THEIR GENETIC RELATIONS 171 
necessary basis either for the older view, that the sporophylls are altered 
foliage leaves, or for the view that the foliage leaves are sterilised sporo. 
phylls: but they do not tell distinctively for either. The decision must 
rest primarily upon the presence of vestigial sporangia, together with broad 
comparison rather than upon details of individual development. Still it 
is necessary that any final conclusion should be in accord with the details 
‘of the individual development, and this is so in the present case, whichever 
of the alternative conclusions be adopted. 
Finally, the interesting demonstration by Goebel, that the sporophyll 
may be experimentally converted into a foliage leaf, does not serve as 
a decisive proof of either view. It demonstrates, however, the close relation 
of the two which either hypothesis will demand. It shows also that 
sterilisation of a sporophyll such as our hypothesis requires can actually 
occur. Such a process of sterilisation, carried out continuously in the 
‘course of descent, and involving either whole leaves or only parts of them, 
would result in the differentiated character of the leaves of Ferns which 
is actually seen in nature. 
The leading types of Pteridophytes have thus been reviewed as regards 
the relations of their sterile and fertile regions. In the individual life 
of them all, there is at first, as their physiological condition demands, a 
more or less extensive vegetative phase, succeeded sooner or later by 
a fertile phase, though this is often not clearly differentiated from it. By 
comparison, it may be concluded that vegetative leaves have been derived 
by sterilisation from sporophylls; and it is not difficult to realise how a 
vegetative system may thus have been increased, and the production of 
spores have been delayed in the individual life. 
On the other hand, the unlimited apical growth seen in many of the 
Pteridophytes, acts as a set off against the progressive sterilisation, for 
it tends to preserve the balance of the sterile and fertile regions which 
the sterilisation would disturb, and still provides for the initiation of an 
adequate number of spores. In the simpler strobiloid forms, such as 
L. Selago, it is easy to conceive how progressive sterilisation and continued 
apical growth combined would lead to a larger vegetative system and 
an increased final output of spores. In the more complex Ferns a progression 
of a parallel nature may be traced, though with less exactitude, owing to 
the fact that the large individual leaves do not develop as units. Any 
individual Pteridophyte plant may thus be regarded as being the resultant 
of two progressions: advancing sterilisation below, and apical growth, 
with or without branching, which provides for additional spore-producing 
capacity above; and it may be ‘pictured to the mind, especially in the 
strobiloid forms, how the fertile zone, which is limited below by the limit of 
sterilisation, may thus have been raised progressively higher on the axis as 
development proceeded, and the time of spore-formation may have been 
correspondingly delayed. But it is essential to remember that however long 
it is delayed, the spore-production which eventually happens is the same 
