CHAPTER XLII. 

 EMBRYOGENY OF THE PTERIDOPHYTES. 



No great difficulty is experienced in recognising the sporogonium of the 

 Bryophyta, in its various forms, as the result of the working out of the 

 requirements in respect of increasing spore-production and consequently 

 of nutrition, under conditions of sub-aerial life. They are believed to 

 present a sequence of forms for the most part caught in the up-grade 

 of evolution, though showing occasional evidences of reduction. 1 But in 

 the more complex Pteridophytes the case is different : they have, according 

 to our hypothesis, proceeded so far in the elaboration of the sporophyte 

 that the steps of earlier evolution are less easily grasped : and as the 

 area of fact involved is very much greater than in the Bryophytes, and 

 the application of the theory of antithetic alternation, with sterilisation as 

 a leading feature, has never till now been fully formulated for them, it 

 will be necessary to summarise the evidence which has been derived 

 from the comparative study of their sporophyte generation. This summary 

 will be arranged in order of the events of the individual life, starting 

 with the embryology, and proceeding to the vegetative, and finally to the 

 propagative system. 



From the criticisms of the older methods of comparative embryology 

 advanced in Chapter XIY., it will be gathered that at the moment the 

 study of the earliest phases of the individual, as an avenue to an opinion 

 on the morphology and phylogeny of Yascular Plants, stands in a dis- 

 credited position. Modern analysis has disproved the conclusions drawn 

 from the primary segmentation, and shown that there is no constant 

 relation between cell-cleavages and the genesis of the several parts. 



'It is possible to make out a case for the converse view of the Mryophytes as a 

 series in which the dependence of the .s|ior<>phyte has been .secondarily acquired, and 

 reduction widely effective; but that idea i> not .seriously entertained here, as it is not 

 based upon observation o any actually existent organisms indicating thai such a progression 

 took place: nor has any physiological ground been advanced a> a Millirieni reason lhat 

 the presumed reduction should have been carried out. 



