SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 837 
In the impressions of Thursophyton ( Lycopodites ) Milleri, a plant of the Middle 
Old Red Sandstone flora of Scotland, which also occurs in Norway, we have a similar 
general habit to what is here assumed for the leafy shoots of Asteroxylon. 
The repeatedly dichotomous leafless branches, which are believed to have borne 
the dehiscent Sporangia, are represented in the reconstruction as the fertile region 
of the plant. Since, however, the connection of these structures with the vegetative 
organs of Asteroxylon is not established, a break is left in the restoration at this 
point. 
It is with regard to the general habit of the plant as a whole that direct evidence 
is lacking. We have met with nothing pointing to the branched leafy shoots having 
been horizontal or prostrate, and have therefore represented them as erect. The 
transition from the leafless rhizome to the base of the leafy shoot also appears to 
have stood vertically. In order to connect these parts naturally, we have assumed 
the existence of a more or less horizontal branched rhizome, giving off, on the one 
hand, downwardly growing root-like branches, and, on the other, erect leafy shoots. 
This assumption seems natural and justified in the light of our knowledge of the 
mode of growth of other plants with somewhat similar characters {c.g. Psilotum and 
various species of Lycopodium ). 
It is further supported by the large specimen of a rhizome of Asteroxylon repre- 
sented in fig. 24 on PI. V. This specimen dips downwards and penetrates a large 
stem of the plant lying in the peat. The basal region of the rhizome is of interest 
since it has a well-marked epidermis and bears a number of scale leaves ; this region 
has the structure intermediate between a leafy shoot and a rhizome. On tracing it 
on to the more distal portion, the well-defined surface is lost and the structure is 
that typical of the rhizomes of Asteroxylon. This specimen thus affords proof of 
the direct passage from a leaf-bearing axis into a rhizome, while the transition from 
a rhizome to a leafy shoot was followed in Part III (p. 647 ff). Fig. 23 on PI. IV is 
a photograph of a portion of epidermis of a stem of Asteroxylon in surface view. It 
shows the median line or ridge on the outer wall of each of the epidermal cells ; this 
feature of Asteroxylon was recorded on p. 653 of Part III, but not figured. The 
similarity in this detail of structure between Asteroxylon and Rhynia Gwynne- 
Vaughani will be evident if fig. 23 is compared with fig. 31 of Part I. 
The remaining figures on PI. V are from a series of transverse sections cut 
through the terminal region of a small leafy shoot of Asteroxylon. The crowded 
tips of the leaves were noticed at the surface of a block of the chert and a complete 
series of nine sections prepared in the hope of obtaining the actual growing point. 
Unfortunately the apical meristem itself was ground away in the process of cutting ; 
it came between the second (fig. 25) and third (fig. 26) sections of the series. 
Fig. 25 shows a complete and almost perfectly transverse section of the bud just 
above the growing point. The leaves of the bud arranged in a complex spiral (much 
as in a corresponding section of Lycopodium Selago ) diminish in size on passing 
