2 INTRODUCTION TO VOLUME II 



(i) The simple shoot, composed of an axis and an acropetal succession 

 of leaves, is the unit of construction of the sporophyte. In primitive types 

 it is commonly unbranched, upright, and radial. The prone position with 

 dorsiventral symmetry is probably derivative. The branching of the shoot 

 gives distinctive features. All the distal branchings (exclusive of adventitious 

 buds) may be referred to dichotomy. Shoots which dichotomise equally 

 may be held as primitive in that feature, and any departure from equality, 

 so as to produce some form of dichopodium, may be held as derivative 

 (Chapter iv, Vol. i). 



(2) Comparison as regards cellular constitution of the apices of stem, leaf, 

 or root, or of the wings of the leaf, or of the sporangia, shows variation from 

 types having regular segmentation of a single initial cell, as in Lepto- 

 sporangiate Ferns, to those where the segmentation is not referable to a 

 single initial, but to several, as in the Eusporangiate Ferns. Comparison 

 with related fossils shows clearly that the latter type is relatively primitive. 

 Hence the provisional hypothesis is entertained that the more robust 

 Eusporangiate Ferns appeared first in descent, and that the less robust 

 Leptosporangiate Ferns are derivative and specialised forms, and appeared 

 later in descent. In fact that there has been a progression from a more 

 robust and less exact to a less robust and more exact type of organisation 

 in Ferns (Chapter vi). 



(3) The leaf is traceable in origin to an elongated rachis with a dichoto- 

 mous distal region ; and in primitive forms it may have basal stipular growths. 

 In various ways sympodial development of the distal dichotomy may give 

 dichopodia, the pinnae being essentially branches of the distal region. But 

 in advanced types where a phyllopodium is strongly established the lower 

 pinnae may arise monopodially, the later only arising by dichotomous 

 branching. In primitive types the segments may be all separate, each with 

 a single vein. All webbed expansions are held to be derivative. The primitive 

 venation is always open with free endings: looping of the veins in various 

 ways to form a reticulum is held to be derivative (Chapter v). 



(4) The primitive vascular structure of the axis was the protostele. Those 

 Ferns which in the adult state are structurally nearest to being protostelic 

 are held to be relatively primitive in respect of that feature: those which 

 have departed from it, showing solenostely, dictyostely, polycycly and per- 

 foration, are held to be relatively advanced. Similarly the primitive leaf-trace 

 is an integral strand : the undivided horse-shoe curve of the Osmundaceous 

 type which follows is probably the prototype of later Fern -petioles, while in 

 yet more advanced types the meristele breaks up into parts still disposed in 

 the original curve. The marginal pinna-trace is relatively primitive, and the 

 extra-marginal derivative (Chapters VII to x). 



(5) The dermal appendages may take the form of simple hairs or scales. 



