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DR JOHN M‘LEAN THOMPSON: A FURTHER CONTRIBUTION 
although the larger sporangia were commonly upon the distal branches of the 
sympodium, no very definite arrangement according to size was recognised. The 
capsule was globular, ovoid, or pear-shaped, and was frequently lopsided. The 
annulus was variable in position and form ; it was seldom, if ever, perfectly regular, 
and was typically very irregular. In a few cases it was almost horizontal, passing 
round the capsule as an irregular broken ring, and having no clearly organised 
stomium. In the large majority of the sporangia the annulus was almost vertical, 
though typically irregular, and a definite stomium was present. The annulus 
usually began at the back of the stalk. It passed obliquely and with irregularities 
upwards, and, having surmounted the capsule, descended to the stalk once more as 
a well-defined stomium. It was evident that in the majority of the sporangia the 
annulus was interrupted by the, stalk. The stalk was of variable construction. In 
the small sporangia it usually consisted of three rows of cells, but a four-rowed stalk 
was not uncommon. The stalks of the large sporangia showed the same variation. 
The large sporangia contained small numbers of large spores, the small sporangia 
larger numbers of small spores. The largest number of spores observed in a large 
sporangium was 16 ; 14 was a common count, and 12 was a rare spore-number. The 
largest spore-number in a small sporangium was 32, and such numbers as 30, 29, 
28, and 26 were frequently counted. There were also spores intermediate in size 
between the large and small forms. These were not restricted to either type of 
sporangium. They were present in both large and small sporangia. Thus in one 
large sporangium 14 large spores and 2 of intermediate size were found; a small 
sporangium contained 3 intermediate and 28 small spores. Both large and small 
spores were found in the same sporangium. 
Such remarkable differences in the size of sporangia and spores are unknown 
outside heterosporous plants, and it is accordingly desirable to know if Platyzoma is 
truly heterosporous. 
It would appear that in homosporous Leptosporangiate ferns marked differences 
in sporangial size in any one species are rare. When such variations do occur the 
spore-output per sporangium may be affected, but uniformity of spore-size and form 
remains. This condition is seen in Stromatopteris {Trans. Roy. Soc. Edin., 1917). 
In modern heterosporous Pteridophytes sporangial size is in general a guide to the 
nature of the spores. The size of the sporangium is indicative of the size, number, 
and sex of the spores. The large sporangia contain small numbers of large female 
spores, the small sporangia a larger number of small male spores. In possessing 
both large sporangia with few large spores, and small sporangia with more numerous 
small spores, Platyzoma is in line with most heterosporous Pteridophytes. 
But heterospory may occur without recognised distinctions of either sporangial 
or spore size. Thus in Equisetum, which produces male and and female prothalli, 
spore-sex is not indicated by differences in the size or form of either sporangia or 
spores. The nature of the spores is not manifested until germination, and the well- 
