4 INTRODUCTION TO VOLUME II 



many thousands to a single one gives a numerical index of the progressive 

 simplification that cannot fail to be impressive (Chapter Xlll). 



(lo) The vegetative features of the prothallus are chiefly negative, and 

 for phyletic purposes stand far behind those of the sporophyte. The structure 

 is uniformly parenchymatous, and the form is plastic under varied conditions 

 of lighting and moisture. The persistence of the cordate type is noteworthy, 

 but it may under certain conditions take a filamentous form, while the latter is 

 characteristic in certain genera. The massive mycorhizic type may very well 

 be a special modification following on fungal nutrition. The Eusporangiate 

 types have relatively massive prothalli as a rule, but it is difficult to found 

 any consecutive phyletic argument on the vegetative characters of the pro- 

 thallus (Chapter Xiv). 



(i i) The sexual organs of Eusporangiate Ferns are apt to be more deeply 

 sunk than those of Leptosporangiate Ferns, which habitually project from 

 the prothallus. In this they compare with the sporangia. The archegonia 

 appear highly standardised for the Class, presenting little of comparative 

 value in Ferns. But it is not so with their antheridia. These are not only 

 deeply sunk in the Eusporangiate Ferns, but are also massive: while in 

 Leptosporangiate Ferns they project and are relatively delicate. The sperm- 

 numbers in each antheridium run roughly parallel with the spore-numbers 

 in the corresponding sporangia, the primitive Ferns having relatively large 

 numbers in each of these distinct organs. This parallel cannot be pursued 

 into strict numerical detail, but along the lines thus indicated the sexual 

 organs provide useful confirmatory material (Chapter XIV). 



(12) The embryology of Ferns took a new value for comparison when a 

 suspensor was discovered in certain Ophioglossaceae and Marattiaceae. It 

 seems probable that this is an archaic feature stamping a primitive character 

 of the plants in which it occurs. In all others it is absent, and they may be 

 held as derivative in this feature. Its existence in these and other plants 

 suggests that the embryo was originally a spindle-like structure, one pole of 

 which is the suspensor, the other the apex of the shoot : while the root is an 

 accessory organ of lateral origin. From a phyletic point of view the presence 

 or absence of a suspensor appears to be the most important comparative 

 feature relating to the embryo (Chapters XV, xvil). 



The strength of the field of comparison thus widened by new criteria, and 

 supplemented by those already in use, does not lie simply in the number of 

 the lines upon which the comparisons are based, but upon the degree in which 

 those lines run parallel. In so far as they do so they mutually support one 

 another. This may even be held as a test of the validity of the criteria them- 

 selves. As the application of the method proceeds in the present volume it 

 will appear that the parallelism is impressively uniform. But it cannot be 



