SHOWING STRUCTURE, FROM THE RHYNIE CHERT BED, ABERDEENSHIRE. 851 
A very important and striking feature of the sporangia of Rhynia and Hornea 
is the indication they afford of the sporangium being the transformed and specialised 
tip of a branch of the thalloid plant-body. This was shown especially in some less- 
defined specimens of the sporangia of Rhynia Gwynne-Vaughani, which appeared 
to have a group of spore-tetrads developed within a structure like the tip of a 
vegetative branch (Part II, figs. 10, ll). It is also shown in the way*in which 
the dichotomy of the tip of a branch in Hornea affects the sporangia developed 
from it ; we meet with both simple and branched sporangia. While from one point 
of view and as regards some specimens the sporangia of the- Rhyniaceae can be 
regarded as specialised organs contrasting with the vegetative axes, from another 
point of view they are clearly seen to be the modified tips of the latter. 
These considerations are significant when the sporangia of the Rhyniaceae are 
compared with the corresponding structures in the Bryophyta and the Algae. 
As regards the Bryophyta, it is only possible to recognise that the types of 
sporangia in Rhynia and Hornea facilitate comparison with certain sporogonia. 
This applies to the enclosed position in which the spores are produced, protected by 
a more or less massive wall, and to the presence in some cases of a sterile columella, 
so that the spores lie between this and the wall. In the absence of any indication 
as to whether the sporophytic generation in Bryophyta is to be regarded as reduced, 
or as illustrating an independent line of progression, it seems inadvisable to pursue 
the comparison in greater detail. The comparisons to be made with the Algae 
may, however, apply to the Bryophyta as well as to the Rhyniaceae on which 
they are based. 
When the comparison is extended to the Algae it evidently bears on the primitive 
nature and origin of the structures which in the Rhyniaceae and throughout the 
Pteridophyta are known as sporangia. These have for long been regarded as organs 
sui generis and not explained on the grounds of possible derivation w T ith modification 
from any part of the plant-body of the Algae. It has, however, been pointed out by 
Schenck *■ that, when the comparison is extended to what we term sporangia in 
various Algae, the strictly comparable structures are the spore-mother-cell and 
resulting tetrad in the Vascular Cryptogams and the sporangia (tetra-sporangia, etc.) 
in which reduction and spore-formation takes place in the Algae. Schenck therefore 
suggested employing the name “sporotheca” for the organ usually known as the 
sporangium in Pteridophyta. On this view the sporangium (or sporotheca) of the 
Vascular Cryptogams (and the sporogonium of Bryophyta) would correspond to an 
association of a large number of sporangia of the thallophyte type enclosed within a 
special region of the plant-body. 
The type of sporangium (sporotheca) in Rhynia and Hornea appears to 
strengthen and support the view based on the above comparison. It has been 
pointed out in Part II (p. 622) how their construction suggests comparison with the 
* Engler’s Bot. Jahrb ., Bd. xlii (1908), p. 31. 
