SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 773 



Sporangium. 



The aerial stems of Rhynia Gwynne-Vaughani have been traced for at least 

 6 inches from the surface of the soil, but the block shown in PL II, fig. 5, unfortu- 

 nately did not contain the' terminal portions of the stems. Both dichotomous and 

 lateral branching were occasionally observed in the upper regions. No reproductive 

 organs have, however, been seen attached to these stems, though presumably they 

 would occur on the higher portions. It is impossible therefore to say whether the 

 main axis terminated in a sporangium or whether sporangia were borne on special 

 lateral branches, though, judging from the size of the sporangium, the latter sup- 

 position appears less probable. 



A number of specimens of sporangia have, however, been met with in the sub- 

 stance of the silicified peat (PL IX), while free spores were disseminated throughout 

 the matrix. 



It might have been inferred with reasonable certainty from the purity of the 

 vegetation that these sporangia were the reproductive organs of Rhynia. The 

 small specimen shown in figs. 63 and 63a fortunately places the matter beyond 

 doubt, for the sporangium is here borne terminally on a slender axis with the 

 characteristic vascular strand and tracheides described above. 



Another and larger sporangium was cut in a series of transverse sections 

 (slides Nos. 2417-2422), and following the series down, the basal region of the 

 sporangium was found to be continuous with a badly preserved but characteristic 

 axis of Rhynia (cf. PL IX, fig. 69). 



The sporangium attained a length of at least 12 mm. and a breadth of 2 '5 mm., 

 and the axis it terminated was about 1'5 mm. in diameter. More precise measure- 

 ments cannot be given, as the sporangia varied considerably in size. It was 

 cylindrical in form, though as enclosed in the peat it is generally slightly flattened 

 (figs. 64 and 65). The form of the base of the sporangium and its junction with 

 the stalk is best shown in fig. 62. The apical region of this specimen appears 

 somewhat more rounded than was actually the case, owing to the section being 

 slightly tangential. As fig. 64 shows, the sporangium was more pointed. 



In several cases a pair of sporangia have been met with lying side by side in 

 the silicified peat (figs. 65, 69). This is suggestive of a possible junction of their 

 stalks, but proof of this is wanting. 



The wall of the sporangium is J mm. thick, and is differentiated into several 

 layers (figs. 66-67). In fig. 66, which represents more highly magnified a portion 

 of the wall of the sporangium shown in fig. 65, the wall is seen in true transverse 

 section. The cells of the epidermal layer are narrow, and greatly extended at right 

 angles to the surface. They have thick- walls, and are covered by a well-marked 

 cuticle. In this specimen the middle layers of the wall had wholly decayed, their 

 position being represented by the clear space which separates the epidermal layer 



