158 THE MORPHOLOGY OF PTERIDOPHYTES 



such species, the sporangia are produced simultaneously, 

 but, where the receptacle can grow, new sporangia arise in 

 basipetal sequence. Surrounding the sorus is a cup-shaped 

 indusium in Trichomanes (Fig. 25F, where the broken Hne 

 indicates where the indusium was cut away to show the base 

 of the receptacle) and a two-lipped indusium in Hymeno- 

 phyllum. 



The sporangium has a relatively thin stalk and an oblique 

 annulus, which brings about dehiscence along a lateral line 

 (Figs. 24J-L), by a process of slow opening, followed by 

 rapid closure as a gas phase suddenly appears in the cells 

 of the annulus. This mechanism is found throughout the 

 more highly evolved members of the Fihcales, and results in 

 the forcible ejection of the spores. The spore output varies 

 from 128 or 256 in Hymenophyllum to as low as thirty-two 

 in some species of Trichomanes. 



The prothallus of Hymenophyllum is a strap-shaped 

 thallus, often only one cell thick, but, by contrast, the few 

 species of Trichomanes whose prothalH have been studied 

 have a filamentous structure which, like that of Schizaea, is 

 mycorrhizal. 



Dicksoniaceae 



The first recorded occurrence of a fossil member of the 

 Dicksoniaceae is of Coniopteris, from Jurassic rocks of 

 Yorkshire. Like modern members of the group, it had highly 

 compound fronds with marginal sori, protected by two flaps 

 (the upper and lower indusia). In the modern genus 

 Cibotium, the fronds are borne on stout creeping stems or on 

 low massive trunks, while some species of Dicksonia are tall 

 tree-ferns (e.g. D. antarctica), with a crown of leaves at the 

 summit of a tall trunk. All are characterized by a profuse 

 hairy covering over the stem and the base of the stipe, the 

 hairs being as much as 2 cm long in Cibotium barometz. 



The stems are solenosteUc or (in species with erect axes) 

 dictyostelic, and the stele is deeply convoluted around a 



