Bower. — Studies in the Phytogeny of the Filicales . 3 1 7 
so that it took a horseshoe form, the indusium following suit. And so the 
characteristic Nephrodioid sorus may be held to represent a flattened 
modification of the Cyatheoid type. A further step is seen in Polystichum , 
by the junction of the two lateral arms of the receptacle on the side next 
the midrib of the pinnule to form a complete ring. The indusium again 
following suit, it now appears in the well-known shuttlecock form. The last 
step is in tfie abortion of the indusium in certain derivative forms, giving 
again a c Polypodioid * sorus of simple circular outline, almost like that of 
Alsophila. It would be out of place here to attempt to trace the various 
evolutionary lines within the Aspidieae . 1 It must suffice to have recognized 
the probability that they constitute a great group of Filicales, phyletically 
related with the Cyatheaceae, and ultimately with the Gleicheniaceae. 
The biological advantages of the Nephrodioid type of sorus over the 
Cyatheoid are obvious. First there is the general advantage which 
a mixed sorus has over a gradate one, that while it spreads the physiological 
drain over a long period, it makes the best possible use of a limited length 
of receptacle. But a second advantage not hitherto recognized is the 
greater convenience in packing during the circinate condition of the young 
leaf. In the gradate sorus pressure of the next outer coil of the spiral on 
the apex of the sorus would tend to crush the first formed sporangia, 
and evidence of this is sometimes to be seen. This is avoided by the 
flattened form assumed by the sorus, as seen in Cystopteris or Nephrodium. 
But a sacrifice of sporangial space is made in Cystopteris. This is amply 
made up for by the ring-like extension of the receptacle in Nephrodium 
and Polystichum , as well as by the continued production of sporangia 
in their mixed sorus. Thus it appears that the modifications, which are 
believed to have been phyletic advances, show that physiological advantage 
is gained by them. This increases the probability that they are rightly so 
recognized. 
The relation of Hypoderris to Woodsia has always been admitted, 
and the facts here given tend to strengthen the affinity. But Woodsia has 
a gradate sorus, though the sequence is not long continued, while Hypo - 
derris , which is more advanced in vascular structure, in venation, and 
in foliar character, has a mixed sorus. There can be little doubt that 
a like soral progression from the gradate to the mixed state has occurred 
here to that in Peranema and Diacalpe , or in Nephrodium. There is, how- 
ever, no reason to think that any of these genera had any place in the 
descent of Hypoderris. The conclusion must then be that Hypoderris 
illustrates another and a distinct progression from the gradate to the mixed 
sorus. It is, in fact, becoming plain that this transition has happened not 
once only, or twice, but by many parallel progressions. 
There does not appear to be any sufficient reason for holding that any 
1 Compare C. Christensen, Biologiske Arbejder, Nov. 1911. 
