SHOWING STRUCTURE, FROM THE RHYNTE CHERT BED, ABERDEENSHIRE. 771 



The xylem strand, whether composed of few or many tracheides, is always solid, 

 no parenchyma being mixed with the tracheides. No distinction between proto- 

 xylem and metaxylem can be drawn, all the tracheides being alike. The thickening 

 of the tracheides was annular (PI. VII, figs. 47 and 48). It had the form of rather 

 broad rings, which give the tracheides the appearance of being transversely barred. 

 Occasionally two of the bars converge and, uniting at one end, take the shape of 

 a Y or V, but neither definite spiral thickening nor the passage to a scalariform 

 type of thickening has been seen. 



Hemispherical Projections. — In PI. Ill, fig. 7, a number of the definite little 

 bulges which occur on the stems are seen in the round. Several of these projections 

 are seen in the transverse section of a stem on PI. V, fig. 27. Their structure is 

 illustrated in greater detail in figs. 49-51 on PI. VII. In fig. -49 a small 

 bulge is seen from the outside on an obliquely viewed epidermal surface. In 

 this view the surface cells of the projection are isodiametric. They fit closely 

 together without intercellular spaces. They contrast in form with the surrounding 

 epidermal cells and have a much thinner outer wall than these, but we have 

 satisfied ourselves that there is unbroken continuity between the two. Two bulges 

 in longitudinal vertical section are represented in figs. 50 and 51. As is especially 

 clearly shown in fig. 51, the bulge is due to periclinal division in the epidermis 

 and outer cortical cells. The thick outer wall and the cuticle cease to be marked 

 over the projection ; this often exhibits a brown discoloration of its outer cells, 

 which are sometimes broken down. 



Though much smaller, these projections agree in their relation to the tissues 

 with the large bulges on the rhizome (PI. Ill, fig. ll). In connection with this it 

 is interesting to find that the superficial cells of some of the small projections 

 grow out as rhizoids similar to those on the rhizome. Examples of these rhizoid- 

 bearing projections will be found on PI. V, figs. 25 and 29, and more highly magnified 

 on PI. VIII, figs. 52-54. 



Lateral Branches. — Many of the small projections were the seat of a further 

 development of considerable interest, adventitious lateral branches being produced 

 from them. This is illustrated in figs. 55-61 on PI. VIII. The tissues at the base 

 of the branch are continuous with the epidermis and outer cortex of the parent 

 axis. The branch in fig. 57 has its own vascular strand, but this does not exhibit 

 any connection with the stele of the parent axis. Such transverse sections through 

 the main axis and branch did not, however, exclude the possibility that the stele 

 of the branch might have continued obliquely downwards as a trace through the 

 cortex of the main axis to join with the stele of the latter. Where, however, as in 

 fig. 61, we see the bases of two fairly large branches in a longitudinal section that 

 includes the stele of the parent axis throughout, and there is no indication of traces 

 passing out, it seems safe to conclude that these lateral branches had no vascular 

 connection with the main axis. 



