I02 LEAF-ARCHITECTURE OF FERNS [CH. 



degrees of webbing of the lobes to form broad expanses with dichotomous 

 venation. But where the blade becomes elongated sympodial development, 

 with or without webbing, makes its appearance. In large leaves the dicho- 

 podium becomes massive below, appearing as a direct continuation of the 

 robust leaf-stalk, but fading out distally with transition of the branchings 

 to the primitive equal dichotomy. In such cases the lower pinnae arise 

 laterally from the phyllopodium by monopodial branching, as in the leaves 

 of P'lowering Plants. Varying distribution of the intercalary growth through- 

 out the length of the phyllopodium, and differences in the time of its develop- 

 ment as well as in the extent and duration of its apical growth and branching, 

 give that great variety of character to its outlines which is seen in the leaf 

 of Ferns. Where the segments are narrow the veins end free. This is some- 

 times the case also where the segments are webbed. But in more advanced 

 forms than these the veins are apt to be joined by vascular loops, or com- 

 missures, so as to constitute a network which is coarser and less complete 

 in relatively primitive types, but more finely reticulate in later and derivative 

 types. The external characters of Fern-leaves when analysed along the lines 

 thus indicated are found to provide material for comparisons which are useful 

 as leading to their phyletic seriation. 



One general result of the analysis which has gone before will have special 

 interest in the later discussions. It is that the basal region where the 

 "stipular" outgrowths are, and the distal end where the phyllopodium runs 

 out into the primitive dichotomy, may be recognised as those regions of the 

 leaf which are relatively archaic in their character. But the middle region, 

 and especially that where the origin of the pinnae has become monopodial, 

 is a part of later origin : in a sense it has been intercalated in descent. This 

 conclusion will be found to coincide with certain anatomical facts which will 

 be disclosed later. 



The steps in advance in complexity of leaf-architecture thus briefly 

 sketched harmonise with biological probability. A ready way of elaboration 

 of a simple foliar structure is by distal dichotomy, which is illustrated in 

 Sphenophyllum and in Asterocalamites (Fig. 98), as well as in the Filicales: 

 and it appears that it was widespread among the leaves of early Vascular 

 Plants. A lateral webbing together of the lobes thus produced is progressively 

 illustrated both in the Sphenophylls and in the Calamarians, and it leads to 

 a more effective expanse of photo-synthetic tissue (Fig. 99). But the modern 

 Filicales show it in higher degree than they, in proportion to the larger size 

 of their leaves. A mere fan-like expansion of dichotomy is less effective in 

 producing a large photo-synthetic organ than would be gained by a longer 

 form. This further step is secured by inequality of the forking, which leads 

 to various types of dichopodium with or without lateral webbing. In the 

 latter case a separate course of the veins has served well enough in relatively 



