THEIR GENETIC RELATIONS 171 



N. 



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. Setago, 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 y 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 



