338 ORGANOGRAPHIC COMPARISONS [ch. 



Ferns of the present day have not only their sporangia complex in segmenta- 

 ^ tion — which is the feature of Eusporangiate types — but all their apical meri- 

 stems are also relatively complex. In the primary segmentation of each of 

 their parts more than a single initial cell is the rule. This indicates a general 

 robustness of organisation of the whole plant. But in the Leptosporangiate 

 Ferns there is a relative simplicity and regularity of all the apical regions, 

 with a single initial cell in each, a state which runs parallel with their precise 

 sporangial segmentation. This precision is regarded as secondary and 

 derivative. Naturally the apical meristems of Palaeozoic Plants can only 

 occasionally be examined in detail, and cannot be expected to give exact 

 results. But such comparison as is possible suggests that tlie primitive Fern- 

 types would probably have been relatively robust in the primary organisation 

 of all their parts, as are the living types to which they ai^e related. 



The vascular system gives a better foundation than any other structural 

 features for comparison, and its features are specially applicable in the phy- 

 letic treatment of the later and derivative types of Ferns, in which more 

 complex primary structure is seen in the adult shoot than in any other class 

 of plants. Passing backwards in Descent from these elaborate and derivative 

 states, the vascular system is found as a rule to be of simpler structure, until 

 in some of those which in other respects are held as primitive, a condition 

 is seen where the axis is traversed by a solid cylindrical column, or proto- 

 stele. A like structure is found also in very early fossil Ferns, and a similarly 

 simple structure appears in all Fern sporelings, however complex the adult 

 structure may be. Thus the evidence from comparison of adult living Ferns 

 held as primitive, whether the facts be derived from early fossils, or from 

 the first steps of the ontogeny of living Ferns, all points to the same con- 

 clusion; that the protostele is the primitive vascular construction for the stem 

 of the Filicales. Similarly the leaf-stalk is traversed by strands of the leaf- 

 trace : and comparison along similar lines shows that the primitive vascular 

 supply to the leaf was a single undivided strand, and that in the earliest types 

 it was oval or almost cylindrical in transverse section, thus resembling that of 

 the axis. 



The non-vascular tissues give less useful material for phyletic treatment. 

 The most constant features about them are the facts that they serve for 

 nutrition and storage, and that they embed the conducting tracts. But it is 

 otherwise with the dermal appendages. All living Ferns have them of one 

 sort or another, though sometimes only sparingly: and they are frequently 

 absent from the adult parts. Simple hairs are found in primitive types such 

 as the Osmundaceae, and they are recorded for Botryopteris. Flattened 

 scales are an indication of advance, and they are found in most of the later 

 Leptosporangiate Ferns. TJie most archaic Filical types ivould therefore be 

 expected either to bear simple hairs, or possibly to be glabrous. 



