THE “SELAGO” CONDITION IN FERNS 169 
Fern (Oxoclea Struthiopterts) (Fig. 89). There may be some degree of 
regularity in the succession of sterile and fertile leaves, which may be 
correlated with season; thus in Blechnum boreale the leaves first expanded 
in the spring are sterile, and they are followed by a series of fertile leaves. 
The condition of the shoot as a whole is, in fact, comparable with that of 
Lycopodium Selago or of Lsoetes, with their successive sterile and fertile zones. 
But the commoner case for Ferns is that where leaves are not sterile or 
fertile as a whole, but many or even all the leaves of the mature plant are 
fertile at least in part, and frequently show a correlative reduction of area 
as compared with the rest of the leaf, which is sterile. In the distribution 
of the fertile and sterile parts of the individual leaf there is great diversity, 
and differences may be seen in species of the same genus, or even in 
individuals; thus in Osmunda regalis the lower parts of the fertile leaf 
are broadly expanded and sterile, the apical region is fertile and correlatively 
exiguous; but in O. javanica the fertile region extends irregularly over the 
lower pinnae, and the apical region is expanded and sterile (Fig. 90). 
-It has been shown by Goebel that the mode of development of such Fern- 
leaves may be experimentally altered: by removing from a plant of Onoclea 
Struthiopteris the foliage leaves which are first expanded in the spring, 
the later expanded leaves, which are normally sporophylls, were induced 
to assume the character of foliage leaves. Similar results were also obtained 
by Atkinson. 
The facts thus briefly summarised for Ferns are evidently comparable 
with those noted for the Lycopods, and the differences in detail which exist 
have their relation to the megaphyllous character. But in Ferns the facts 
are less cogent; for though abortive sporangia and imperfect sori are at 
times found on Fern-leaves, still the evidence that they are vestigial is less 
clear than in Lycopodium, Isoetes, or Ophioglossum, owing to the less 
definite position and number of those parts in Ferns. The conclusion that 
the foliage leaves or parts of leaves in Ferns are phylogenetically sterilised 
sporophylls, or parts of sporophylls, is therefore based rather on broad 
comparisons and on analogies with other Pteridophytes than on the direct 
observation of parts which may be held to be vestigial. That such a 
transmutation may take ‘place in the individual life is fully demonstrated 
by the experiments of Goebel above quoted. It seems therefore reason- 
able to hold for Ferns, as for other Pteridophytes, that sterilisation of 
sporophylls has been effective in the course of their evolution. 
A converse view to that thus stated has been habitual in the past, 
and is maintained by some to the present time. By them the evolutionary 
history is read in direct terms of the ontogeny, and the sterile leaf is thus 
assumed to be-the primitive leaf, which has become a sporophyll by the 
superposition upon it of sori and sporangia. Those who take this point 
of view have brought forward in its support the facts that the develop- 
ment and structure of the sterile and fertile leaves is closely alike, and 
that intermediate forms exist frequently between the two, so that the 
