164 
DR JOHN M'LEAN THOMPSON: A FURTHER CONTRIBUTION 
tinctions between large and small sporangia are indicated when the spore-mother 
cells are formed. The evidence is still insufficient for the determination of the 
nature of the spores, but the facts now advanced seem to strengthen a belief in the 
heterosporous nature of Platyzoma. As is commonly the case in heterosporous 
Pteridophytes, the small sporangia are most numerous. But if Platyzoma is 
heterosporous it is probably an upgrade fern type in which segregation of the 
microsporangia and megasporangia is not yet completed, and in which the 
megaspores do not declare their female character until they are shed. A point 
of contrast with the modern heterosporous Lycopods may thus be provided. 
On the other hand, should sporangial development and spore-germination prove 
this plant to be homosporous, its anomalous structure and unique position among 
living Pteridophytes will be accentuated, and a warning provided against the 
employment of spore-size alone in the determination of the sporangial conditions of 
fossil forms. 
Summary. 
In this paper are set down additional facts relating to the mature sporangia and 
spores of Platyzoma. Confirmation is given of the disposition of the sporangia upon 
the pinnse, and of the well-marked differences in sporangial and spore size and spore- 
output already recorded for this fern. 
In the absence of knowledge of the sporangial development and spore-germin- 
ation it has been impossible to determine if Platyzoma is truly heterosporous. 
The large majority of the sporangia are small, and the spore-output per sporan- 
gium in these is approximately 32. 
The remaining sporangia are large, and have an approximate spore-output of 16 
per sporangium. 
The sporangial stalk is typically three-rowed, but four-rowed stalks occur. The 
annulus is usually irregular, but is interrupted by the stalk. 
The small and large spores are the dominant types, and the spores of inter- 
mediate size are developed when the spore-output in any sporangium is greatly 
decreased. 
The three types of spore have similar spore-markings. Their walls are thick, 
and no sign of germination is found in any spore-type while still contained within 
the sporangium. 
The balance of the evidence seems in favour of a heterosporous condition for 
Platyzoma. Should a homosporous condition be demonstrated at a later date, the 
anomalous condition of this remarkable fern will be further accentuated. 
