48 



PTEROID FERNS 



[CH. 



characters Loiichitis lies intermediate between Pteridiuni and Pteris: and 

 not the least interesting feature is that L. aiirita bears no scales but has 

 a reticulate venation, though L. hiisiita has both hairs and scales, but its 

 venation is open. Thus such advances as they show do not march parallel 

 in the different species of the same genus. But still a general trend is 

 maintained. 



A 



Fig. 6i 



Lonchids atirita L. ( = Z. Lindenimia Hk.). {A) one pinna, natural size, and 

 {B) a segment from it, enlarged. (After Hooker, from Christ.) 



HiSTiOPTERlS (Agardh) J. Smith 



A further step in the progression towards the typical genus Pteris is 

 supplied by Histiopteris {Pteris) incisa (Thunbg.) J. Smith, a Fern of very 

 wide distribution in both hemispheres, and of variable habit. On account 

 of its occasional reticulate venation it was placed in the section Litobrochia 

 of Hooker's comprehensive genus Pteris, but it is now (with a Philippine 

 species) restored to generic rank under Agardh's naine oi Histiopteris. The 

 usual type of the species resembles Pteridimn in its creeping habit, with 

 underground rhizome and deltoid leaves. But its venation is variable, some- 

 times free but frequently with coarse anastomoses. The surface when young 

 bears simple hairs, but there are also scales upon the rhizome. The adult 

 leaves are glaucous. These vegetative characters point to Pteridimn, but 

 with a suggestion of advance. 



The adult anatomy discloses a solenostele in the rhizome: it is liable to 

 corrugation in large rhizomes, a device that gives an enhanced proportion 

 of surface to bulk in the conducting tracts (Fig. 613). The relation of the 

 stele to the foliar trace has been worked out by Tansley and Lulham, and 

 it is found to correspond essentially to the Hjpolepis-ty^e, but the vascular 

 tracts are more deeply folded, and the structure is complicated by the fact 

 that each leaf in the specimens examined arises in the angle between the 

 shanks of a dichotomy of the rhizome (see Tansley, Fil. Vase. Sj/st. Fig. 65). 

 The leaf-trace is at first undivided, but it soon breaks up into separate 



