392 
DR JOHN M'LEAN THOMPSON ON THE 
more directly on anatomical grounds to Mohria and a Schizseoid source than to any 
other group, a reasonable suggestion as to the ultimate origin of the Ferns discussed 
in this memoir is that they may similarly have sprung from some Schizseoid source. 
It is admitted that such a source has not been demonstrated for them, but the evidence 
available points more clearly to a Schizseoid origin than in any other direction. 
In this comparison with the Schizasacese the most interesting point is the 
variability of spore-size disclosed in Nothochlaena and Pellsea. This is also to be noted 
in relationship to a similar variability which I have demonstrated in Platyzoma, 
R. Br. (Trans. Roy. Soc. Edin., vol. li, 1916 ; vol. lii, 1917). In this connection the 
acknowledged Schizseoid affinity of the Marsiliacese which show marked heterospory 
is significant (Campbell, American Naturalist, 1904 ; Bower, Origin of a Land 
Flora, 1908 ; and Ann. Bot., 1918). Such a variability as is now seen in the Ferns 
for which we contemplate a Schizseoid affinity would give such conditions as suggest 
the origin of heterospory. But before it can be held as demonstrated that the 
variability really represents an initial step towards heterospory, the germination of 
the spores would have to be known and the typ’e of prothallus demonstrated. 
The author is indebted to the Carnegie Trust for their assistance in the produc- 
tion of the illustrations of this memoir. 
Summary. 
I. Jamesonia. 
1. The dermal appendages are simple hairs and irregular sclerotic mounds. 
2. The cortex of the rhizome possesses a peculiar parenchymatous tissue im- 
mediately outside the stele. This tissue is well ventilated. 
3. The stele is a simple solenostele. The leaf-gaps are long and narrow. 
4. The leaf-trace is undivided, and originates from the base of the gap. It is of 
a simple construction throughout. 
5. The pinna-traces are of marginal origin. 
6. The typical leaf is unbranched, and its pinnse are small and leathery. The 
leaf-apex shows continued apical-growth. 
7. Reduced leaves are not uncommon. These show reduced vascular tissues. 
8. The leaves and roots are specialised against xerophytic conditions. 
9. The venation of the pinnae is a sympodial dichotomy. 
10. Collectively, the anatomical characters constitute a primitive state. 
11. The sporangial arrangement is “ Acrostichoid,” and the sporangia are of an 
upgrade type, in which irregularities of the annulus still persist. 
12. The spore-count is above 64. This is considered an important primitive 
feature. 
13. The anatomical state and the sporangial characters are held to indicate for 
Jamesonia an ultimate origin from some Schizseaceous source. 
