212 PRINCIPLES OJ]' PLANT-TEEATOLOGY. 



to exist between peltate sporophyll and vegetative 

 leaf, and in showing the method by which the one 

 type of leaf is able to pass into the other by means of 

 certain modifications of the structure. 



Sporangiody of Spouophyll-segments. — As an ex- 

 ample of the appearance of sporangial tissue on 

 normally sterile portions of the sporophyll, the 

 interesting case of Botrychium (Ophioglossacese) may 

 be brought forward, in which some of the segments of 

 the vegetative portion become sporangiferous, or even 

 the entire blade (fig. 138). In the former instance 

 this may be regarded as a reversion to the ancient 

 type of fern-sporophyll in which sterile and fertile 

 parts of the frond were more evenly distributed ; in 

 the latter we see merely an extreme example, repre- 

 senting an exaggerated swing of the pendulum, which 

 can have no reversionary value. 



Poisson mentions and figures an instance of the 

 same phenomenon in the elks-horn fern {Platycerium 

 biforme). The rhizome being very short, the fertile 

 fronds, presumably, were unable to develop owing to 

 lack of space. Apparently in compensation thereof, 

 portions of all sterile fronds jsresent, instead of remain- 

 ing in the normal adpressed and prostrate position, 

 grew upwards and produced sporangia on their upper 

 surfaces. 



There may, perhaps, be introduced here a case of 

 androgyny. 



Heinricher and Grray describe abnormal sporocarps 

 of Salvinia natans (HydropterideEe) in which both 

 micro- and megasporangia occur. This is obviously a 

 reversion, for in Azolla vestigial developments of the 

 one kind are usually found in company with the other, 

 and in Marsiliacese it is a normal feature for both kinds 

 of sporangia to occur equally well-developed in the 

 same sporocarp. 



General Gonclusions. 

 Already, at the end of each section, generalisations 



