848 DR R. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 
plant-body of the Rhyniacese there is, as has already been pointed out, a distinct gap. 
We have kept our minds open to the possibility that thfe hemispherical projections 
on many stems of Rhynia Gwynne- Vaughani might be regarded as indicating a step 
towards the origin of small and truly lateral leaves ; this was indicated as one of 
two alternatives in Part III (p. 674). There were always difficulties in such an 
interpretation apparent to us. The late origin of these hemispherical projections 
underneath the stomata that we have now been able to demonstrate (pp. 832-834) 
seems to further weigh against entertaining it, although the specimen represented in 
fig. 10 lends it some support. The only possible conclusion at present appears to 
be that the Rhyniaceae afford no clear indication as to the first origin of leaves. 
Another question raised by speculative morphology on which the bearing of 
the Rhynie plants must be considered is as to whether a distinction should be drawn 
between the small leaves (e.g. of Lycopodium ) and the large leaf or frond of such 
plants as the Ferns. The clearest statement of such a view is that by Lignier, who 
derived it partly from a consideration of the reconstruction of Psilophyton. He 
distinguished the small leaves of Lycopods as phylloids from the specialised branch- 
systems or cauloids which gave rise to the fern-frond. 
On this question also the morphology of the Rhynie plants does not afford 
direct evidence — though, as has been pointed out in Part III, it is not inconsistent 
with such ah interpretation. The most significant fact ' provided by the newly 
discovered plants is the demonstration in the Rhyniaceae of Vascular Cryptogams 
with a thalloid plant-body without small leaves or phylloids. It is, therefore, not 
necessary to assume that such phylloids have been lost from the branch-systems 
forming fern-fronds, though this possibility is open to evidence as to whether in 
particular examples it was or was not the case. 
On the further question of whether a distinction is to be drawn between the 
origin of relatively small leaves and of larger subordinated branch-systems the 
Rhynie plants also do not give any clear evidence- On the whole their organisation 
seems to weigh against drawing too sharp a distinction and to support Tansley’s 
modification of Lignier’ s view. 
It should be borne in mind that there is evidence in the Early Devonian period 
of stems bearing definitely arranged, large, lateral appendages that can clearly be 
regarded as specialised subordinated branch-systems or fronds. With fuller know- 
ledge of such plants more definite evidence on the comparative problems indicated 
above may be anticipated. 
( c ) The Vascular System. 
All the Rhynie plants had a well-developed, though simple, vascular system 
with characteristically thickened tracheides forming the xylem, which was 
surrounded by a tissue differing from the parenchymatous ground tissue and 
corresponding to the phloem. 
