A Unit of Construction in the Pteridophyta. 145 
If these two figures be compared with Fig. 26, which is a dia¬ 
grammatic representation of a tangential section of Mazocarpon 1 
from the Upper Carboniferous Rocks, one sees again a relatively 
enormous development of sterile tissue which has given rise to the 
name proposed for this form (,aa^=a loaf). This extends into 
ridges on the sporange wall which have been compared with an 
incipient indusium, but which may be a vestigial structure. The 
tetrads are here ranged along the two sides, and the space around 
them was filled with tapetal cells which have perished. One seems 
Fig. 26. Diagram rrom tangential section of a megasporophyll of Mazo- 
carpon. x30. Royal Holloway College Collection of Dulesgate Plants, C.N. 
30. 
to see in this sporange an explanation of the saddle form of many 
of the “ sporangia ” of recent Lycopods, accompanied as such a 
form is by a well developed “ archesporial pad,” e.g., in Lycopodium 
clavatum. It would be a very natural sequence that the sporogenous 
regions of a single sporangiophore should become confluent, and 
the gradual reduction of the sterile tissue to a mere “ archesporial 
pad ” and pedicel would next follow. The inclusion of the Lycopo- 
diales among the “ Sporangiophoric Pteridophyta ” would make 
this term as extensive in application as Jeffrey’s term Lycopsida. 
But have we even then really reached the full extent of the 
legitimate application of the expression ? Have we no representative 
1 Scott. “ Studies,” second edition, 1908. 
