SSN a oo ae, csi T TEES 
HOOKER LECTURE, 1917. 117 
near circles of affinity. The perispore is absent in most primitive 
types ; but it is present in many derivative types. 
xiv. Form of prothallus : relatively massive in primitive types, relatively 
delicate in derivative types. But the character is unreliable. 
xv. Sexual organs: sunken in primitive types, projecting in derivative 
types. 
xvi. Number of spermatocytes in each antheridium : large in primitive 
types, smaller in derivative types. 
xvii. Embryology : with suspensor present in a few primitive types, but 
mostly without. The embryo relatively massive with axis vertical 
in primitive types, more delicate with axis prone in derivative 
ty pes. 
These are the most important criteria for the phyletie seriation of the 
Filicales. Many others are possible, and those stated raise many side-issues. 
Their value individually is undeniable; but it is vastly enhanced by the fact 
that the progression which each criterion shows in its own individual feature 
runs parallel, as a rule, with progressions in the features of other criteria. 
For example, Eusporangiate Ferns have a relatively complex cellular seg- 
mentation, hairs as dermal appendages, usually an open venation, a simple 
type of sorus, without indusium, massive sporangia, deeply-seated sexual 
organs, and an upright embryo, sometimes with a suspensor. The more 
advanced Leptosporangiate Ferns have a relatively simple cellular con- 
stitution, scales as dermal appendages, frequently a reticulate venation, 
gradate or mixed sorus, often indusiate, small sporangia, exposed sexual 
organs, and a prone embryo with no suspensor. Such parallelisms of 
progression in a plurality of criteria may also be traced more or less 
clearly, and with a high degree of constancy, within nearer circles of 
affinity. 
On the other hand, occasional exceptions occur. Thus Cheiropleuria, 
which is Leptosporangiate, has a protostelie stem with dermal hairs; but 
its leaf shows an advanced type of reticulation, and an Acrostichoid sorus. 
The axis in this case has retained its archaic characters, while the leaf has 
advanced. But the converse is seen in the Marattiaceæ; for in them the 
typical eusporangiate sorus is retained while the vascular system of the 
stock is broken up into a complex plexus of reticulated strands. Here 
the sporophyll remains archaic, while the axis has structurally advanced. A 
somewhat parallel case is seen in Ophtoglossum (Cheiroglossa) palmatum, 
where also the stock is swollen and sappy, and the stele is disrupted into 
numerous strands. Such exceptional cases do not negative the method ; but 
they serve to remind us that it is Organic Nature, with all its resources of 
adaptation, that is being dealt with, not a mere mechanism designed after 
fixed rules. 
we e RS 
