92 



LEAF-ARCHITECTURE OF FERNS 



[CH. 



A scorpioid dichopodium naturally results in a midrib with apparently lateral veins, 

 a condition which is seen in most Ferns. Where there are separate pinnae or pinnules 

 borne upon a rachis, their margins are distinguished respectively as cuiadromic, which is 

 that directed towards the leaf-apex, and catadromic, that directed towards the leaf-base. 

 Mettenius (66, iv, p. 2) found that in the venation of Ferns there is some degree of constancy 



Fig. 86. Young leaves of Angioptcris. ^ = apex of leaf bearing 

 six alternate pinnae. The phyllopodium ends abruptly in a cone, 

 ap. j9 = apex of a younger leaf with alternating pinnae 2^9, 

 formed by monopodial branching. a/ = apex. The pinnae 2, 3 

 have begun to form pinnules, also monopodially. 



Fig. 87. Young leaves from seedling 

 of Cycas. A shows a younger state, 

 where the pinnae are appearing mono- 

 podially, on the flanges of the phyllo- 

 podium (x 14). i? = a later stage, where 

 the two distal pinnae suggest a rever- 

 sion to dichotomy ( x 2J. 



Catadromic 



Anadromic 



Fig. 88. Anadromic and Catadromic sequence of branchings 

 of veins in leaves of Ferns (after Christensen). Note that 

 the distinction applies to the pinnules. But it is only 

 available for leaves which have branched at least twice. 



in the relative position of the first or lowest vein. He designated the venation as anadromic 

 when the lowest lateral vein of a pinna or pinnule lies on the acroscopic side, and cata- 

 dromic when it is on the basiscopic side (Fig. 88). Potoni^ indicated that this character 

 had a phyletic bearing (72, p. no). He drew attention to the preponderance of the cata- 

 dromic venation among Palaeozoic Ferns, while in the majority of modern types it is 

 anadromic. That this criterion can only be applied in general terms and not in detail 



