SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 765 



stratification of the silicified peat layers in different degrees of decay is still more 

 clearly visible ; two thin bands a and b occur in which the stems are much more 

 decayed and crushed together. These two photographs give an idea of the 

 abundance of the stems of Rhynia in the peat. From the fact that they are 

 practically all cut transversely in these "sections, it can be inferred that for some 

 reason or other the stems lay horizontally and more or less parallel. 



A horizontal surface of a block, on which the closely crowded and flattened 

 stems of the plant show that the whole growth had been laid down and compressed, 

 is represented in PL II, fig. 3. The small portion of this, enlarged two diameters, in 

 fio-. 4 shows at a a median line on the flattened stem. This will be seen below to 

 indicate the position of a central vascular strand.* 



In contrast to this mode of preservation, the unaltered cylindrical stems of the 

 free aerial portion of the plant can be exposed by fracturing the chert. Examples 

 of such stems, which have preserved their shape perfectly in the almost pure 

 silicious matrix, are shown enlarged fourteen times in PI. Ill, figs. 6-8. 



External Morphology. 



Owing to the favourable nature of the material, it has been possible to arrive 

 at a clear conception of the morphology of the plant as a whole. It will assist the 

 reader if we deal briefly with the general organisation of Rhynia before entering 

 into the details of its structure. 



The plant formed a practically pure growth, and its erect cylindrical stems stood 

 closely crowded. These stems probably attained a height of 8 inches or more,t and 

 range from 6 mm. to under 1 mm. in diameter. The plant was rootless and had no 

 leaves, being composed entirely of a system of cylindrical axes or stems. Its lower 

 portion consisted of branched underground rhizomes attached to the peaty soil by 

 numerous rhizoids. Branches of the rhizome turned gradually or abruptly up and 

 assumed the characters of the aerial stems. The latter were occasionally branched 

 dichotomously and tapered gradually upwards. They bore small hemispherical 

 projections which were more or less closely placed without apparent regularity. 

 On some of these bulges tufts of rhizoid-like hairs were borne, while in other cases 

 the projections developed into adventitious branches, usually attached by a narrow 

 base. Some of these branches appear to have been readily detached, and their 

 occurrence free in the peat suggests that they served to propagate the plant 

 vegetatively. 



Even in the most complete specimens, preserved as they grew from the peat, 

 the terminal parts of the aerial stems are wanting. The reproductive organs have 

 not been observed attached to the complete plants, and it is impossible to say 



* Such impressions, with or without an evident midrib, according to their preservation, might easily be 

 described as linear leaves or even algae. 



f The stems in PI. II, fig. 5, measured 6 inches, but were incomplete. 



