XIX] MIADESMIA 277 



of larger cells. The stem of Miadesmia is not uncommon in 

 sections of the Lancashire calcareous nodules, and may be 

 recognised by the delicate crushed tissue of which it mainly 

 consists and by large hypodermal parenchyma. The spirally 

 disposed leaves bear a conspicuous and relatively large ligule, 

 3 mm. long, in a deep pit (fig. 219, B and C) roofed over by a 

 few layers of tissue corresponding to the velum in Isoetes 

 (cf fig. 133, E, v). The fairly thick central region of the lamina 

 is expanded laterally into thin wings, which in the living 

 state probably bore delicate hairs. These delicate leaves, 

 apparently without stomata, were attached to the stem at an 

 acute angle, and Miss Benson suggests that their form ana 

 arrangement may have enabled them to hold water by surface- 

 tension. As seen in fig. 219, B, C, which represents part of 

 a transverse section near the leaf-base, the ligule is a very 

 characteristic feature, and the size of the single vein is in 

 keeping with the almost filmy nature of the lamina. 



In addition to the sections in British collections, I have 

 been enabled by the kindness of Prof Bertrand to see photo- 

 micrographs of the sections on which he founded the genus. 

 One of these sections, transverse to the stem and leaves, 

 illustrates in a striking manner the relatively large size of 

 the leaves and ligules in proportion to the delicate axis of the 

 shoot. 



The megasporangiate cone has an axis which agrees in its 

 structure with that of the vegetative stem and bears several 

 megasporophylls approximately at right-angles. As in the 

 foliage leaves, the ligule is prominent and large, and lies in a 

 groove which contains also the megasporangium ; both ligule, I, 

 and sporangium, s, as seen in the transverse section represented 

 in fig. 219, D, are covered by an integument or velum which 

 arises in the proximal part of the leaf and leaves a circular 

 micropylar opening at the beak-like apex of the sporangium. 

 The circular micropyle is surrounded by numerous hairs borne 

 on the integument and which presumably played the part of a 

 feathery stigma. A single megaspore with a thin membrane, 

 •TO, abuts on the fairly strong sporangial wall, s ; in some cases 

 the sporangium and megaspore walls may be indistinguishable. 



