850 DR R. KIDSTON AND PROF. W. H. LANG ON OLD RED SANDSTONE PLANTS 
facilitate comparison on the one hand with the spore-bearing organs of the more 
specialised Pteridophyta, and on the other hand with Bryophyta and especially 
with Algae. 
As regards the position of the sporangia, it is clear that in Rliynia and Hornea 
they occupied the ends of some branches of the thalloid plant-body. There is 
strong reason to hold that the position of the sporangia on certain branch-systems 
of the more complex plant of Asteroxylon and Psilophyton was similar. If, as 
appears to be the case, we are here dealing with generalised early plants representing 
the precursors of the more specialised groups of Pteridophyta, it is reasonable to 
expect that the position of the sporangia in the Psilophytales should be consistent 
with and explain the diverse specialised positions they occupy in later derived groups. 
This is as a matter of fact the case ; the position of the sporangia of the Rhyniaceae 
at the tips of thalloid branches appears to fit naturally with conclusions arrived at 
from other points of view by a number of investigators of the Pteridophyta. 
Thus this conception applies readily to sporangiophores bearing one or more 
sporangia such as are met with in what have been termed the sporangiophoric 
Pteridophyta and in some others. The sporangiophores would appear to represent 
the last persisting remains of the original leafless branch-systems of the Rhyniaceae. 
It is unnecessary to follow this comparison into detail, but it may be pointed out 
that it applies to such cases as the sporangiophores of Helminthostachys , of the 
Equisetales and Sphenophyllales, in a somewhat different fashion to the fertile 
appendages of the Psilotaceae, and with greater difficulty and obscurity to the 
position of the sporangia of the Lycopodiales. 
In the case of the Ferns there is no difficulty in the comparison of the terminal 
position of sporangia on the ultimate ramifications of a branch-system with the 
marginal position on a webbed cladode-system of pinnules forming a definite 
flattened frond. This was suggested in detail by Lignier (taking Psilophyton as a 
starting-point), and he further followed the change in position of sporangia to the 
lower surface of the frond. Bower’s more recent conclusions envisage the marginal 
position of the sporangia as the probable starting-point in the ease of Ferns and 
support these comparisons from the side of the spore-producing organs of the higher 
Ferns. 
As regards the structure of the sporangia themselves, there are two very distinct 
types or grades in the Rhynie plants. The fairly large sporangia referred to 
Asteroxylon (which are, however, relatively small compared with those of the 
Rhyniaceee) had a definite dehiscence depending on the structure of the sporangial 
wall. They are thus comparable with the specialised sporangia found in most 
existing and extinct Pteridophyta, and are very similar to those of Stauropteris. 
The sporangia of Rhynia and Hornea , on the other hand, which in Rliynia major 
were of relatively huge size, show no arrangements for dehiscence although the 
epidermal layer of cells is specially thickened. 
