rhizome, bearing leaves spirally or alternate. The 

 leaf has as a rule a shining black and brittle petiole: the blade may be 

 entire {A. renifonne L. or ParisJiii Hk., Fig. 633, ^) ; or simply or repeatedly 

 branched. It is often of delicate texture in relation to the moist and shaded 

 habitat, and is traversed by forking veins, which are usually free: but in 

 some species they anastomose to form a reticulum {Hewardia J. Sm.). The 

 branching of the leaf is clearly related to dichotomy (Fig. 633, G). The 

 ultimate segments are often deltoid in outline, and when these are borne 

 upon the polished black branchlets of the highly divided rachis they give 

 the characteristic appearance of the Maidenhair Fern (Fig. 633). 



Anatomically Adiantum is closely related to Gyvinogrmmne. Those 

 species which have an elongated rhizome may show actual solenostely, as 

 in A. pedatum L. and hispidulum Sw. But commonly the stele is gutter- 

 shaped, owing to the great elongation of the leaf-gaps (G.-V. MS. notes). In 

 those with short internodes, and particularly where the rhizome is ascending 

 or upright, the leaf-gaps overlap, and the stele appears in transverse section 

 as several meristeles disposed in a ring. This condition is obviously based 

 upon solenostely, from which it is not far removed (Gwynne-Vaughan, Ann. 

 of Bot. XVII, p. 695, A. trapeziforme L. ^\\A petiolatum Desv.). In this there 

 is near correspondence to what is seen in Pellaea and Notholaena. In 

 A. capillus- Veneris L. the number of meristeles in a transverse section may 

 be 5-7, of circular form (Luerssen, Rab. Krypt. Fl. ill, p. 81). The leaf-base 

 receives two closely related strands, which unite upwards to form a single 

 four-angled meristele, a condition not uncommon in contracted leaf-stalks. 



